Ringland breaking at intake relief

[mmitchell]

I have a 4.155 bore 393cid 18-degree SBC. Heavy (42 pill in fogger) NOS. I put about 10 nitrous runs on engine and it started smoking. Pulled head expecting damage from lean NOS tune up, but found the pistons in various stages of damage, looks as though the ring land is flexing and breaking at perimeter where it is thinned for the intake relief. It has a 2.20 valve so it really crowds the edge. What can be done to prevent this from happening? Rings are dropped .29 inches from face, gas ported JE flat tops. Need help, I'm afraid it will ruin another $$$ pistone set.

[perfconn]

Too deep a valve pocket or top ring to close to top will cause it.

[billhendren]

Send the pistons to Swain Technology and have them ceramic coated on top,this really helps pistons that are thin in the valve pocket area.we do this with all our N20 and turbo engines.Bill

[slowman]

mmitchel, how long are the rods 6"? to get 393 you most be running a 3.630 stroke with the 4.155 bore? you problem isn't heat related right? the piston isn't melted at the ring land right? if the piston isn't melted and the problem isn't cause by heat then coating the pistons is a wast that will not fix the problem. move the ring pack down ..050-.060 more and that should do it. the oil ring will need a support rail but from my math it should have one now. the coating a good idea for some safety margain from a missed tune up but wont fix the ring land being to thin at the edge. the extra .050-.060 will give you a piston that is .350 down from your .290. good luck with it

[topsportsman1]

Send the pistons to Swain Technology and have them ceramic coated on top,this really helps pistons that are thin in the valve pocket area.we do this with all our N20 and turbo engines.Bill

I'd have to agree here with Bill the problem is:

Valve size, 2.200

Piston relief,is cut deeper to accept the 18 degree valve,which inturn means a lesser distance to the 1st ring land.

Nitrous means more heat on the top of the piston,coating I believe will help.Also oil sprayers from the connecting rods will help cool the bottom of the piston.

Do your home work on a better ring stack on the piston.

All piston qualitys are not all alike,different type grades of aluminum alloys.

And I am prolly forgetting more but thats off the top of my head,maybe this will bring more to the discussion.

Gas port the piston to get a better ring seal.

Use a good hell fire ring with plenty of ring gap= atleast .007 to the inch

[cboggs]

I'd move the ring pack down, ..

My experience with coatings hasn't proven them to fix this type
of problem, .. but it sure can't hurt.

Curtis

[slowsbc]

I have a 4.155 bore 393cid 18-degree SBC. Heavy (42 pill in fogger) NOS. I put about 10 nitrous runs on engine and it started smoking. Pulled head expecting damage from lean NOS tune up, but found the pistons in various stages of damage, looks as though the ring land is flexing and breaking at perimeter where it is thinned for the intake relief. It has a 2.20 valve so it really crowds the edge. What can be done to prevent this from happening? Rings are dropped .29 inches from face, gas ported JE flat tops. Need help, I'm afraid it will ruin another $$$ pistone set.

.290 should be fine in my opinion. I have run them as high as .275 and as low as .350.. I found .300 worked best for me. What is the weight of the piston? What is the ring spacing from top to second ring? Does the first ring land have an expansion groove in it? What were the rings gapped at? Is this a 3mm oil ring setup? Are the skirts cracked on other pistons? Why JE? :D

[Racing1968]

I've seen this problem in some circle track engines that had signs of bad detonation. Guys used a lower octane fuel than we recommended.