MAKING PETROL ENGINE PISTONSby “Chuck”

CHUCK BELIEVES it is true to say that the modelengineer generally has little use for a reproductionprocess. He acknowledges that the meticulousbuilder may well undertake the production of hisown bolts and nuts to scale proportions and, forthis purpose, sets up his lathe temporarily forrepetition work. And, although Chuck reallyknows very little of the requirements of the“steam man”, he can well imagine that a rapidlyproduced quantity of handrail knobs, all to theidentical profile, and a succession of quicklythreaded stay rods may well be worth an hour ortwo’s prior tooling up. Jigs to assist in repetitiveboring are also common enough in the amateurworkshop. But how many components are therethat your model man can reproduce for “stock”,as it were, and which may be acceptable for usein models to different designs?

Since Chuck’s interest lies largely in the direc-tion of internal combustion engines it is under-standable, perhaps, that it is with the requirementsof these machines that he allows his imaginationto dwell. Few model engineers, unless theyentertain aspirations towards a small factory, and

the possibility of selling their end product, willbe interested in developing one engine and pro-ducing a run of identical models. As a rule, ifa modeller is interested in producing his owncastings, for example, he will be satisfied withpouring one or two good copies only, and shelvingthe pattern for possible future use. He mayalso, possibly, make an odd set or two for hisfriends. Outside of that, however, he is unlikelyto find that a component designed for oneengine is going to fit, sweetly, his next brain-child.

But is not a piston “blank” a valid exceptionto the foregoing?

By a piston “blank”, Chuck refers to a castingin light alloy; cast hollow, with suitable internalprofile, and carrying gudgeon pin bosses in thetraditional manner. And a properly formed pistonis extremely difficult, if not impossible, to pro-duce by any other means than casting. It iscertainly no end of a juggle to machine out theinterior to any resemblance of the desired shape !

No ! no! no ! Not even Chuck in the momentof his wildest fancy would consider the possibility

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of reproducing a piston blank which could beutilised at one end of the scale for an .049 in.,and at the other for a 30 C.C. racing engine!

Within what he bel ieves to be acceptablelimits, however, it will be seen that it is, in fact,practicable to design a piston blank which isquite capable of being machined to suit enginesof widely varying bore and stroke. The one youare about to read of has, indeed, been fitted toengines of from 3 to 6 C.C. per cylinder. Ifyou are fortunate enough to possess a copy ofthe late Mr. E. T. Westbury’s little book on“Model Petrol Engines” , turn to page 219,where he has provided us with a useful chartindicating the relationship between bore andstroke and cubic capacity. A straight-edgeacross from 20 mm. bore to 20 mm. stroke givesyou a little over 6 cc. A square configurationreduced in the piston radius by only one milli-meter to 18 mm. bore and stroke indicates acapacity of less than 5 C.C. Take off another milli-meter in the radius and your engine is now littleover 3-1/2 c.c.!

True, if you are to machine the same pistonblank in this way to reduce the diameter, thepiston wall thickness is also going to be reduced.In this respect it is the skirt thickness left bythe core which is going to dictate the smallestdiameter of piston for which a blank can beutilised. In the same way the upper limit willbe governed by the outer diameter of the casting.

Y/L_4_- PISTON DIE

MK. I

In the case of the larger piston the extra wallthickness below the gudgeon pin bosses can beregulated by skimming away the interior until thedesired skirt thickness has been achieved. Thecrown of the piston will have more meat thel a rge r t he bo re , but this should not bedisproportionate as, in all probability, it will beexpected to accommodate thicker rings. A happymedium has also to be struck in the outsidediameter of the gudgeon pin bosses. Practically,however, there need not be so marked a differencein gudgeon pin diameters between an engine of,say, 3-1/2C.C. and an engine of 6 C.C.

Chuck illustrates an actual piston casting whichhas been finished off in various sizes and withsuccess. These pistons are die cast and havebeen used in a vertical twin with a total capacityof 6-1/2C.C. and in his V twin, which is 12-1/2 C.C. ; aswell as in various motors in between and a four-cylinder engine of 21 C.C.

