Not Scott related but worth a shot…
I am looking for a new piston for my 1912 2,5 HP Premier. The piston now used is a cast iron one and I would prefer the new one to be cast iron too. But this proves impossible to find an aluminium one would be fine too. Below a picture of the piston and the most important dimensions.
Does anyone have an idea who could provide me with such a piston? Or could make me one?
Cast iron pistons! I bet your eyeballs must go round like a fruit machine after a ride!
The most obvious thing to do is to make a pattern, (well within your scope), and get a couple cast.
It is possible to make one out of the solid; I have done so myself for a much smaller engine, (a 42mm bore twin), but getting the weight anything near the original is all but impossible.
If you are not too precious about the design then you could make a multipart one. S.G. (spherical graphite) iron would do nicely for the crown and skirt, skimmed lengths can be purchased from any good materials suppliers, and steel could be used for the gudgon pin bosses. Because of the guaranteed homogeneous nature of SG iron the extra mass needed for bolting the parts together can be saved elsewhere with thinner sections thus keeping the weight near original.
Such multipart pistons have a long history, from a published DIY motorcycle design that I have from the early 1900’s, (and I do mean DIY! Frame, engine, the lot, from scratch!), right up to today with some very twentyfirst century ideas.
Hi Erik Some time ago I bought via the internet, a CD copy of the Hepolite piston and ring data for British and continental motorcycles from about 1930 to 1955. If I want an ususual size ring or piston, I have a look through in case there is a more popular model where spares might be more likely to be available. There is no certainty you will find what you need, but there is a chance and pistons that are slightly oversize can be ground down.
In any case, an aluminium piston will have much bigger clearences than the iron one which would run with about 0.035mm running clearence.
As regards castings, many years ago I was involved in making a production milling machine of about 100 total hp and to give the best possible performance, we made the machine bed from grade 14 cast iron as this absorbs cut vibrations quite well. As the machine was a “One off” special, the pattern cost would have been very high, so we had a none reuseable pattern made from expanded polystyrene that stays in the mould and is burnt out by the molten iron. It makes a lot of smoke but it is cheap and simple and you could carve out the pattern in two halves and stick it together. You should check with a small friendly foundry first, but it is an idea. It worked for us! Best of luck Roger
Thanks guys for the answers so far!
To be honest I think making a pattern/mould is beyond my capabilities… And besides that, I simply do not have the time at the moment. Two small kids and a very busy job have kept me from the workshop for weeks now…
And in Holland there are hardly any (small) foundries left…
@Roger: if you can find some time I would appriciate very much if you could have a look through the Hepolite catalog. The most important dimensions are in my first posting. But only when you have some time, I know you are a busy man.
I looked for a piston for my 1927 Humber 350cc for ages and in the end gave up and looked through the Mahle and KS piston cataloques to find something I could modify to use. I ended up with a Ford Fiesta piston which works fine, but remember to leave smaller piston to bore clearances with modern low expansion alloy pistons.
I am just rebuilding the engine on my 1923 Abingdon King Dick 500cc after advertising in Old Bike Mart and the VMCC mag for 3-months I decided to do the same as I did on the Humber and use a modern piston. The original cast iron piston is cracked and has a lump out of the side. I am using a VW deisel piston in this engine and the weight is very similar to the original piston.
If you do find somebody to cast a new cast iron piston for you, please let me know as it may be useful to me in thefuture.
Trust Roger to come up with that particular “cunning plan”, one that I’d almost forgotten about! Years ago I made a polystyrene mould for a ¾ ton lead keel by much the same method; there wasn’t a church roof in Essex that was safe!
Erik: Could you elaborate on why you want/need to replace the piston? It might help in stopping me offering daft ideas if nuffink else – like that’ll stop me!
Roger’s suggestion is a good way to go; surely it is possible to find a foundry within reach? Braintree College (in Essex) made and operated their own small blast furnace a while ago, probably the Health & Safety Mafia have put a stop to it by now but it goes to show that sometime it pays to cast your net wide.
