Ringless piston question

borna · Feb 5, 2012

Hello all,
I'm designing a 4 cycle single cylinder slow running engine with 1" bore X 1.5" stroke .
Planning to make a ringless piston out of cast iron and for sleeve, I'm using A513 type 5 steel tube DOM.
Since the sleeve will NOT be tapered smaller at the top, is it going to have compression problem when reaches operating temperature?

Thanks
Borna

MachineTom · Feb 5, 2012

As I assume you know, a piston is not round. And a ringless piston even more so. The expansion rate of the piston will match the cylinder, BUT the temperature will not. The piston head will be much hotter than the skirt, and the skirt hotter than the cylinder walls. So the how do you intend to machine your piston to account for all those conditions, most of which you have no way to measure.

For that reason Charlie Ring designed Piston Rings, and said let them pistons be free.

chuck foster · Feb 5, 2012

i have several model hit and miss engines that run just nice without rings, but the cylinder and piston are both made out of cast iron
so the expansion rate should be the same.
i lap the bore and then lap the piston to fit, and i give the piston about .0001" to .0002" clearance.

the engines that i have range from .750" to 1.375" bore.

tom is right about full size pistons not being round and i think they very in diameter from one end to the other (i could be wrong on that).

hope this helps

chuck

bigrigbri · Feb 5, 2012

If I may add my 2 cents worth on the subject of piston shape.

The crown(top of piston) is slightly smaller than the bottom(skirt)dia,
and the roundness is slightly oval with the smallest measurement across the
gudgeon pin(wrist pin)hole due to the excess of material at this point.
As the piston heats up the crown is subjected to more heat and expands
more giving a near parrallel piston and the same with the ovality giving a
round finished shape.

Brian.