I found this on enginebuildermag.com and found it informative so I though I would share.

Piston Scuffing
This is a sure sign of overheating and/or insufficient clearance. When an engine runs hot, the pistons swell up reducing the clearance between the piston and cylinder. The cylinder bore can also distort from localized hot spots, further adding to the problem. If a piston gets hot enough, it will scuff the wall wiping material off the sides of the piston. Where this occurs will give you a clue as to the cause.

If overheating is involved, the scuffing will be primarily on the upper ring lands and on the sides near the wrist pins. There may also be oil carbon and lacquer burned onto the underside of the piston indicating it was unusually hot.

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.

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So applying this to my issue... Possibly overheating? The bottoms of the pistons have a very light varnish inside the rod connection area but all 8 are consistent. The scuff marks on the bad pistons are more on the corners of the skirts where at the light scuffing on the other 6 are in the center of the skirt. Even on the bad pistons I can still see the machining on the bottom-center of the skirts so I presume that would rule out a lubrication issue. I will also add that when this happened at the track, I pulled the plugs and the ground strap on #2 had started to melt. No obvious signs of detonation though.

 

Unfortunately, I can't return it to the builder. Shortblock was build by a past sponsor on tech which is now defunct. We have a couple small machine shops locally so I will be making some phone calls tomorrow.

 

Bummer...

The sounds of your last post about the strap burning sounds like it was running lean.

 

Wideband reads 11.2-11.5 on WOT but the plugs were br6ef. I have 7's to put in now. Unfortunately, I was not logging when this supposedly happened. Not saying there isn't a problem with those two injectors or the hand ported heads or who knows what. I am thinking while the engine is down, sending off the injectors and having them cleaned and flow tested.