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Help wanted with piston design

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Steve Crow28/11/2021 17:37:47
310 forum posts
191 photos

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.



noel shelley28/11/2021 18:22:02
1442 forum posts
23 photos

Pistons of ali, with narrow V grooves to hold oil ! bore liner of cast iron. You could chrome plate the pistons. It then all really depends on how much you use it as to lube. I would place the gudgeon pin mid way down the piston to steady the piston. Howard or Tug may your best bet ? Noel

JasonB28/11/2021 18:24:04
23050 forum posts
2769 photos
1 articles

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.


Steve Crow28/11/2021 22:21:45
310 forum posts
191 photos

Thank you Jason, you answered pretty much all of my questions there!

I intend to introduce a bit of oil so I'll go for the aluminium.


Steve Crow29/11/2021 18:50:11
310 forum posts
191 photos

After taking on board your advice, I have come up with this design -


Can anyone see any potential problems or means of improvement with this?

Also, I would be interested in any methods for introducing oil to the air supply.

In the intake manifold (plennum?) I have made, I've left space for bleeding off some air in case I need to inject some oil any where but mixing with the air supply sounds a lot simpler.



JasonB29/11/2021 18:57:58
23050 forum posts
2769 photos
1 articles

Looks OK, probably does not really need the two flats either end of the wrist pin hole as you are counterboring the head and hopefully won't make the thread on the pin too long.

An air tool oiler should do it though they don't work quite so well if pressure is low so depends on what you are likely to run at. Otherwise just add a drop into the air line or inlet every few minutes. A light clock oil is what I usually use as thicker oils can make valves stick

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