RE ENGINEERING THEMAMOD LOCOMOTIVE

he main purpose of these notes is todescribe the way in which the Mamodsteam locomotive can be significantlyupgraded in performance. But there ismore to it than that. Although this

will give the dimensions and sequences ofoperations to the experienced modelengineer, it has been written with thebeginner in mind; someone who hasmodeled and worked with materials andenjoyed the work. But specifically it isintended as a gentle first exercise forsomeone who has recently acquired a latheand who may still be a bit nervous of usingit.

The operations can be carried out on anylathe, right down to an inexpensive miniatureone. The writer uses the Hobbymat MD65,and it seems just right for the task. On theother hand the whole job was done by anexperienced machinist on a Myford ML7, inno time at all.

As well as a lathe, you will see thatseveral tools are called for. These have beenkept to a minimum and all of them will givelong and faithful service on a variety ofprojects in the future. If you have a table ofequivalent drill sizes, you will need toconsult it. Various dimensions are given invarious types of measurement. This reflectsour habit of using the most convenient scaleat the time.

Peter Jones describeshow to upgrade this

popularmass-produced

locomotivePart I

The garden gauges offer a whole range ofinterests, and many of these are fairly lighthearted affairs which call for little seriousinput. But there are times when we needgood old-fashioned accurate machining. Oneplace where good quality work paysdividends is when it comes to uprating theMamod steam locomotive.

The manufacturers are most helpful, butpoint out that their product is aimed at thetoy market and that they must work to acompetitive price. By the enthusiast’sstandard, the locomotive is very simple andperformance is poor. But as well as themany cosmetic improvements, there isfortunately a modus operandi which willturn it into a respectable little runner whichpleases our engineering soul.

The first thing to do is to identify theindividual shortcomings. The locomotive asbought will exhibit some or all of theseinconsistently, but if we are going to

upgrade the beast, we might as well make adecent job of it. Hence problems are likelyto include:Wheels poorly secured on axles andoccasionally slightly eccentrically mounted;Crankpins not properly secured to wheels;Steam Valve leaking;Cylinders leaking steam;Piston leaking;Lack of gland on rear cylinder covers;Cylinder pivot pin a slack fit in the valvespace.

This is quite a collection, but they can allbe overcome. I’ll be describing the workthrough the medium of the locomotive assold in kit form, but the notes are readilyadaptable to redeeming a ready-builtversion.

Special ToolsWe need to make a couple of simple jigs.

The first is made from a bar of YB in. X 3/8in. mild steel and is shown in Fig. 1. Youwill find plenty of uses for this little toolanyway. Actually the dimensions of the bararen’t all that critical, but those given makethe tool useful for other jobs.

The second tool needed is a punch asshown in Fig. 2. The dimensions of theturned shoulder are critical. We need to

T

know the diameter of the small shoulder ofthe axle bush. It is important to check allfour with a micrometer and use the smallestsize for reference. The depth of the 0.035 in.cut is best measured on the lathe scale, butthe diameter needs to be exactly 0.002 in.less than our micrometer reading, so taketime to get this right.

If only one locomotive is being rebuilt,then the hardness of the silver steel will besufficient. But for more than one, somehardening and tempering will be called for.The newcomer will find this an excellentintroduction to the subject. the job is just anice size for heating up until the metal ischerry red, quenching, polishing andreheating to a specific

The purpose of this punch is to CLOSEUP the holes in the main frames so that thebushes are a snug fit. This seems acontradiction at first sight; punching a holesmaller. What’s more it is a technique whichdoesn’t often appear in print. We need adecent anvil. This doesn’t mean somethingthat looks like one and is very expensive.

Nor does itmean one of those dainty little things on saleat shows, etc., these days (although theyhave their uses). Nor does it mean the lathebed! What we want is a big flat chunk ofmetal. The underside of an old smoothingiron does nicely. Incidentally, for some jobs,a shoemaker’s last is handy to have around.But many things can be pressed into serviceprovided that they are flat and have enoughmass to give the job some “bounce”.

