Piston Clearances / Ring Gap

[cb23]

Hi! I'm rebuilding the top end of one of my snowmobile racing engines. I figured that I'd ask here, as the depth of technical knowledge seems better here than on my snowmobile forums. . .

Two cylinder, two stroke, water cooled. 72mm bores. 8200 rpm. Always warmed up slowly and completely before runs. New Wiseco pistons and rings (Swain PC9 coating on skirts - first time), sleeved cylinders are to be honed by one of the big shops, to specs determined by me. I run very lean (oxygen sensor), mostly full throttle, and lots of oil in the pre-mix. A history of scuffing/melting pistons (less now that I am controlling fuel better), poor cooling design. . .

The manual calls for a piston ring gap from 15 to 25 thousandths, which seems high to me. I plan to use oversize rings and grind/hone them to the desired gap.

Piston skirt clearance is 6 to 7 thousandths in the book. Pistons are forged, and the factory clearance may have been determined with cast pistons.

I have software that lets me use the coefficient of expansion to determine hot clearances from cold, piston and cylinder growth being measured by me in a commercial oven to check these values. I am smart enough to play with the numbers to see clearances when one part is hot and the other is cold. . .

So. Top ring (2 rings) to be nearly closed when hot? With just 2-4 thousandths beyond minimum (hot rings cold cylinder) for safety? Bottom with a bit closer to spec gap, say 6-8 thousandths? How much slop should the piston have in the cylinder? I am guessing 4 thousandths.

I hear top rings should be tight, but never butt. Bottom rings a bit more open. My fear of piston clearance is scraping off oil, don't want to depend on that skirt coating. . .

This engine will only run in a break-in event and then national championships, about 10 minutes a year. . . < g >

Thanks!

 

[marcusgunby]

for that type of info you need to speak to eric gorr IMO, he knows stuff like this better than most, the link is on the left.

 

[DirtIsMyName]

When you find the answers you are looking for I would be interested in reading them here. I agree that those clearances sound loose, especially for a liquid cooled engine. But this engine may need those clearances.

You said lots of oil in the premix. Remember that more oil means less gas and less gas is a leaner mixture. This is backwards of what intuition might lead you to believe.

 

[cb23]

Thank you, Dirt. This is a racing only engine. My fuel mix is very precise, and is meant to achieve certain things. . . Note that I run an oxygen sensor, which along with EGT's, piston wash, plug color (last and least, mostly just to determine plug heat range), and several other things lets me control 'the burn' very precisely.

Also, the oil I run is. . .quite strange. < g >