I'm attaching pictures of something I have not seen before. 360 race engine with .040 KB silvolite pistons in it. I built this engine maybe 10 years ago for a dirt track Dakota. Has maybe 10 nights on it-30 laps max per night. After a teardown, clean up, and re-hone, the cylinders show a dark spot that feels pretty deep. Its on all 8 cylinders.

Any idea what causes this? It ran 3-4 races in the last couple months with no known issues. thanks in advance.

Thrust side wear. Does the other bank have it also?

Very common on LA motors, that is the piston rocking at TDC when reversing directions making the piston rings dig in

looks like the bores are wore out to me,.. . get a dail bore gauge and check it.

Too much piston to bore clearance. What is it now?

Aka piston slap.

Nothing new there. I had a .030 (360) 408 combo with Diamond pistons that on its third rebuild the bores were so bad I wore out two sets of stones somewhat cleaning it up then hit it with my dingle berry hone. I saw some of the strangest marks in those bores Iv’e ever seen. That engine ran 9.73@135mph at 2880 pounds in my Duster and 10.10@3200 pounds in my sons car.

My first thought was, would Line 2Line skirt coatings help eliminate this? L2L coatings are applied about . 003 too fat, and they wear in as the motor is run through a set of warmups, then increasing loads at running temp, increasing rpm and load in steps. This wears in the coating to exactly fit at full temp. I used it to cover a gap in a rehoned block, 4.5043 size, reusing 4.500 pistons. Over 900 hp out of that 550 inch 440-1 head motor. I am sold on it, seeing great results. Cost $300 plus the ride to L2L.

The only difference here is that these are cast pistons, should be around .001. No more than .002 max. What happens is the skirts collapse. Run a mic down the skirt if it doesn't keep getting wider, the skirts are collapsed. If it keeps getting smaller, a cracked skirt is in its future.These pistons have the oil supplied to the rings through a large window behind the oil ring. The silvolite line is a stock replacement piston like 80 bucks for a complete set.

I don't know how many times I freshened my stuff. Every winter. Polish the crank, berry-ball the cylinders, rings and bearings.

No idea how big the bores were. Never measured it after the initial build. Never saw marks like that. I did have it Hardblok'd to the bottom of the water pump holes. Maybe that helped.

Or just get a set of forged pistons, mic the bores and hone the block as needed. These dog gone blocks are getting harder to find every year and some are scraped for silly issues. It’s not a spaceship going to the moon so don’t sweat the small stuff.

A ball hone will still leave the low spots

Usually not
That was one of the reasons I quit using them
I bought and use a Lisle mechanic hone that will straighten out the cylinders with enough work as already mentioned above Those handheld drill motor driven hones take forever to remove material A little better than nothing though

you can see the ridge at the top of the cylinders. #5 and #7 look like scuffs or water erosion at the top of the cylinders. if this engine was run on a dirt track without very good air filtration i'd think the dust would grind the cylinders up. there also was a deal back in the '70's were chrysler bought a bunch of inferior cast iron and used it. could be a soft block,...? i've ran and beat the snot out of hypers without wear issues, but i put them in just as the literature said to do. i think the block is just plum wore out.

i did a 360 for a friend some time back and the block had over 100,000 miles on it. the oil was changed frequently and the engine was just a driver. it only had about .001" of taper, no ridge, wear marks or nothing; i think it was a '90's block.

Thanks for all the input. Correction on the pistons, these are the KB107 Hypereutectic ones, not silvolite.

I measured the marks and they appear to be about .002" deep. It was bored at .040 over. bores measure .045 now, the marks measure .046~.047

It did sit for several years, hence the couple of cylinders show some pitting near the top of the bore.

thanks again for the replies

Whats your P2W? Also what was the combo? Boosted? Nitrous? The marks at the top remind me of a 302 I took apart a year or so ago, foxbody motor (forged pistons stock) guy was running boost with probably too much timing. Bent the top ring lands down on 4 cylinders, rings then dug grooves in the bores near the top of the bores around the peak pressure location. They were like .005" deep.

that's probably going to put piston to wall about .007". that's pretty loose for those pistions. my KB literature gives unlimited circle track clearance between .0025"-.0045" for a 4.10" bore. Top ring end gap should be .032+".

Forged piston time and send it.

so 300 laps, in dirt, that will do it , was it running Methanol ?

KB107 has a slipper style skirt (with drilled holes) .They will take a little more abuse. But still a cast piston designed to run tighter. I would still check the skirts.