The punch is held vertically over theframe and given a sharp tap with a decentsized hammer. The action of this is to forcemetal downwards and “fill” around thespace of that shoulder. If our 0.035 in.dimension was right, the punch should nowjust touch the anvil. The hole will be smallerand there is a slight depression on theoutside of the mainframe.

Punch the other three holes in a similarway. Immediately the 4th one is punched,mark the mainframes left and right, as thisdepression means that they are now handed.Just touch the outside of these holes with a90 deg. countersink to make sure that thebushes will have an easy entry when theyare pressed in.

colour — in this case purple - and thenquenching out. if the word gets out thatyou can work magic with poor runningMamods, there will be a path beaten toyour door by people with work for you!

e can now put the frames to oneside and turn our attention to thewheel assemblies. Put the jig inFig. 1 into the vice, with the Ushape projecting out from the

side. Use a 2mm pin punch to tap the axleout of the wheel, whilst the assembly sits in

the cut out portion. If you don’t have such apunch, it is a very, simple turning job for thebeginner to make. Turn a 2mm parallelportion on a length of 3mm dia. silver steel,leaving a generous radius on top. Hardenand temper as described in Part I.

Don’t let the other wheel/axle drop ontothe floor. It was written in the stars that, if itdoes, the bushes will slide off the axle andbe lost to mankind for evermore. With allfour wheels separated from the axles, turneach one upside down, put the crankpin intothe hole at the other end of our jig, and tapthem all out as well. Be warned thatcrankpins are even better than bushes fordisappearing

Note that there are two short crankpinsand two long ones.

The wheels need to be drilled out to3.9mm diameter. The alloy is quite soft and,illogically, this makes the need to drill theholes truly even more important. A hand-

drill invites problems. We need a drillingmachine or the job done in the lathe. I amafraid that you are going to need to buy asmall tool here; there is just no way round it.The beast in question is a 5/32 in. dia. handreamer. It will see a lot of use in this job andwill find plenty of gainful employment inyour later model engineering career.

From the back face of the wheel, enterthis reamer slightly. You will see that aparallel hand reamer has a slight taperedportion at the business end. This is importantfor our needs. The tool is entered just enoughfor 1/16 in. depth of the hole to be opened outto 5/32 inch. It isn’t as fiddly a job as itsounds. The purpose of all this is to give aperfect press fit for the wheels onto theaxles.

Wheel TurningThe next task is to turn the wheels true

and improve their profile. For accurate

machining, we can’t rely on the chuck alone.We can muddle along, doing all sorts of jobsin a haphazard sort of way, but soon or laterwe come to a job that has to be done right;and this is one of them.

All wheel turning needs to be done ona mandrel - a little jig (Fig. 6) which is turnedin a chuck. We try not to disturb it once it is

in place and, if we have to, we make a littlemark on the bar, adjacent to Jaw No. 1 onthe chuck. This way we can take the jig outand put it back in exactly the same place.This ensures that we continue to eliminateany error in the chuck. Work held in oneposition will always run truly to the spindleof the lathe once it is turned true.

The mandrel is made from a piece of l 1/8in. dia. mild steel, about 1¼ in. long. Turnthe end face true, whilst you are cuttingaway metal, leave a short spigot projectingfrom the centre. This projection only needsto be 1/8 in. long. With that pop mark madeagainst No. 1 jaw of the 3-jaw chuck, we canremove the job with confidence that it willgo back in the right place.

Push a wheel onto the projection. TheMamod wheel has two lightening holes castinto it. Mark the face of the jig through oneof these, (this is referred to as “spottingthrough”). Remove the wheel so that you candrill and tap 4 BA at this point. The drillingsizes for all threads are given in tables. Mineare the much thumbed back pages of an oldA.S.P. Plans Handbook. The jig can now bereturned to the 3-jaw, lining up that popmark. The wheel is pushed onto the spigotwith a short piece of 4 BA studdingprojecting through one of the holes. This willprovide a positive drive to the work.

The tailstock is brought up and arevolving centre presses the job tight to thejig. If a live centre is not available then asimple little tool can be turned up as shownin Fig. 7. This is a pressure pad and will dofor us if we are only doing one locomotive.It will need lubricating with EP oil.