My only reservation in this case would be that while burning the polystyrene out with the molten metal might be OK with a big mass like a machine casting, for something of small mass with thin sections I think I’d want to burn out the mould before pouring, it’s hard enough to get a really sound casting without adding a source of gasses!
What about coming at the problem from the opposite direction? How about sleeving the barrel and having it bored to a slightly smaller size? The existing piston can then be ground to fit or even the bore honed to suit the piston “as is”, the photograph may be flattering it but it looks servicable in the picture. My thinking here is that at least it saves the problem of having a new casting made and even if an off-the-shelf liner cannot be found good quality iron to make one from is readily available from metal stockists.
That would leave just the problem of what to do about new rings.
Taking one step back for a moment, if wear is not too serious a new piston and/or liner might not be needed anyway, you might just get away with new rings only. Now making rings is not a “garden shed” exercise if you think in terms of today’s hi-tec methods but then when your machine was made they didn’t have access to them either! They will have used a rather more lo-tec method and given a halfway decent lathe it is entirely possible to make good serviceable rings in the “shed”. It is only a matter of method and a bit of careful turning but the method is a bit convoluted and a description rather too long for this forum.
The piston and cilinder are pretty worn. The clearance between the piston (bottom of skirt) and cylinder (halfway) is about 0,40 mm (total so 0,2 mm all around) and that is quite a lot for a cast iron piston. You can see on the piston that it has been rocking in the cylinder. Arround the piston there is a recess machined in that helps lubricating the bore. The edge of this recess is also worn away completely at the front and rear thus indicating the rocking. And also the engine smoked quite badly as I was told from people riding behind me.
Of course I could re use it and just fit new rings. It is not that I will do hundreds of miles on this machine. But being quite a perfectionist I would like to have things right. And I could have a new piston made (aluminium) for about 150 euros which is a fair price I think.
I also investigated the sleeving option but there is only room to fit a thin sleeve and that will probably deform on pressing in. And of course the cylinder will not be a homogenous part anymore and those sidevalves get very hot so a good heatransfer is important.
To be continued!
I note the concerns about a liner but consider this; I did a pair some years ago for my Moto Rumi. Now the original 42mm bores for this little beast were chrome on aluminium. When I got it there were various bits of cylinder skirt broken off, clearly welding was going to further ruin the chrome and the prospect of getting it re-chromed after welding and retrieving a working surface was pretty well nil.
As a result and with nothing to loose I made a pair of liners from S.G. iron but due to the design of the original casting I could only manage a 2,5mm wall thickness.
Concerning the fear of distortion due to local heating as a result of this engine being a side valve job; I think it doubtful if it proved to be the case. Compare it with the Rumi, a piston timed 2-stroke with the customary exhaust port with a bar occupying 1/3rd of the cylinder circumference plus inlet and two transfer ports. Then there is also the matter of the unequal expansion of the iron liner and the aluminium casting to be considered, a recipe for unequal expansion if there ever was one! Despite all those possible pitfalls there has been no indication over many miles that there have been any distortion gremlins at work. Indeed the machine was used in this form with homemade pistons and homemade rings for many months doing a 40 mile each way commute in all weathers with no problems – must have got something right! That is save for the Rozzers for ever pulling me over to get a closer look at this odd little machine — “Ello! Ello! Wot we got ‘ere then?” — when all I want to do is to get home for some grub!
Regarding the possibility of a reduction in heat rejection due to the sleeve/cylinder interface; if the job is done well this will be so small that it can be ignored particularly if close attention is paid to the finish in the bored out cylinder and the liner, the finer the finish the better. I’d be inclined to have the liner shrunk in rather than pressed, an easy enough job if a source of liquid nitrogen can be found and careful attention is paid to the dimensions for the interference fit. At least you don’t have just the one chance and a few seconds to align the ports in the liner with those in the cylinder as I did with the Rumi!
