Piston to bore clearance gain after break in?

[Pablo]

How much piston to bore clearance does one gain on a bored and honed cylinder after break in?

I have been measuring up all clearances on my build and have found piston to bore clearance as low as .0031 to as high as .0042 (one oddball cyl) most are in the mid .0030s. I told my machinist specifically to machine for .004-.0045 clearance. I thought I had read somewhere that you'll gain something like .0005 after break in... true?

 

[RmvBfrFlght]

Not sure what the gain is after break in. Usually it isn't included in the piston to wall clearance value. I think you need to have a talk with the machinist.

Forged pistons?

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[1chance]

Are you sure you got the right method to get the clearence? Forge pistons need a lager clerance than stock. I think you measue from side to side on piston, then do cylinder, subtract the two numbers, then divide by two.

 

[bison]

To measure clearance you need to know where to measure the piston. The manufacturer will tell you where to measure the diameter. Then its simple math from there.

 

[Pablo]

Yes, I measured everything correctly with a dial bore gauge and an outside micrometer. I will be re-measuring to ensure I didn't inadvertently introduce any errors.
I did re-measure a couple of cylinders and I got the same numbers to within a couple ten thousandths though.

These are TRW forged pistons. The data sheet calls for .0035 but everything I have read points to using a larger clearance on our engines.

Think I need to have it re-honed?

 

[RmvBfrFlght]

I was hoping someone with more TB experience would have responded.

My belief is that the .0035 clearance is for a N/A motor. I've seen this before with a different brand of forged pistons in a TB. Clearance set to the manufacturers spec ended up with 5 scuffed pistons and one seized in the bore.

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