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.
Cheers
Steve