In the handHis first attempt at a die cast piston was the use

of a “hand-held” die! Such a mould becomestricky to handle when hot, and the interior ofhis workshop, during the process, was apt tobecome somewhat cloudy from sizzling spit onhis finger ends! The die itself tends to get dam-aged, too, from the action of clamping it in thevice, after the pour, for the purpose of extractingthe core and so on. Pistons could be made in this

M O D E L E N G I N E E R 1 5 J u n e 1 9 7 3

Ey&-N%SrGSIDE CHEEK OF CORE

w a y - a n d g o o d p i s t o n s , t o o - b u t t h e w o r kwas uncomfortable to say the least.

It may have become clear to most readers bythis time, that Chuck prefers, as far as possible,to prepare his own material for his models. Hehas been heard to boast that he purchases only theplugs and the ball races but, in actual fact, heis known to utilise ready-made screws and nutsas well. In truth, however, he does provide allhis own castings, where such can be employed.But he machines blocks of dural or s imilarmaterial to produce connecting rods and, perhaps,cylinder heads, rocker pedestals and so on. Butsmall cylinders, if not actually cast to shape, aremachined from sticks of his own crucible-castiron and, at one time, he made his pistons andtheir rings from the same material.

Chuck’s first vertical twin of 10 cc. capacity hadcast iron pistons. This engine, which I will showlater, h a s p e r f o r m e d m a n y y e a r s ’ m a r i n eduty powering Ducky a Fairey “Swordsman”.(Give credit where i t ’s due! -An Aerokit . )D u c k y ’ s engine can idle or yet push the boatalong at a fair rate of knots. But at certain r.p.m.along the speed range the vibration is simplyterrific. So much so that for successful operation

the engine has to be supported in very resilientmountings indeed! Clearly some improvement hadto be made to the balance of future engines andChuck could see little else to blame than theheavy pis tons. Thus, even before his fourcylinder engine had arrived at the drawing boardstage he had already decided to try his luck withlight alloy pistons. Thus the first tentative ven-ture into the “hand-held” diecasting technique!For the record the “four” doesn’t waltz a milli-metre when running stood free on its base-boardon the bench at any speed. A duplicate of thevertical twin, made at a later date, merely screamsat speeds in excess of 12,000 r.p.m.

ToolmakingDrawings of the two dies, the hand-held one

and the machine die, are illustrated for compari-son. In the case of the former, the outer part wasa mild steel cylinder, bored on the inside toprovide draft for withdrawal. The core, which, ofcourse, constitutes the main part of a die of thissort, was machined from three mild steel flats each1 in. wide, the outer “cheeks” being 3/8 in. thick

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and the inner “sandwich” a 1/4 in. thick. The gud-geon pin bosses, of course, are moulded in nega-tive in either cheek and the centre section is the“key” to withdrawal. It is the first part of the coreto be removed from the casting, after the pour, sothat the cheeks will collapse inwards to clear thebosses on the way out.

But aluminium alloys contract rapidly as theysolidify and, while the metal clears the inside ofthe cylindrical part of the mould happily enough,it tightens like a hot vice on the core, making thewithdrawal of the centre very difficult indeed.Chuck’s original idea was to draw this out bymeans of a bridge and a screw tapped into theunderside of it. But the “sandwich” as aforesaid,was only 1/4 in. thick and the largest thread it waspossible to tap into this just would not stand thestrain. He had to resort to striking out the coreswith a hammer. This, sometimes, required manyheavy blows and the die number one suffereddamage. Nevertheless, and in spite of this handi-

Below: Zgnition by glow-plug. Runs on pure methanolwith Castrol “M”. 10,000 r.p.m. plus!

Chuck’s 21 C.C. 4-cyl. engine built by Alan Normanof Ellesmere Port, showing die-cast pistons.

cap, it produced many pistons; all of which cannow reciprocate!

Another fault with Mark 1 was that Chuck hadprovided for the core for the gudgeon pin locationto be a taper pin passing right through the dietrom one side to the other. But here again thecontracting metal made the pin very tight and ithad to be driven out by force. It was found, too.that the alloy tended to seize onto the pin and asmear of metal came away with it each time.This in spite of a liberal coating with soot from agas flame inside the mould before each cast.

