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Piston damage... what would you do?


Steve G

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One of my goals was to do a big block swap that looked close to factory... not perfect but able to fool 95% of most GM fans. I scored big time when I came across a 402 car with very low miles and never tampered with. Just about every plastic part, every hose and clamp was original. Ran smooth and quiet no oil leaks. Since the owner was doing an LS swap I got it all from radiator to driveshaft for $1500. In my opinion well worth it. So my big plan was to just replace the freeze plugs, some gaskets and a tad of paint and call it done. When I pulled the valve covers i was pleased with the cleanliness of the heads but one pushrod caught my eye, I could tell it was bent. So like any engine cleanup it snowballed into me ripping the entire engine apart. I kinda got mixed feelings about doing it, the engine is like brand new inside... no it was a bonehead move on my part.. haha. Other than the bent pushrod and the piston with the mother of all valve gashes there is no other damage... I’m still going to redo the heads just in case (that valve does not appear to have any damage!) and new cam and lifters/pushrods.  
I’m leaning towards just doing a ring and bearing job on the bottom end and calling it good (never done that before) but I need to be able to sleep at night. Yes or no... would you trust this piston?

thanks!

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I would absolutely trust this piston. The valve hit the small dome, thicker aluminum there. 

If it looks as good as you say, I would do rings, bearings, gaskets and have the heads checked, cam, lifters like you said and all pushrods for insurance.

I would also file where the valve hit the piston to remove any "high" spots. Make it the same height as the dome. Otherwise you could have hot spots that could cause an issue. 

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I'm with Tom, I would file it smooth and run it. That being said I would also be checking piston to valve clearance real well when putting it back together. It has been a while since I had my old 402 apart, but that looks like it might be an aftermarket piston.

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I know its hard to purchase just one stock piston, but if you already have it broken down to parade rest like you do why not swap it? For nothing else just for peace of mind. Its far enough disassembled you can put new bearings and rings in anyway. I don't see a good argument not to change it other than a slight cost difference. 

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Go with a roller cam and roller lifters the difference is well worth it. My engine builder quoted a price of $1,500 to rebuild my 350 with a .0020 overbore, flat top pistons using my cam that came In The 290 horse crate engine. He was really leased when I told him I wanted a roller cam and lifters and the price only went up $600. I am really glad I went that rout the engine has a nice lope to it and perform s well on the street. With the Flowmaster 40’s and 2 1/2” stainless steel exhaust it sounds really healthy.

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1 hour ago, Leghome said:

Go with a roller cam and roller lifters the difference is well worth it. My engine builder quoted a price of $1,500 to rebuild my 350 with a .0020 overbore, flat top pistons using my cam that came In The 290 horse crate engine. He was really leased when I told him I wanted a roller cam and lifters and the price only went up $600. I am really glad I went that rout the engine has a nice lope to it and perform s well on the street. With the Flowmaster 40’s and 2 1/2” stainless steel exhaust it sounds really healthy.

I built a lot of motors back in my 20s but always went with new parts and a clean machined block. Never did a basic cleanup before so I’m a tad paranoid.. also times have changed and now the internet car scene requires that every motor be built with the most expensive parts on the planet plus an extra 10k invested or it will instantly blow up the first time it hits 4,000rpms. Thank you car building TV shows for misleading everyone.   I learned back when I was 18 that a motor can be an internal disaster and still be a good performer. I bought a 64 falcon that had a built up motor from the 60s. The guy warned me the motor was tired and needed a rebuild. This thing didn’t even start putting out power till 5500 and I was doing 7000+ rpm shifts constantly.. the oil pressure gauge never went over 20psi so I assumed it didn’t work and the crankcase pressure was so bad it would pop the breathers off the valve covers... still went strong. After a few months I took it out to refresh the motor and found bearings worn past the copper, the 20psi oil pressure was probably generous. Rings and ring lands broken, part of a piston skirt missing, all the valve spring retainers were cracked and about to let go and a cylinder wall ridge that was so deep it looked intentional.. if I had known it was that bad I would have never even drove the car in the first place. 

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