For liquid nitrogen if an industrial source fails try your local University. You are looking mainly for one with an Engineering, Chemistry or Physics Department especially if they have an electron microscope as many use liquid nitrogen for cooling EDAX equipment and the like. The Chief Technician is probably the person to chat up.
You could do no harm by having the whole thing soaked at about 250ºC for two or three hours followed by a slow cooling just to release any stresses caused by fitting the liner. It is assumed that you will have left enough meat in the bore so that the last job is finishing the bore to the desired size. I am assuming here that the existing piston will be re-ground and reused.
Concerning the piston I suspect that the recess is there more to reduce friction than to retain oil although of course it will do that. It would be well to maintain the depth of the recess if the piston is reground. Depending on the amount of metal that has to be removed to render the piston circular it would also be wise to have the ring grooves deepened by an equal amount. As new rings will have to be made in any event it would be wise to widen the ring groove by enough to remove any witness of previous wear. This will ensure a good fit and thus effective working of the rings; it is a build up of compression pressure behind the ring that does the bulk of the sealing, the outward pressure caused by the springiness of the ring itself making a minimal contribution.
Don’t do it! don’t even think about fitting a liner to a cast-iron veteran barrel!
Many years ago I owned a 1912 Bradbury which had been fitted with a liner before I bought her. I don’t like to think about the trouble that this caused me. In the end I was lucky enough to find a replacement barrel, these days I doubt you would be so lucky.
These early blind-head cast iron barrels are just not robust enough for such treatment even with high-tec resources. Far better to source a replacement piston.
Why not, as far a I know, it has been done many times, as long a the liner is machined of cast iron
Thanks guys for all the input! Especially being not Scott related…
Some persistent googleling (is that a word?) showed that cast iron pistons are still in use today in air brake compressors for trucks etc.! Like this one:
Of course a compressor piston has a somewhat other purpose than a combustion engine piston but they sure look the same. And a compressor piston will get pretty hot too I guess.
I have contacted a few manufacturers for more info. I will post the follow up here later.
Maybe I make a mistake in thinking a compressor piston could also work in my Premier engine. If so, please tell me!
malcwebb’s note of caution should be given due consideration but without a first hand knowledge of that particular engine it is impossible to come to a definitive conclusion as to the reason for the problem experienced and how that might bear on Erik’s Premier.
I still consider that if a liner is well done there is little risk. Furthermore there are very real attractions in a liner; the material is guaranteed to be homogeneous and the composition can be selected with particular attention to its wear properties. When a casting is made there are other factors that need to be taken into account, no such compromises are needed when selecting a material for a liner.
The condition of the cylinder, the quality of the original casting and the amount of metal around the bore must be considered but as it has survived more than an average lifetime already it is likely to be a pretty sound casting!
As a rule of thumb I’d say that if there were enough metal around the bore to make a liner of sufficient wall thickness that it takes up no more than 1/3rd of the thickness of the cylinder wall then all will be well. It is further worth bearing in mind that the fins act as hoops and help stiffen the cylinder so the above proportions are probably conservative.
Turning now to C.I. pistons, in many ways it is an ideal material having excellent wear properties and a coefficient of expansion compatible with iron cylinders. It was only because of the advent of higher and yet higher engine revolutions that the mass of a C.I. piston became a very serious problem and as a result the much lighter aluminium piston came to reign supreme but it is really only its low weight that is in its favour.
Lets face it, when it comes down to it a piston is just something moveable that plugs a hole! I cannot then see why a C.I. compressor piston could not be able to do the job; both seal a bore, both are subjected to similar loads. Compressors don’t as a rule run as hot as an I.C. engine, (although I’ve known some that you wouldn’t want to keep your hand on), so clearances might be a bit tighter for a given bore but that is about it and certainly worth a try.
There is nothing wrong with the priniple of fitting cylinder liners. What most of you may not appreciate is that Erik’s Premierhas a ONE-PIECE
cylinder – the head and barrel are ONE casting.
