Can a piston be crooked?

So I'm measuring my piston to deck height so that I can figure out what thickness head gasket to order, I put a straight edge over the center of the #1 piston, and I can just barely fit a 25/1000 feeler guage in there. I move the straight edge to the top of the piston and try to fit the feeler guage under there, but I can't. So then I move the straight edge to the bottom of the piston and the feeler guage fits in there loose, meaning there is more than 25/1000 between the deck and the piston at TDC.

I move to the other side of the engine to check the other piston that's at TDC, and it is fine. 25/1000s top, bottom, and middle. I'm tripping out here! Is this normal?

BTW, one of the head gasket fire rings on the drivers side did blow out.

eh...turn the crank and measure two others?? then do it twice again to catch all 8....25 mills seems like a LOT to me....summfin rong....

wonder if the skirt is ok or broken?? :confused2:

Mount an indicator like this and rock the piston...see what you get.... something is wrong, you should not have .025" from one side of the piston to the opposite...



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It's not a difference of 25/1000 more like 5 or 10/1000 I was just saying that the .025 feeler guage would not fit top, but it would fit bottom. What kind of indicator is that Karsten? And how does it mount?

The engine ran fine, other than having a blown HG. And it put down plenty of power. The pistons are forged SRP, so I doubt that I've managed to break them somehow.

Edit: And engine guru at speedtalk said this:

"What you are seeing is skirt clearance
Take your fingers & push on the piston one way then the other
It will rock in the bore
If the intake side were even & the exhaust side were at .025 down
You have a nominal deck height of about .0125"

Check that bore. If something broke, it would be scored. If it's not, put it back together and drive it.:wink:

enkeivette said:

BTW, one of the head gasket fire rings on the drivers side did blow out.

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I want to know what you found on #2, That's where I saw it leak on the dyno.

Well I pushed on the piston and it rocks back and forth! Haha, I guess they all do that, they pivot on the rod. I didn't know that they had that sort of clearance.

I think the leak was from the VC wasn't it? And the #2 looked like it could have been leaking. #8 might have been leaking too, but #7 was leaking for sure. That one was leaking bad. Crazy how fast the car ran with leaking HGs.

There are some factors in this (the piston btw is barrel shaped), rod pin height is a major factor in this, the lower it is, the more the piston can rock. Obviously having forged pistons with larger tolerances will cold rock a lot more than hypers.

enkeivette said:

What kind of indicator is that Karsten? And how does it mount?

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It's a dial indicator and a magnetic base. You can find these for less than $20 at HarborFreight and up to $200 for a good quality product (Mitutoyo, Starrett)

With the cheap chinese stuff you might have to use pliers to clamp the joints tight but for what you're doing it'll work.... if you plan on using that tool for a few years I'd buy quality.

You need one of these to check rotor runout btw...

Too much piston clearnace and it'll rock in the bore. Try it with no rings and see how sloppy it is. You can get a rough idea by using a long feeler gauge alongside the piston at 90* to the pin. Most accutate is a micrometer on the outside of the piston and an inside mic in the bore. A cylinder that's tapered may show more clearance too.

Twin_Turbo said:

There are some factors in this (the piston btw is barrel shaped), rod pin height is a major factor in this, the lower it is, the more the piston can rock. Obviously having forged pistons with larger tolerances will cold rock a lot more than hypers.

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I've got Hypers, I use .004 clearance. Yes, .004.