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Help wanted with piston design

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Steve Crow:
Hello, I'm 16 months into building a 1/12 scale V8, powered by compressed air/gas. The bore is 7.2mm and the stroke is 5mm. I will not be using rings. There is thread on here about it but I haven't updated it in ages.

I've made nearly all the parts but I haven't even started designing the pistons yet. I made the con rods and the crank nearly a year ago!

I haven't been able to find any help on the net as regards proportions of the pistons. I'm sure somebody here can help.

Here's a diagram to help me explain:-

My questions are-

Is there a rule of thumb for the proportion of D to x? I realise that having a bigger x will help keep the piston parallel.

Is it more advantageous to have the centre of the gudgeon pin (p) between a and b as in the first diagram or is it ok below as in the second?

Grooves as shown on the left. Are they a good thing?

Finally, materials. I haven't made the bore liners yet and have been thinking of using brass. For the pistons, I have no idea. I would like to keep the weight down, how about ali with a steel sleeve for bore contact?

Any ideas, advice or experience would be very welcome.



Roger B:
Some thoughts:

The maximum length of the piston below the gudgeon pin is limited by the length of the con rod and the diameter of the crankshaft.

Modern high performance engines have very short pistons but also very tight tolerances.

If you are not using rings a longer piston with some labyrinth sealing grooves will probably give the best seal.

I have a 16mm bore and stroke petrol engine with a brass liner and aluminium piston with a single cast iron ring. This revs to 7-8000 rpm and seems to work ok.

A lot will depend on how much clearance you have before the piston skirt starts to collide with crank or conrod but I tend to try and get the pin al least in the middle if not slightly closer to the head of the piston. You can always cut away part of the skirt if there are issues but still keep some length to help line things up.

I would tend to use V grooves to retain a little oil rather than the rectangular notches you show.

Aluminium will run fine with a small amount of oil in the air supply or a drop or two every few minutes of running

I've just made this one for the current high speed steam engine that will only run on air, 18mm dia x 15mm long the pin is 8.25mm up from the bottom.

On the little compression ignition engines we use CI pistons which have smooth outsides - no labyrinth seals or rings  :hellno:

When Eric Whittle designed his V8 engine (10cc air cooled glow plug) back in ME in 1995 he used HE15 with no seal/rings. The pistons are about 10mm tall and slightly over 10.5mm high but that includes the curve where it goes over the crankshaft at BTC.

If you have not read Eric's article it is well worth a read it will give you lots of ideas - including how to make the tiny bits.


You do have the advantage of a high oil content of the fuel on the compression ignition engines to keep every thing slipery


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