RE ENGINEERING THEMAMOD LOCOMOTIVE

Peter Jones describeshow to upgrade this

popularmass-produced

locomotive

W

We are going to true the wheels up andturn them to a better profile. In real life,narrow gauge eschewed a common standardmagnificently. In model form, this traditionis being upheld. For the serious engineer itseems peculiar that there is no commonwheel standard. There is vigorous debateabout the subject in the 16mm Society, butthat doesn’t help us much at this stage!However, I take courage in both hands andoffer you Fig. 9. The dimensionsshown should give a good all round

performance on most narrow gauge tracks.Check through your wheels and find the

one with the thinnest flange - they can and dovary, so check with the micrometer. Mountthis wheel in your jig and set the lathetopslide over to 4 degrees. The tool needs tobe ground so that it will give a front face tothe flange which is at 10 deg. to the vertical.This tool is wound into the work until theface of the flange is cleaned up. Note thereading on the topslide and wind in the toolslowly until the wheel has been profiled.

At this stage the topslide is wound backand the cross-slide handle is not touched.The first wheel is removed from the jig andthe next one replaces it, making sure that allfour wheels are the same is now just aquestion of winding in the topslide until wehave reached our reading. When all thewheels are done the tip of one flange isskimmed and, without altering the setting ofthe cross-slide, pull the tool away from thework. Then repeat the procedure for theother wheels. Finally, with a file, just roundoff the tip of the flange and break the edge ofthe front face of the wheel. Using a small filefor such a light job in a spinning lathe isnearly good engineering practice, but don’ttell anyone I said so. For safety’s sake ensurethat there is a handle on the file, a tang cangive a nasty injury.

We are going to need new axles. Theseare made from 5/32 in. dia. silver steel orprecision ground mild steel. Fig. 10shows that we just file a slight taper on eachend and radius the corners off a smidgeon.this will make the axle just the right push fitinto the wheels.

For a really deluxe job we can drill theends of the axles about 1/ in. dia. X /16 in.deep. A 1/64 in. hole is drilled radially tomeet this hole. By means of an hypodermicsyringe the bushes can he lubricated. It isworth keeping this idea tucked away forprojects in years to come. See Fig. 11 fordetails.

Next time we will deal with the crankpinsand reassembly of the wheelsets.

RE ENGINEERING THEMAMOD LOCOMOTIVE

he crankpins, even though they havebeen tapped out from the wheels, willhave belied ends and this will needattending to before we can replace

them. Fortunately the 3-jaw chuck seemsto have been invented expressly for thispurpose. The pin is held in the drill chuck,which is held in the taiistock. This is thenbrought up until the other end is located inthe 3-jaw, now all that is needed is a smalltightening of the jaws and the job is done —

see Fig. 13.

The pins are a slack fit in the crankpinholes and need securing with Loctite “Nut-lock” or similar. Now, provided that youhaven’t lost them, the circlips need to be putin place on the crankpins. This is important atthis stage; otherwise when you come to rivetthe pins in place, the circlip groove maycollapse.

A 2.6mm dia. hole is drilled somewhere inour “dismantling jig” (Fig. 1). A little pip ofa punch (Fig. 12) gives the first stage ofriveting — you will note that the wheel has tobe packed out from the jig. The second stageof riveting is to peen the ends out using amore conventional punch. If you wish, youcan turn up a punch of such a shape as to doboth operations in one go, as shown in Fig.14.

Remember when you refit the wheels, thatthose on one axle have short crankpins andthe other has the long ones. Yes, I know it’sobvious and you won’t need reminding, but.

The wheels are reassembled on the U-shapedcut-out on our jig. The area of the framesadjacent to the axle hole must be supportedwell and so the work is done at the deepestpart of the “U” (Fig. 15). Also remember thatthe wheels carrying the long crankpins go atthe front end of the frames.Insert 2 pairs of wheels in one frame and thenfit the bushes into place. The bushes aretapped in place with a hammer and a 1/2 in.dia. brass drift, see Fig. 16.

