Hello, I just received my pistons back from you after being coated and discovered that they no longer fit in the cylinders. They received PC-9 on the skirts.

Prior to sending the pistons in I took the following measurements:
Avg. Piston Diameter = 3.9364"
Avg. Cylinder Bore = 3.9376"
Avg. Piston to Cylinder Clearance = 0.0012"

Now that they have been coated, the average piston diameter is 3.9390".

The engine is a Porsche 944 with an alusil block. Factory specs require a minimum of 0.0003" (0.008 mm) clearance between the pistons & cylinders.

I spoke to someone at your shop and was told to use sandpaper to buff down the thickness of the PC-9 coating. If I do this I would end up with an average thickness of the PC-9 coating of just 0.0004" in order to get my required clearance. Is that ok? Is there a minimum thickness of PC-9 I should maintain? Would I be better off enlarging the cylinders? Any thoughts or advice you can share would be greatly appreciated. Also, I was told to use 400 grit or coarser sandpaper to buff the coating. Is that correct? It seems like it should be 400 grit or finer so as not to rough the coating up too much.

Thanks in advance,

You can go either way. If you sand down some coating you risk sanding right through the coating and ending up with bare aluminum again. If you hone the cylinder you have the time and expense of the cylinder work. If it were me I would open up the cylinder but you could go either way.

I would use 400 or finer to reduce chances or removing too much coating too fast and being right back to the aluminum. The down side of 400 or finer is the paper loads up with coating quickly. Either way is acceptable but I would lean towards 400 or finer.

Regards,
Richard
Swain Tech Coatings, Inc.