Piston wall clearance and Coating skirt
 

I have a new engine I am putting together and it has Ross/Butler pistons. I have the piston to wall clearance set to .004 per the tech sheet that came with the pistons. Now I am considering having the piston skirts and tops coated. Problem is the thickness of the dry film coating is .001-.0015. This would reduce my clearance by .002 to .003 and I wouldn't have enough clearance after coating. I don't want to have my block honed again so I suppose I should just forget about coating the skirts.

Most of the coating shops say to let the coatings wear in. Do not adjust clearance because of the coating.

Is that the same for rod and main bearings?

No, the coating is thinner on bearings. Well at least most of the time.

Under extreme pressures and rubbing like the side of a piston molly coatings eventually will wear away

one may ask why bother then?

the molly coating reduces galling and wear of the piston mainly during break in when the cylinder hone is most abrasive

on rod and main bearings less surface abrasion and a more robust hydrodynamic oil wedge and probably less surface speeds vrs piston skirt all leading to a longer coating life on bearings

If a cylinder wall has been plateau honed after been final honed, or already broken in then sending pistons out to having a skirt coated will only serve to lessen the green out of your wallet.

One reason a person may want to coat the skirts on the pistons is to take up excessive clearance.

One reason a person may want to coat their bearings is to simplify the decision whether the bearing should be replaced or not.

After listening to everyone's comments including from my machinist and Butler performance I have decided not to use any coatings. My rods are at .0025 and the mains are .0032 and piston skirt to wall is .004. I am happy with the clearances not going to mess with any coatings that would add $500.00 to this already expensive build.

Are the pistons Ross 2618 or some other 4032 piston?

Ross only uses 2618 material

I agree with leaving it as is, especially with the bearings !

Face the facts, if a running motor is relying in its coated bearings to stay alive then that time span of staying alive is going to be short anyway!

eventually that coating gets worn thru and your done but you don’t know it until bearing clearance noises and loss of performance takes place.

Steve Morris has a few videos on the a coating he has started using on his builds.