We now have two frames held togetherrather tentatively by the axles. This needs tobe improved on rather promptly and so the

middle spacer tube is screwed in place. Thechassis is stood on a flat surface and checkedfor wobble. Rectification at this stage is easy.One of the screws holding the frame spaceris slackened off and slight pressuredownwards on the assembly will force theframe square. Without removing thepressure, the screw is tightened up again. Ifonly all model locomotives could he trued upas easily as this.Crankpin Check at Quartering

Before proceeding further, let’s check thatwe have nothing in the wrong place. The twofront wheels have the long crank-pins andone frame isn’t upside down relative to theother. Nothing has been said aboutquartering the wheels until this stage, butnow is the time to tackle it. Again, it turnsout to be a very simple task. Place thecoupling rods and washers on one side onlyand slip the little circlips into place. Now tryto fit those on the other side. Unless“someone up there” thinks very highly ofyou, they won’t fit. Take the trailing axle androtate the wheels against each other until thecrank pins are set at 90 deg. relative to eachother. The perfect tools for this job are acouple of circlip pliers, but long nosed plierswill also do. You could make up a couple oflittle tools yourself. These would simplyconsist of handles with a couple of pegs apush fit into the body of the tool.

You will find that because you have gotthe axles a really good tight fit in the wheels,you will just be able to move them, but at thesame time, once they are set right, theywon’t move again on the workinglocomotive. The front axle is now attendedto. With a little trial and error, a position willbe found where, once the coupling rod isslipped on, all four wheels revolve withoutbinding. At this stage slip the rest of thefittings onto the crankpin. Could I justremind the reader that whilst

T Peter Jones describeshow to upgrade this

popularmass-produced

locomotive

quartering should mean that the crankpins onone side should be at exactly 90 deg. to theother, one degree more or less won’t makeany difference. It is primarily the fact that theangle should be the same on both axles thatcounts. This work leaves us with a rollingchassis, with coupling rods fitted, that willrun up and down a piece of track “nice andsweetly”.

Truing-up the Reversing BlockLet us now turn our attention to the

reversing block. As supplied, it is anythingbut flat and will usually spill steam at analarming rate. To true this up a surface plateis the ideal tool, but failing that a 10 in.smooth hand file will do the needful job quitenicely. Lay the file down and work the faceof the two components up at down with asteady pressure (Fig. 17). This is a traditionalmethod used by brass finishers and gives agood finish in its own right. Keep inspectingthe job. What you are looking for is thesurface covered entirely with the fine scratchmarks. When this is achieved, the face istrue, although still rough. Glue or tape apiece of 880 grade wet and dry paper to a flatsurface, again it can be a piece of glass. Thepaper could be taped to a drilling machinetable if one is available. Actually, whiteMelamine faced Contiboard gives a nice truesurface provided that it lays flat on a bench. Iusually keep an offcut handy with a couple ofself-adhesive sanding discs stick down. This,combined with fine wet and dry, makes avery useful tool in the workshop.

To clean up the scratches on the faces, justwork the job up and down on the abrasivepaper (rather than the usually suggestedfigure of eight motion). Once more, keepinspecting the job. Once the last of thescratches has disappeared, the job is done

THEMAMOD LOCOMOTIVEPart IV, from page 293

Peter Jones describeshow to upgrade this

popular

B efore going further. there is a smallmodification to he done to the valveitself in Figs. I8 and 19. You will see

mass-producedlocomotive

the two semi-c i rcu lar channels have thing to hear in mind is that we should onlyrounded ends. We want to extend these extend the openings by about 1/16 in. or so.into points. There is no need for any The best tool for the job is a penknife. Thisprecise machining here as we are not carves the shape nicely and we could finishworking to an exact dimension. The only off with a small oval file.

Fig. 18VALVE AS PRODUCED BY MAMOD

(Note that dimension “B” should be 1/64 in. greater than thediameter of the hole on the valve block. Check with job).

Fig. 19

RUN OUT THE SEMICIRCULAR INTOA POINT

430

Fig. 20EXHAUST STUB

The purpose of this job is to give a lesssharp entrance to the channel as the valverotates. This will give us a slightly moresensitive throttle opening. Touch the faceof the valve on the wet and dry to removeany slight protrusions we might havethrown up whilst hacking away.

