I just took the heads off my boat to change the valve springs and found two pistons that were broken/burnt in the vavle releif next to the piston wall, I can clearly see the top ring. I Did a comp check before I removed the heads and didn't notice low comp in those holes. The motor is a 496 with 10.2:1 srp pistons. I'm running a msd dist with the lightest springs and I set the total advance at 34deg. What besides detonation will do that to pistons? Any thing else I should look for on reasembly?
Spark knock did the damage...I run a B&M 174 blower on my truck and even with moderate timing and a MSD timing boast control and 92 octane have knocked the top off of two pistons twice. One trick that helps is to smooth all the sharp edges off the pistons and chambers to help prevent hot spots. Your there so give it a shot. It's seemed to help on mine.
Mike
The area that it happened to is the sharpest on the whole piston. Why does heat effect these areas so much more?
I just took the heads off my boat to change the valve springs and found two pistons that were broken/burnt in the vavle releif next to the piston wall, I can clearly see the top ring. I Did a comp check before I removed the heads and didn't notice low comp in those holes. The motor is a 496 with 10.2:1 srp pistons. I'm running a msd dist with the lightest springs and I set the total advance at 34deg. What besides detonation will do that to pistons? Any thing else I should look for on reasembly?
Do you run a blower?
The area that it happened to is the sharpest on the whole piston. Why does heat effect these areas so much more?
It gets the valve relief area because that is a very thin area. Usually blown motors have the problem. Most of the time too lean. Cutting torch time. Heat area and add air = hole.
The area that it happened to is the sharpest on the whole piston. Why does heat effect these areas so much more?
The area cannot shed heat fast enough to the thicker areas leading to melting of the aluminum. Also, The relief for the valve is deep and the top ring land is very close to it....leading to not being able to shed the heat fast enough from that area to keep from melting or cracking. :boxed:
Any thing else I should look for on reasembly?
What holes were the problem cylinders?
You should grind and smooth off the sharp edges (intake valve relief)(nearest the cylinder wall) on the remaining pistons. Not very much removal as far as weight is concerned and will help keep it from occuring again.
Clean up ANY sharp edges. Blower motor or not detonation was the cause. The edges on the combustion chambers also. doing the pistons is #1 but all of the area "SQUISH" holds heat. Just one way of cutting down a repeat fudge...
Mike :idea:
The motor is not blown. But the thickness in that area makes sence to me. So it looks like I did get the blow torch effect. Sorry if I confussed anyone but I am not reussing these pistons. Although the compression was still alright I'd rather error towards the safe side and just replace them. It seems like a weekend at the lake cost about the same as the pistons/rings do. My old pistons weight 560 grams, if my new ones don't weight the same, how much of a varience is acceptable before rebalencing is in order?
The problem cyliners were 4 and 5.
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