Pistons broke at valve reliefs. Any ideas

This is my buddies engine. Only 5 runs on grudge 521 deal. Two pistons broken and rest cracked at the valve relief. These are Diamond pistons. Looking for answers so he can decide what direction to take. He only hit it with a 350 shot. He found problem during normal inspection.  Engine was running fine before teardown. He did experience a backfire.

cobra501 wrote:This is my buddies engine. Only 5 runs on grudge 521 deal. Two pistons broken and rest cracked at the valve relief. These are Diamond pistons. Looking for answers so he can decide what direction to take. He only hit it with a 350 shot. He found problem during normal inspection.  Engine was running fine before teardown. He did experience a backfire.

Sometimes the most obvious questions still need to be asked... is your buddy sure he had the ring gap set right? I've seen too tight of a ring gap pull the top off many pistons once you spray the gas on em.

I should have thought of that. Good point. I will let him know and from there. Thanks.

cobra501 wrote:This is my buddies engine. Only 5 runs on grudge 521 deal. Two pistons broken and rest cracked at the valve relief. These are Diamond pistons. Looking for answers so he can decide what direction to take. He only hit it with a 350 shot. He found problem during normal inspection.  Engine was running fine before teardown. He did experience a backfire.

Detonation or not enough ring end gap or maybe both.

Is the 2nd ring pinched in the ring land? What does the skirt look like scuffed?

If someone will PM me your phone number I will text you two pics and some more tonight when I get them. I never could upload pics here.

If the intake valve relief machining left a "wall" , the cylinder pressure spikes caused that wall to flex and the piston cracked at the inside radius of the ring groove. IF the valve notch machining broke through that "wall" , the lateral pressure would be reduced and the failure could be rings butting as Randy mentioned. That is the "cooler" side of the piston and is not known for detonation until a section breaks out and the hot ring causes localized melting of the surrounding area. Detonation would "normally " appear around the top ring land and usually on the exhaust side. It creates a texture like shot peening. Pictures will help the diagnosis.

https://s13.photobucket.com/user/cobra501/library/?sort=3&page=1#

I tried posting a pic. No luck so far. I got more pics lastnight and they tell a better story than he described. This damage has to be improper ring gap. Pics show that the area above the ring groove is busted from a ring bind. This was new motor and 1300 spent on a new set of pistons.

No pics.....private

On the outside by the intake pocket?

Lean