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I'd like to start out by saying that I have the Helms manual. I'm just a little unclear about a certain point. I've been measuring a Z6 for cylinder roundness. The book says to measure in the X and Y directions (perpendicular and parallel to the length of the motor for those w/o the Helms) at 3 depths within the cylinders. I'm clear on that part.
I get a difference between X and Y of about .03mm. This makes sense as the piston rocks in the bore, etc. My question is, how much of a difference, or out of roundness, can be tolerated before the cylinder has to be bored oversize? Thanks in advance.
 

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in the helms your using there should be specs for what it is originaly blueprinted to and what is considered "service limit" meaning the absolute most it can be out. If you cannot find it, let me know what engine you have and i will try to look for it in one of mine.

m.
 

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marcin said:
in the helms your using there should be specs for what it is originaly blueprinted to and what is considered "service limit" meaning the absolute most it can be out. If you cannot find it, let me know what engine you have and i will try to look for it in one of mine.

m.
Thanks for the reply. The Helms gives a spec of 75.07 as the service limit in the X direction. This X direction is in line with the axis that the piston would rock in the bore. Should I assume that the cylinder is only going to wear in this direction and that this amount of wear is acceptable? It is within the limits, but I'm worried about excessive noise when this is running. Thanks for your help.
 

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when you put pistons and new rings in any engine, the wear characteristics change, so it may wear in a different pattern. I myself would have the block machined to the next oversize, so i would have to do it again in the future, this also would give you more security that the new rings will wear uniformly. If your going to make spec clearances, its best to do them all at one time, and save you trouble down the road.
 
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The X axis will almost always be the first and most noticeable direction for wear on a engine because of the rotation of the crank. I would measure your Y axis and compare it to the X if there is a considerable difference (chances are it will be) you definitely need to get it re-bored.
 
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