355 SBC that isn't
- Thread starter FastCraig
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The edge of the piston broke off. I don't know off the top of my head how angled the plugs are or where in relation they are to the valves and such, I assume pretty centralised. I don't think the plug broke the piston. I'm in the "ring gap too tight" camp.
Alloy + silicon = strong but brittle? KB134-030 = Description: Keith Black hypereutectic pistons are made from 390 aluminum alloy 16 % silicon and heat-treated to T6 standards, which makes them 30 percent stronger than ordinary untreated hypereutectics.
They are fancy cast pistons is what hypereutectic is. They are very hard, easy to crack, don't change size much due to heat and are quiet running. And cheap to make. Make a spot thin, like the valve relief area and it can crack easily.
I'll throw my two cents in here - that piston broke due to something other than ring gap. If it was indeed ring gap as you suspect, then there would be heavy evidence on the cylinder bore. If the bore looks mostly unscathed, then that's a either a tuning issue or an extreme cylinder pressure issue coupled with the ring gaps inverted (lower ring gapped smaller than the upper).
How do the bearing shells look on that cylinder? Is there a small area the size of a dime or nickel that's worn significantly on the upper shell? Have you pulled the remaining to check? If no, then I'd STRONGLY recommend it.
Next, in your second photo the headgasket is hanging over the edge of the bore. If that is actually the case, then get a better set of headgaskets. Those blue paper gaskets are stock replacement units - get something along the lines of a FelPro 1003 or simialr. Also, this a great time to verify piston to deck height, tdc, cm specs and degree the ca to make sure that the all is well.
Edit : should have checked before, but those pistons sit about .017 in the hole of an undecked block. If the block has never been, then you'd be best with a .0020 to .030 thick gasket. GM performance makes a .026" gasket that would work great and is relatively inexpensive. Again, this is if your block is undecked.
How do the bearing shells look on that cylinder? Is there a small area the size of a dime or nickel that's worn significantly on the upper shell? Have you pulled the remaining to check? If no, then I'd STRONGLY recommend it.
Next, in your second photo the headgasket is hanging over the edge of the bore. If that is actually the case, then get a better set of headgaskets. Those blue paper gaskets are stock replacement units - get something along the lines of a FelPro 1003 or simialr. Also, this a great time to verify piston to deck height, tdc, cm specs and degree the ca to make sure that the all is well.
Edit : should have checked before, but those pistons sit about .017 in the hole of an undecked block. If the block has never been, then you'd be best with a .0020 to .030 thick gasket. GM performance makes a .026" gasket that would work great and is relatively inexpensive. Again, this is if your block is undecked.
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There is 1 very light scuff on the cylinder bore. I did not remove any other pistons but rotated the assembly to check each cylinder. For an engine with nearly 14K on it I think it looks very good. The crank and rods are Eagle. The crank and block has 383 stamped on them. The block does not appear to have been modified for a 400 crank. There isn't much space between them. There is no evidence of any contact. I am not a mechanic and my knowledge of 383's has been aquired in the last few weeks.
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What I don't understand is side view shows piston material over the top of the ring remaining. The top view looks like the top compression ring is exposed???
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