Got a problem,i built a 351 stroked to a 393 and used keith black flat top pistons. I was in low gear a (c4 tranny) on the track wasn't paying attention and tacked about 5000rpms so the car lightly back fired. Went home and pulled the plugs and all 4 plugs on the right side were closed. I put in a new set of plugs the car ran but missed a little. Pulled the motor and started taking the engine apart and found the number 1 and 6 piston had a crack in it. The crack was near the side just above the rings. The thing that bothers me is the crack in both pistons are almost in the same spot and same type of crack. Neither one of these had scars on the plugs or vavles from hitting the pistons. Before i took the motor apart it still ran but just had the slit miss. The car never smoked or had no signs of a broken piston. Could this be a set of weak pistons. The motor was farely new when this happened i only had about 300 miles on it and it was very tight. I am going to finish taking it apart to check the crack and bearing but the piston issue has me puzzled. Who has a answer please help for i think it is a bad set of pistons. The motor was bored 30 over the heads shaved .20 the block decked .20 and the crack was turned 10/10. Hope that will help find a answer.:16suspect
Sounds like detonation. What gas do you use? Also, is timing set a bit too high? Just questions to ask.
It ran on pump gas 93 octane. Thats the only kind i could use to stop the spark knocking. THe timing was pretty well right on not for sure of the exact timing if i recall we were at about 2 degrees back.
If your quench is too tight or border line, it can be possible to damage only one side of the block especially if it was decked. I have seen stock blocks as far off as .008" inch per side add on top of that machine shop variance. The fact that all plugs were closed on one side makes me think its a clearance issue. I always install the crank and use the same rod and piston at all four corners and measure the deck height before having the block decked. Are your pistons sitting above the deck height of the block?
Are your Keith Black pistons hypereutectics? If so, they are known to be very particular about the top ring gap which needs to be set a good deal wider than other pistons to keep the ring ends from butting.
Ding Ding Ding, I think we have a winner..... Too tight a ring gap and mix in some detonation = unhappy pistons..... You always need to check your ring end gaps when putting a motor together..... The last one I assembled I used a basic Speed Pro's with the moly top ring, these were basic use as the are(not file fits)..... When I check the gaps the top ring was only .005 out of the box and the 2nd was .009, neither was near enough for even the mildest of stock engines..... Also the timing, earlier bad418 said he thought it was 2 degrees back..... Where the initial timing is set doesn't tell the whole story, where the total timing is set is what really matters..... If the distributor is not curved properely it can cause major problems in a motor with some decent compression with pump gas.....
The plug problem could be the wrong reach plugs installed coupled with a bit of excessive piston rock on the "over rev". Every piston I've ever seen where the rings have butted bad enough to damage the piston have torn the top of the piston off I think it's probably more likely it was detonation. Hypers are good pistons but they are brittle and don't tolerate detonation well. Since the motor is apart it's time to check all you clearances to see what is wrong. Look at the ring ends and if shiny they were butting. Check the plugs to make sure they don't extend into the chamber too far. Check the piston to wall gap to ensure you are not getting excessive rock. Look at the rod bearings to ensure you are not getting rod stretch, evidenced by excessive wear at the parting line. Check deck height and compressed gasket thickness to ensure you are in the .040 to .050 range. Things can seem just fine until you get the right combination of RPM and heat then your tolerances go away and you have problems.
