I had problems with my old pistons contacting the cylinder walls scuffing up both the pistons and the walls. I fiqured it was because of lack of oil pressure.

I went out got the same pistons except .040 over. I had a head gasket fail so I pulled both the heads and the walls are already fuc*ed. All 8 cylinders, and my oil pressure is fine??? I checked the piston to wall clearance before installing the pistons and they where at about .005, which was recommended Is there something that im doing wrong or is it the bunk *** machine shop?

 

Cut and pasted from http://www.babcox.com/editorial/ar/ar80125.htm

The presence of scuff marks on the lower skirt area would tend to indicate a lubrication problem rather than an overheating problem. Scuff marks on the edges or corners of the thrust sides of the piston would indicate bore distortion as the culprit. Scuffing on both thrust sides would indicate binding in the wrist pin.

Any time you find evidence of scuffing, therefore, it’s important to diagnose and correct the underlying cause to prevent a repeat failure.

If the engine was recently rebuilt, the scuffing problem may be due to insufficient assembly clearances. The only way to know is to measure the pistons and cylinders. Measure the pistons at the top, center and bottom of the skirt area in two directions (parallel to the wrist pin and perpendicular to the wrist pin) to see if the pistons are within the manufacturer’s specifications.

If the pistons are the correct size, then measure the cylinder bore at the top, middle and bottom, also in two directions (parallel to the crankshaft and perpendicular to the crankshaft). This will tell you how much taper is in the bores, and if bore distortion is a problem. By subtracting the piston dimensions from the bore dimensions, you can figure the piston-to-cylinder clearance. If the clearance is within specifications, the underlying problem is overheating.

Many aftermarket replacement pistons today are available with moly coated skirts to protect new pistons against scuffing. The coating also allows you to run slightly tighter clearances to reduce piston noise when the engine is cold, and blow-by. Upgrading to coated pistons is probably a good idea for any engine that has a history of scuffing or may be prone to detonation or overheating. Think of it as insurance against piston problems.

Rich Krause

 

sleeper-
obviously oik preussure has nothing to do with piston scuffing.. unless your pistons are pressure oiled or something


Sounds like the piston to bore clearance is too tight. what was it set at when new?

 

What brand of piston? Ross, JE, and BRC all recommend .006 piston to wall clearance for their forgings on a turbocharged motor. CP is the one brand that I've seen that can run tighter at .0045 to .005 .

 

Were the bottom of your cylinders chamfered after boring?

If not, the sharp edge at the bottom of the cylinder can gouge the piston skirts as they rock in the bore.

 

Thanks for the replies. The piston to wall clearance is set at .005 right where ross told us to set them. I checked the clearance myself before installing the pistons. As for the wrist pin binding that is not a problem. I checked all before putting the oil pan on. motor turns smooth and no contact between the bosses. My oil temp does get a little on the extreme side, which is the only reasoning I can come up with. It would hover around 280, in hot weather. The scuffing on the walls is present on both sides not just the thrust. Im thinking the turbos are getting the oil too hot removing the film between the pistons and the walls. The scuffing was on the lower skirt of the other pisons.

This problem didnt happen until the twin turbo setup. It was fine with a single