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Replace piston or pistons?

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#1
Lee Tilton

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I missed a shift at VIR this weekend and bent a #1 exhaust valve. Head is being replaced. The valve made slight contact with the piston - enough that I feel better replacing it. The question is: can I just replace one piston or should I do all four? The motor is a crate motor with about 10 weekends on it, so it is just getting loose. Also, what about the rod/main bearings? Since that is one of the components that makes for a loose bottom end, I am hesitant to change them. Obviously I will inspect the #1 rod bearing very closely. Any advice is welcome.
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#2
dstevens

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If you want to be safe open it up and look at it all. Anything obviously galled or showing heat damages needs to be addressed. You can measure your bearing clearances as well. There's a good chance that number 1 is all that needs to be addressed. These are tough little four bangers. There is also a possibility that something else happened that isn't as drastic as the bent valve but might lead to issues in the future. If it were mine it would be worth it for me to pull it and spend the time checking it out for piece of mind. If a valve contacted a piston in a race engine I'd swap at least it out. Depending on the age and measurements I might do all the cylinders.

#3
davew

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We have all missed shifts, but very rarely will it result in a bent valve. As mentioned above, these motors are pretty strong. I would look very closely at what really happened.

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#4
Lee Tilton

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We have all missed shifts, but very rarely will it result in a bent valve. As mentioned above, these motors are pretty strong. I would look very closely at what really happened.


The exhaust valve broke just below the keeper. It must have bent the valve stem just a little because the valve will not fall out of the head. I had Mike Rossini look at the head while at the track and he said he had seen similar failures on a missed shift.

So back to the original question: IF the number one piston is the only lower end damage, is it OK to just replace one piston. How close are OEM pistons in weight?
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#5
FTodaro

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I have missed lots of shifts and reved the motor beyond belief so, I guess I would say that is not typical of my experience. Having said that to your question,

I am not sure you can get an answer here that you can take to the bank, You might talk to a motor rebuild shop and get an opinion of an expert.

Drawing from my current experiment with rebuilding my transmission, there is something to be said for taking a few extra precautions with a race equipment. we push this stuff to its limits, if there was any question while you are this far into it I would just do it and be done. But, and expert may have enough confidence to tell you, what you can safely get away with.

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#6
dstevens

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I am not sure you can get an answer here that you can take to the bank, You might talk to a motor rebuild shop and get an opinion of an expert.


Yep, without actually seeing even the best builders won't be able to give a full evaluation. That's a good benefit of having an engine program with a reputable builder. I don't mind doing my own junkyard/shade tree builds but when I get to the point where I make a bigger investment, I'll job the whole thing out.

#7
steveracer

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Wow, how bad did it over-rev?

I got 2nd instead of 4th, data showed 8510 rpm! I raced that motor for a full season after that with no issues, it just got tired.

I'm with Davew, something else happened...

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#8
Keith Novak

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The exhaust valve broke just below the keeper. It must have bent the valve stem just a little because the valve will not fall out of the head. I had Mike Rossini look at the head while at the track and he said he had seen similar failures on a missed shift.

So back to the original question: IF the number one piston is the only lower end damage, is it OK to just replace one piston. How close are OEM pistons in weight?


Pretty close. I've been told not to worry about matching the weight by someone who knows volumes more than me.

I've experienced a bent valve following the dreaded Ooops that's not 4th! shift. I know people also who did the same thing and registered crazy high RPMs on their logger with no ill effects. Was it the botched shift or something else? Who knows for sure. The damage is already done so fixing it is the critical thing now.

I would check it out thoroughly. Nothing more annoying than fixing what you can see is wrong, only to lose several thousand dollars in parts the next outing from what you didn't see. It ruins the weekend and doesn't help the race budget. Once you have the engine out and the head off, tearing down the bottom end really isn't that hard or time consuming.
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