Different Pistons - Different Clearances, Please someone explain - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 11-09-2012, 10:54 AM
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Different Pistons - Different Clearances, Please someone explain

I am doing a stock rebuild of an M52B28 engine. My goal is to give it as much longevity as I can. To that end, I would like to make the piston-to-wall clearance as small as allowable without binding. That said, I cannot find any pistons available that will allow the clearance to be even close to what the stated factory clearance presumably was.

Here are the facts:

1. The piston-to-wall (PW) clearance of the factory engine can be as low as 0.0004". Yes, that is four TEN THOSANDTHS of an inch. Both Chilton and Haynes state the PW clearance of the factory engine to be in the range of 0.0004" - 0.0016". I don't know much about the factory pistons. I think they are an aluminum alloy, and I think they are cast, and I think they are made by a company called Mahle.


2. The only cast aftermarket pistons I can find are the Maxsil pistons. The Maxsil website states that their pistons have

13% silicone content allowing for tighter cylinder clearance (0.0015")

But compared to the factory range that is hardly a tight clearance. The Maxsil pistons are cast, they have 13% silicon, and they have teflon coated skirts. So why can't they have a smaller clearance ? What is so special about the factory pistons that allows them to run with smaller clearances ?


3. Forged Pistons. I called Wiseco the other day. Even using the 4032 alloy which is reportedly 11% silicon, the cold clearance they call for is 0.0025"- 0.003" -twice what Maxsil calls for and as much as seven times the lowest factory clearance ! And forged pistons supposedly run cooler.


Why such differences in the stated clearances? Is it just a liability thing - not wanting to be responsible for ruining an engine? Or is there some other factor at work here that I am unaware of ?

You might be tempted to say that the small factory clearances are actually the clearances that will be there once the engine heats up and are not the clearances you would measure at room temperature. But then, are the stated dimensions for the bore and piston given for a hot bore and hot piston? The stated cylinder bore is 3.307" and the stated piston diameter is 3.306". Difference these numbers and divide by two and you can infer that the cold clearance will be 0.0005" - half a thousandth - at the lower end of the stated clearance, yet presumably for a room temperature engine.

So right now it looks as though no matter what I do I cannot improve the PW clearance over what I currently have with my worn pistons/cylinders.

Please help me understand what is going on here.
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