As made, the Mamod valve lifts off at12-15lb./sq. inch. If we want to upratc theboiler pressure, we will need to improve onthe reversing valve springing. To do thisrun a 5 BA die down the valve bolt. It runsdown for a distance equivalent of thethickness of the die. and so is easily meas-ured as we cut threads. Assemblc the valvedry in order to hcd the two componentstogether. Finally. peen over the end of thebolt and then screw backwards until it jamson the valve.

The exhaust system is modified by cut-ting the pipe back to a stub 5/8 in. long, seeFig. 20. Slip a piece of silicone tube overthis. The top should come just below thetop of the chimney. The effect of this smallmodification is to give us a nicer show ofsteam from the chimney and less conden-sate dribbling down onto the track.

-r

r, /5/8"

With that slightly cosmetic job done, wenow turn to the heart of the project - thecylinders. Engineering miracles are notcalled for, but we will want to take our timehere. It would be useful. perhaps, to list thevarious faults - some or all of which maybe present:

Lack of flatness on the valve face.Oversize pivot pinhole which causesheavy knocking.Valve ports which cut off steam beforethe engine has reached the top of thestroke, and don’t open to steam until thepiston is well past top dead centre. Thislast is usually the main reason for poorperformance. Indeed, if it weren’t forsteam leakage past the piston and valvefaces, the locomotive would barely runat all.We start the job by putting a stub of 3/4

in. dia. bar in the vice. We want a 6mm dia.pin punch next (again, a nice little turningjob), We strike the punch down into the

MODEL ENGINEER 5 OCTOBER 1990

-_. .

THEMAMOD LOCOMOTIVEPart V, from page 437 Peter Jones describes

how to upgrade thispopular

mass-producedlocomotive

in. clear from the chuck jaws. With a wellsharpened knife tool. turn the piston downto 0.394 in. diameter. Pull back th e tool-post and, without disturbing the piston,offer u p the cylinder. It should just fit andno more, it is all to do with the feel of thething really. The best descriptio n I can giveis that it should push on with light handpressure, but without a trace of wobble. Ifit falls in it is too slack and you will have tomake another. Time and material arccheap enough to us. but without apology . 1repeat the dictum that , whe n tu rn ingmetal, it i s eas v to take it off but muchharde r to glu e it back on! The fit we arelooking for is called a sliding fit. The pistonis turned finally to a length o f 6mm . Repeatthis job with the second cylinder. Note thatthe packin g groov e is NOT turned at thisstage. The piston rod is now gripped in thechuck by only a few millimetres; the rest ofthe rod and the piston is standing right outin the fresh air. Apply a little oil to thepiston and then take a cylinder and work itbackwards and forwards on the piston.After only a few minutes of this you shouldhave a beautiful fit of components - areally silky feel in fact.

e will make a new piston andW piston rod. These will be screwedtoge ther and not a l lowed to

‘float’ as per the origina l Mamo d system.Various materials have been tried for thepisto n - bronze, aluminium, PTFE. etc.but given the circumstances of very lightloading and irregular lubrication, brasshas proved itself to be the best.

Making New Pistons and RodsFig. 27 shows the dimensions of the

piston blank. After turning a 1/4 in. lengthof brass rod to 0.420 in. dia., it is drilledand tapped 5BA. This is then counterbored3. I mm dia . X 0.12 5 in. deep. All of this isdone without disturbing the job in thechuck. The piston rod is made from 1/8 in.dia. ground stainless steel. Th e 5BA threadneeds to be cut accurately using the tail-stock dieholder. and i s 1/8 in. long, see Fig.28. The piston rod is now screwed tightlyinto the piston.