Ordinary blacklead, by the way, has now beenfound a more convenient substitute for soot.

Melting the AlloySeveral years, ago Chuck read an advertisement

in Model Engineer for an “Electric MuffleFurnace”. He cannot remember the price but hethinks it must have been pretty low or he wouldnever have sent for one. When his purchasearrived he found he was the better off for a,rather rough, rectangular, fire-clay former; inter-nally 6 in, x 3 in. x 4-1/2 in., and wound on theoutside with fairly heavy gauge element wire. Thetwo loose ends of the wire were formed intoloops and that was all.

No one will ever know what Chuck imaginedhe was going to do with this thing when heordered it. He certainly didn’t know what he wasgoing to do with it when he received it! It hadalso been slightly damaged in transit and this factdid nothing to increase his confidence in itspractical possibilities. Hence the “Electric MuffleFurnace” found its way back into its packing andsmartly under the bench where it remained, for-gotten and forlorn, for quite a number of years.

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And then, one filthy, cold winter’s night, Chuckfound himself contemplating the production ofone or two quite small castings in aluminium.His outdoor furnace was lashed with rain and halfburied under a small mountain of rotting, deadleaves and, in the words of the great W. C. Fields,“ ‘tweren’t a fit night out fer man ner beast!”

A sudden recollection of an erstwhile purchase,a scuffle among the debris of years under thebench, and there, unveiled for the second time andlooking, frankly, as forlorn and useless as ever,stood the electric muffle furnace!

Chuck halfheartedly tried it for size. Had itbeen made cylindrical it might have accommo-dated a No. 2 crucible, a four pound pot, quitecomfortably. It cleared the height by more thanan inch and a half but lacked half an inch in thewidth, a No. 2 being 3-1/2 in. diameter. But itsunfortunate rectangular section would admitnothing bigger than a No. 1, which is 2-3/4 in.diameter. And at this stage it was by no meanspositive that the muffle would, in any case, evenreach the temperature required to melt light alloy.

In fact, to attain this heat the muffle wouldrequire to be well insulated on the outside as well

as being provided with safe electrical connectionsand an earth. If progress was to be made thatnight a satisfactory answer to these problemswould have to come to light as the result of anindoor foraging session.

To cut a long story short the muffle finishedup in the form shown in the drawing, embeddedin a mixture of sand and broken firebrick, theonly refractory materials readily to hand, withinthe enclosure of an old-fashioned biscuit tin.Lead-out wires were passed through the side ofthe tin encassed in secondhand ceramic beads andconnected in an ex-junkbox, porcelain jointboxon the outside. The sheet metal cover for this wasadded at a later date because it was found that,within the limits of its capacity, the electricmuffle furnace was a success!

Clean MetalFor melting, the crucible is placed in the

furnace while cold and a good hour is usuallyrequired to heat the whole thing to red, which isthe kind of temperature required to melt lightalloy rapidly. The pot is charged with bits ofbroken pistons and, once a pool is formed in thebottom of the crucible, added metal fuses intothis quite quickly.

For making miniature pistons great care istaken to retain, as much as possible, the originalqualities of the alloy. Chuck does not in any waylay claim to any metallurgical knowledge, but hehas a kind of pious hope that, if he is a good boyand doesn’t use dirty scrap and keeps unprotectediron plungers and stirrers out of the molten metal(iron will contaminate aluminium alloys) theensuing castings will be reasonably sound. Theproof of this particular pudding is not in theeating, but in the subsequent machining and theultimate usefulness of the little pistons when theyare in the engine. His castings can hardly be saidto be subject to quality control and none has beenanalysed, but Chuck has been able to recognisethe difference in the machining qualities of thesesmall diecastings with-for example-his ownmore usual run of sand castings melted in acoke-fired furnace. And the pistons certainlyperform well!

Having no combustible material inside thefurnace the metal cannot, to any large extent, besubject to deleterious furnace atmosphere. Butthe textbooks tell us that molten aluminiumabsorbs hydrogen from the air, and this, auto-matically, gives rise to a decidedly porous struc-ture in the finished casting. (At one time it wasbelieved that this porosity was inseparable fromlight alloy castings.)

To be continued

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