piston allowance in bore

I am building Brian Rupnow's Rockerblock ic engine and I would like advice on the allowance to leave of the the piston diameter in the cylinder .The cylinder and the piston are aluminium.There is an O ring for the piston ring and I would like to know if I leave a cirular bottomed groove or a square one and side and bottom allowance.It is a 1" bore.I have made cast iron rings before but not fitted an O ring or worked in alluminium.

Frank

Brian usually puts all that info on his drawings, best ask him what allowances he uses. I know he tends to ream his cylinder srather than bore so will depend on whether he uses H7 or H8 reamer. He also has to motor most of his engines to get rid of tight spots befor running them.

As materials are about teh same I would make piston about 0.001" to 0.0015" smaller than bore.

What cross section is the ring as that will determine the groove size but it will be flat bottomed.

Frank, why did you make the cylinder from aluminium, the original was 316 stainless steel, with the option of using cast iron. The Viton O-ring is used with the stainless cylinder, while a cast iron ring is used with a cast iron cylinder.

An interesting little engine.

Ian S C

Jason, thanks for the information,I wiii be using a 1' adjstable reamer and finish with lapping.

Ian, the reason I am using aluminium is because I could not find a small supply of 2 1/4" stainless and aluminium will not rust as the engine is water cooled.

Frank

It's just that plain aluminium does not wear too well, or to put it another way, it will wear quite quickly. Aluminium cylinders either have hard anodized, or chromed bores. See how it goes, as long as it gets a good oil supply it may last quite well, the main problem is the aluminium piston skirt running against the aluminium cylinder, the aluminium tries to weld the two bits together---high friction. If you could get a bit of stainless tube with a 1" (or there abouts) bore, a flange could be silver soldered on that, and the bore polished out and a piston fitted. You might need to adjust the holes through the water hopper depending on the outside dia of the cylinder.

Ian S C

Ian

That sounds like a good idea,I will see what i can find .I never considered that the piston might bind on the skirt,It's what I like about this forum one is always learning from others experience,thanks Ian.

Frank

Briggs and stratton engines use aluminium bores with no coatings but they run a plated piston ( can't remember if it was silver or nickle or tin )to stop any galling .

Edited By XD 351 on 31/01/2018 04:58:14

"Briggs and Stratton engines use aluminium bores with no coatings but they run a plated piston ( can't remember if it was silver or nickle or tin )to stop any galling ."

I believe Villiers did the same with a lightweight range of small engines. As has been pointed out, Aluminium running on Aluminium does not work well. I'm not too sure about stainless steel and Aluminium, galling is not unknown with that combination.

Whatever combination the OP decides to use then I would suggest the first step in considering the clearance would be to consider the relative thermal expansions and go from there.

John

he Briggs piston is chrome plated, and the cylinder is made of a high silicon aluminium alloy, the expected life of the motor is about 250 hrs, about the time it takes for the average mower to fall apart(my mower must be over due to die).

Ian S C