The next job has to be done accurately.This is to turn the piston true to an exactdiameter. This time the 3-jaw just isn’tgood enough. But, inevitably, there is aneasy way out . We will make a little devicewhich has been around since Noah turnedup the thrust bearings for the Ark. My petname for it is a crush collet and it goessomething like this:- Fig. 29 gives the

550

dimensions of a simple shouldered shapemade from 1/4 in. dia. brass bar. make acentre pop opposite number 1 jaw of the3-jaw. Face the end across and then drilland ream 1/8 in. dia. through it. This littledevice will now crush under a tighteningchuck and grip the piston rod on anaccurate centre. The piston rod is pushedin and gripped with the piston just abou t 1/8

Rechuck the piston rod deep in the bushand turn a groove away. This calls for aparting tool ground to a width of 0.050 in.wide. A groove is turned to a depth of 0.047inches. Don’t forget that this countsdouble when turning a bar and, in fact, thediameter of the grooved portion will nowbe twice 0.047 in., i.e. 0.094 in. less than thediameter of the piston.

Piston Groove PackingThe groove is packed with 1/16 in. round

graphited yarn. (Which can sometimes beobtained by unravelling a larger sectiontwist) Packing a piston is easy, providedthat you go about it the right way. The aimis to get what looks like a continuousdiameter of piston with the packing exactlyflush with the outer brass portions. At thesame time the groove must be comfortablypacked full. The trick is to wind thepacking around the groove until it juststands proud of the piston diameter. YOU

then take a flat piece of steel and roll thepiston with it on to a flat clean metalassembly like the lathe bed or the drillingtable. This rolling process tends to cut offany surplus, but an overpacked groovemight need a touch of excess cut off andthen try rolling it again. Once you havepacked your first piston this way, you willwonder what all the fuss was about andYou are set up for life in this department.

MODEL ENGINEER 2 NOVEMBER 1990

-.__

Should the packing still be slightly fat.provided that it is well rolled, pushing itinto a cylinder may well shave off theexcess anyway. Once more we need towork the cylinder up and down on thepiston until it feels right. This techniquevirtualIy guarantees perfect running,without leakage. ‘0' rings may be thebecs-knees these days but everyone shouldbc able to pack a gland or cylinder.

Replacement Cylinder CoversIt is necessary to make the new rear

cylindcr covers.complete with glands.These have to be accurately turned. but ifyou work through the job in the rightorder, it becomes very difficult to get itwrong! Fig. 30 shows just what we arc

2: A re- worked piston and cylinder. seen here partiaiiy re-assembled.

thread). Turn the cylinder register to 0.396in. dia. using a parting tool. Now part off

in. A/F hex brass. This is put in the 3-jaw

the job, leaving that register 0.100 in. long.and drilled 3/16 in. dia. x 7/32 in. deep. Thebottom of the hole is flat and this will cal

As you are parting off, just break the sharpedge to allow for an easy entry for the

for a simple reamer/broach. This can becommercial or home brewed. The internal

register into the cylinder. thread I S, of course, 7/32 in. x 40 TPI and

aiming for. First of all we chuck a piece ofper f i g. 3 0 D r i l l 3 / 1 6 " d i a x 7 / 3 2 d e e p

1/2 in. dia. brass rod in the 3-jaw. A portion Bottom of hole to be f lat Tap 7/32" x 40tpi

7/32 in. long is turned down to a size that willlet us cut a 7/32,in. dia. thread thereon. After making the back covers, the frontCentre and drill a hole 3mm dia.. 7/16 in.

you will need a second tap that wil1 run aones are a ‘doddle’, and are made as per thread down to the bottom. A hole 3.2mm

deep. then open up the hole to 1/8 in. dia. Fig. 31. dia. is drilled through the ‘end wall’ and ausing a machine reamer. Turn that portion The final component in this section is themcntioncd above 7/32 in. x 4OTPI (M.E.

very small chamfer is put on both its ends.gland nut, set Fig. 32. This is made from 1/4 Job done! To be continued

CANYOU HELP?

I wonder if you can helpme? I am trying to locate

e Lea Francis car modelown on the cover of theodel Engineer dated 12arch 1953.This model was built by theotor industry Jubilee

Exhibition of 1946. Maybeyou could provide me withsome leads. I know it may bynow be rather impossible.