All these early side-valves get very hot when they run and the differential between the sleeve and the outside of the barrel can lead to it cracking – you end up with a “detached” head, which is what happened to the Bradbury.
It is quite normal for even the best veteran barrels to crack across the ridge between the two valve seats, I have known it to it happen to several Triumphs (and they are the best of them – in fact I have a theory that Triumph, Rover and Premier barrels were probably all cast at the
same Coventry Foundry).
I would quite happily use a liner in a normal barrel – have done in fact – but don’t ever do it with these early ones!
malcwebb’s expansion on the nature of the danger with these older side valve cylinders is interesting. The trouble with cast iron is of course that while it is a wonderful material in compression it is a complete non-starter when any tension is involved and a liner will impose a bursting force.
Metallurgy has progressed since these castings were made but the founder’s art of that time should not be underestimated. As this seems to be something that afflicts this particular configuration maybe it is something about the shape that has an effect the castings ability to deal with the heat of combustion. It is even possible that it was the way the metal flowed in the mould and solidified that contributes to the weakness but all that is no more than speculation. Only a metallurgical examination of some samples would offer any real answers.
Remember also that the accursed Bean Counters existed back then too so maybe cost over quality might be a contributory factor. What was it that one of the Apollo astronauts said when asked what it was like to sit atop a Saturn5 rocket? Something along the lines of “How would you feel about sitting on an explosive force the equivalent of a battlefield nuclear weapon built by the lowest bidding contractor? ”
By the way we can add MGB cylinder heads to the list; they are not unknown to go between the adjacent exhaust valves of the middle two cylinders.
There are however some steps that can be taken to minimise the risk. The first is to use shrinkage as a method of fitting a liner rather than a press fit.
When a shrink fit is used it is common practice to use a smaller interference, in this case as little as 0.025mm to 0.04mm. That may seem very little but it is interesting to note that laboratory tests have shown that a consistent 3½ times the force is required to extract a shrunk in part compared with one that was pressed in. This is almost certainly due to the elimination of the wiping action when an interference fit is assembled which inevitably alters the surfaces involved whereas with a shrink fit every microscopic imperfection beds in. A bit like Velcro where one strand is nothing but many offer a remarkable degree of grip.
Another step would be to finish the boring of the cylinder with a good radius at the top of the bore. Corners are always a stress point, a radius, particularly if well finished, even polished, will do much to give a crack nowhere to start. The liner would naturally require a complimentary radius.
To further remove strain the top 25mm or so, certainly somewhere below the first fin could to advantage be eased to little more than a push fit to further reducing the risk of bursting forces on the casting.
Well it took me a while and quite some extensive searching and took me over all the continents (well, while surfing that is) but today a fresh cast iron piston arrived! It is a compressor piston and looks very good. Below a few pictures. The compression height, pin diameter etc are all very usable. There are, however, a few things I have questions about…
It is a very heavy lump weighing in at 546 gramms (with rings and pin). The original piston was not a lightweight too with 360 gramms but there still is a 186 gramm weight difference… In order to keep then original balance of the crank-piston assemby I would like to put the new piston on a diet.
Looking at the pics you can see that the new piston has 5 rings instead of two on the original one. I could of course cut of the bottom section just above the second ring. This will sure save me some weight. And this would still leaeve two compression rings and one oil scraper. But it would shorten the piston an that might increase piston slap. Or not?
The original piston has a recess machined around the middle section of the skirt (the dark part in the picture). According to the Premier manual this is to keep oil for lubrication. Should I machine such a recess in the new piston?
The new piston is completely cylindrical. So no taper and no ovalty. Most pistons are not cylindrical and have at least more play at the piston ring lands. Should I give the piston a bit more play at the lands? See the image below where I have quoted the recommended play that Radco mentiones in his fabulous book.
And will the lack of taper and ovalty have negative effects? Remember, it will be doing it’s work in a 1912 250 cc sidevalve that will do about 2.500 revs max.
Any other hints?