If this is carbureted, it is pretty weird to see a lean cylinder 4.

I was tuning on the water and only reading plugs as I do not have a wideband (yet). I know this thing was super fat at the beginning of the summer and eventually I leaned it out over the summer in small increments reading the plugs and going off of the performance of boat. On our last outing of the year I kept putting a degree or two of timing in it to see what I could get out of her on for speed but that was only for a few short pulls. It normally runs at 34 degrees timing.

I'll have to go back and look thru some more of the dyno sheets I have when I get some time, they are buried in a folder somewhere. The 2 that I did find, with an EGT probe in each cylinder, did show #4 and #6 to be the hottest out of all 8 at 4000RPM. By 6000RPM, #2 and #8 were . This was a CNC ported head, NA engine with a custom ported intake manifold. I'd guess that all bets are off when using a vintage set of GM iron rectangles with grossly imbalanced intake ports , and unknown intake/plenum design.

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. Back to Basics: Preventing Piston Problems - Engine Builder Magazine

This was merc's spec on the 540ci for piston clearance.

Got some pics of the cylinders including the 2 in question..

widebands are so cheap. Inexcusable to tune with out one.

Any suggestions or have a brand that is preferred? As I have never used one.

they all use the $80 Bosch sensor

Innovate Motorsports Wideband Air/Fuel Ratio Tuning

you do a little searching and you can get them around 50 bucks.

The way I see it is you potentially have a few things going on. Without going into depth as MT and others have covered at some point through the thread. 1 is original machine shop quality of work. Seems like cylinders are out of round and possibly were not chamfered among other things. 2 is how builder didn't catch the two pistons being a .001 smaller shows he didn't throughly check bores, pitons etc prior to assembly. I don't believe the .001 smaller pistons 4 and 6 would cause this but is coincidental. I would do a little research on the icon pistons. Just wonder how much they're really expanding under normal heat conditions. I know they've been used in a lot of builds but... A wideband o2 is definately in order s you know cause like MT said heat is probably the number one cause of scuffing. What carb and jets did you end up with at end of season? On dyno closer to sea level you'd be plenty rich I'd think at least where you started. Just curious how much jet you went down. Id think a couple 2-3 sizes at your elevation would be close assuming your egts on dyno were on target.

I'm sure your new machine shop knows but be sure and sonic test. My NOS GM blocks tested out fine to go out to 4.600 and still plenty safe if were gonna be NA. Only taking them to 4.530 for SC builds.