I am a life-long Lea Francisenthusiast. I have 3 vintageLea Francis cars, two 1929“P” type 12/40 h.p. and a1932 “P” 12/40 so you see. Iwould be happy to see or evenpurchase this, or a similarmodel, of this the first LeaFrancis and incidentally, itwas also the prototype firstSinger car. So full size carexists as far as is known. onlyan engine. The cam rods areover 3 ft. long!G. Jol leyNr. Sunderland. Ty ne & W ear

I have.

The chuck is made in Indiaand has no other informationthan the name Guru.

As I was given the chuck asa present when I retired andthe person has since died. Ihave no idea where heobtained it.

I would be most grateful ifanyone could give me anyinformation.G. E. MatthewsInverurie. Aberdeenshire

Interchangeablc Steel TypeLetter and Number Set. Onchecking the set. I find thatsome of the interchangeableletters and numbers aremissing. 1 would very muchlike to purchase the missingItems. but I cannot get intouch with the manufacturer.

The makers name. EdwardPryor & Son Ltd. of Sheffield,is in the box, together with atelephone number. but thetelephone Iine is dead andthere is no one at the address.

I wonder if you could tellme if it is possiblc for me toreplace my missing items fromanywhere and does the Pryorfirm still exist?R. W. B e l l

MODEL ENGINEER 2 NOVEMBER 1 9 9 0 551

TMAMOD LOCOMOTIVE

T he piston rods now have to be turnedoff to length to match the old ones.But before the latter are consigned to

the useful scrap box transfer those littlemarks. which Indicate the position of thebig ends. to the new ones. Press the rearcylinder covers in place. Try the piston androt for fit. It should move silkily up a n ddown. Press rhc front covers on. If youhave got the dimensions right. these coversshould be a tight enough fit to withstandcylindcr pressure without any furtherassistance. Should you get one that blowsout then Loctite should retain them. Thenew Loctite 454 is particularly good as it isless pernickety about virtually sterile jointpreparation. If you don’t want to trust toan adhesive which might fail when over-heated, a taste of soft solder will do thetrick.

The gland is packed next. A piece ofgraphited yarn is wrapped round the

Peter Jones describeshow to upgrade this

popularmass-produced

locomotive(Part VI (conciusion), from page 551

piston rod 4 or 5 times. the excess cut offand then the gland nut is screwed up. If youare going to make a mistake, it will inevit-ably be in screwing the thing up too tightand draining away the powcr of the loco-motive. Be it a Mamod or a 4 ft. 8-1/2 in.gauge Pacific. glands should he just tightenough not to blow and no more. Ithappens that. in the case of the latter largeengine there is a lot more power availableto overcome the friction of a tight gland.

The cylinder pivot pins now need to beshortened so that the cylinders when thespring is fully compressed do not lift morethan 1/64 in. from the face. For once. this is atrial and error process. Start by puttingtwo more threads (5BA) on the pin. Thenfile off the end of the pin so that threethreads remain. Try this out. If it is still tooloose. cut another thread and file one offthe end of the pin. On no account over-tighten the thread. Not only can we notspare the power to overcome the tightness,but it is possiblc to cause distortion.

Finally AssembleWe now come to final asscmbly. Place a

gasket between the cylinder port face andthe mainframe and secure the assembly inplace with a couple of screws. nuts lightlytightened. Smear the sides of the valveblock with silicon sealant (sold as bathsea lan t ) making sure tha t the steampassageways are clear. Insert the foursecuring sc rews . Note tha t . dur ingassembly. the big ends need to be placed onthe crankpins. Because we now work totighter tolerances. we can no longer simply“spring” the big ends in place.

If possible. try a steam test here. Some-thing like a Mamod stationary boiler willdo - particularly if it has been fitted for ahigher pressure of around 20psi (see con-clusion). The engine wi l l probably turnover quite slowly at first - expect it to be abit stiff. But after 1-1/2 - 2 hours running itwill be as sweet as the proverbial nu t .During this running-in period put plenty ofoil everywhere: oil is cheap. cylindersaren’t.

