Valve-Piston Clearance

[edge328i]

Does anyone have any knowledge of what kind of clearance exists between the valves and the pistons in a new BMW?

I am rebuilding the engine of a 1996 328i (M52 engine) and need to do some things that will lessen the valve-piston clearance. It would be nice to have some idea of how much space I have to work with before I ruin the engine.

I have Haynes, Chilton, Bentley, and have looked at the TIS online, but the only possibly relevant information I can find is the following from page 2C-7 of the Haynes manual:

"If 0.012 inch is machined off the cylinder head, a 0.012 inch thicker cylinder head gasket must be fitted when the engine is reassembled. This is necessary in order to maintain the correct dimensions between the valve heads, valve guides, and cylinder head gasket face."

Even this doesn’t explicitly say anything about valve-piston clearance. My machinist doesn’t think they would manufacture an engine with only 12 thousandths of an inch before interference became a possibility.

I need to know what valve-piston clearance my engine started with, and what the minimum allowable clearance is. Shouldn’t this information exist somewhere?

When the head was reconditioned, they had to take about 0.004 inches off the gasket surface to clean up some corrosion. To flatten the deck, the machinist is telling me he has to remove at least 0.005 inches. OK, you say, that’s a total of 0.009 thousandths, no problem. But the only aftermarket pistons available for this engine (please correct me if I’m wrong) are the MaxSil pistons. These pistons have a compression height that is about 0.025 inch higher than the original pistons (and the relief notches do not appear to be any deeper). The MaxSil people, by the way, would not tell me this number – I had to purchase the pistons and measure them myself.

While I haven’t removed more than 0.012 inches from the head surface, I have effectively moved the height of the piston travel (0.009 + 0.025 = 0.034) 0.034 inches closer to the valves. I need to know if this will result in valve-piston interference.

Does anyone know?