This completes the engineering of thechassis and the rest of the locomotive canbe assembled. But to do justice to ourwork. there are some improvements to bemade in o ther depar tments . MessrsMamod are obliged to use solid fuel pelletsas a fuel. A spirit burner gives a better heatthroughout its burn (whereas the pellets

drop in cfficicncy as they burn through). Ifthis is combined with a Goodall Valve aloco can be kept in steam permanently. Forlubrication the simplest method is to gethold of a really thick steam oil. (Many ofour advertisers sell this in small tins - Ed.).A couple of drops of this are placed on thecylinder/portface joint. As the engine getshot the oil thins and runs down onto thefaces. It is not aesthetically pleasing but itworks. Obviously a proper lubricatorwould be a better solution.

Revised Safety ValveOne last job that we can tackle easily is

to uprate the safety valve. The thought ofthis can send cold shudders through anexperienced engineer, but in this case weare quite safe. Replacement safety valvesare available commercially but it is quiteeasy to do it ourselves. Mamod themselves.quite rightly. say that they do not holdthemselves responsible for any engine thathas been altered -particularly in regards tofuel and boiler pressure. They are in thebusiness of supplying the children’s marketamid some tight safety regulations.

_I

marks on the newIy cut surface. The stem isnow modified as per figure 34. It is a simpleturning job and calls for no comment.

The modification to the safety valve willuprate it to blow off at about 20 psi whichwill make the steam exhaust upwards.rather than sideways. The valve is dis-mantled by sawing off the valve stem justabove the crimping. This is another ofthose glorious opportunities for com-ponents to fly off into oblivion: in this casethe spring and the washer. Put the body ofthe valve into the 3-jaw, bottom outwards.and shorten the threaded portion to 0.200inch.

The valve is reassembled using a 3/32 in.dia. “0” ring - this must be in Viton orsimilar to withstand the heat. Using theexisting spring and washer. as Fig. 35, youwill now have around the 20 psi in theboiler before the valve lifts. this w ill upratethe available power quite considerably.

Conclusion

Put a scrap of brass in the chuck, drilland tap 1/4 in. BSF in the centre. Now screwthe body into the tapped hole. You willnow need a 1/4 in. dia. centre drill. This isused to drill into the top of the valve body.You will need to go in just far enough togive a short parallel opening which is 0.040in. deep. The shape of the centre drill willtake care of the rest of tht hole (see Fig.33). Run the lathe at about 200rpm whendrilling this hole. and enter the drill gently,It is essential that there are no chatter

The engineering is now complete. It maywell be that you can think of variousfurther improvements. But those we havetackled in these notes will give you a modelimproved out of all recognition inperformance.

The newcomer will have realised by nowthat accurate machining onIy calls fordoing things in the right order. and takingtime to do each job steadily Medals are notawarded for speed: mode1 engineering is agentle occupation. you soon get into theswing of it. If you have managed to do thejobs described in these pages. there is nolimit to what you can build: it is just a caseof tackling things one step at a time.

2: With driver on the footplate. theattractively styled narrow gauge 16mmlocomotive Albert ready for the track. Inthis case a George England design.

As well as the many home-made im-provements that can be made on theMamod steam locomotive, it is useful to beaware of the many commercial products tobe had; such as the Goodall Valve whichreplaces the whistle and allows a boiler tobe refilled under pressure, using a dispos-able syringe.

Several small firms produce replacementsafety valves of increased pressure.Likewise replacement spirit or gas burnersare available. Lubricators and regulatorsare also available. Messrs Kenversion pro-duce a range of cosmetic improvements -including complete drop on fabricatedbodies which look very well indeed. Infor-mation is available via the 16mm Associa-tion. details below.

A combination of some of the abovecomponents and a re-enginered chassiswill turn a toy with mediocre performanceinto an attractive looking model which willhaul a steady load all day. Perhaps men-tion should be made too of the bolt-onradio control outfits available.

Anyone taking an interest in Mamods wouid be well advised to join the 16mmAssociation. This splendid organization teems with upgraded and rebuilt Mamodlocomotives. At the AGM there is a section of the modelling competition devoted tothem For mcmbership details contact .

00 ST19 Great Britain. 8

MODEL ENGINEER 7 DECEMBER 1990 687