Knock on fresh overhaul, high RPM warm

I just rebuilt my Super M with a Redpower major overhaul kit with 4 1/8 bore domed pistons with the large wrist pins.
Everything was rebuilt/replaced: new rod & main bearings, crank ground, new wrist pin bushings, 8060 head was resurfaced with new valves and guides, block was NOT surfaced, & copper coated the Felpro head gasket. Bearings were all plastigaged and piston and ring gaps were checked. I retorqued the head after first bringing it up to full operating temp and set valves to 0.017" I then took it on an extended tractor drive varying throttle along the way. I noticed a slight knocking sound when the engine was fully warmed up and rpm's were above 3/4. So I drove it at a lesser RPM where it didn't, thinking the valves may need adjustment. After reaching our destination and letting the tractor cool down, I started the tractor up and it no longer knocked at high RPMs, but again as the tractor reached normal operating temp the high RPM knock came back.
I proceeded to adjust the valves, varied the timing, changed to a colder spark plug, looked inside at the pistons, compression was all 130lbs. Nothing seemed unusual. I could turn the engine over easily by hand. Drive it around for 5 minutes and the knock would come back. I researched the web and determined the piston must be hitting the head. Thoughts were the dome is contacting the head chamber since I had it planed.

Upon tear down I found the pistons were contacting the head. (photos) Not the dome, but the word "front" was easily seen in the head on 3 of the 4 cylinders. The "front" and arrow extend above the flat portion of the piston by 0.015 to 0.020". Two of the three marks easily wiped off when cleaning off the carbon. #1 cylinder actually left a slight mark in the head. I filed the "front" and arrow from atop the pistons and ordered a new head gasket kit today.

I know aluminum piston expand when hot, which explains why it knocked only at operating temp. Also the Felpro head gasket is metal on one side and fiber on the other. A metal/fiber/metal may have provided just enough clearance.

Has anyone else experienced this issue? How much deck height clearance do I need?

photos

How much did you have the block deck planed?

Well you found the cause of the knock,which was what I suspected,but I would talk to my machine shop,if it was me.You may need pistons with different wrist pin location.I would look at piston height at TDC,or protrusion above the deck.I was thinking maybe .025 to .050 piston to head clearance,maybe more,but your machine shop ought to know.Mark

Block was not planed, only the head

since the head was shaved that looks like where the problem lies. and if .020 was machined off that might be enough to make the pistons hit the head. usually the amount milled off the head is stamped in a corner of it. and if thats the case you would need to look for another head if too much was milled off.

You might also be able to find a thicker head gasket.

For what it will cost, I would try Chevys fix for a high performance engine, and selling it as a low performance engine. Chevy sold the 409 with two head gaskets(Duel quad),the racers just removed one to raise the compression up(I think it was 10 to 1, but it has been a while)
I really would try two gaskets, unless you can find a thicker one somewhere.

Doesn't mater what was removed from head at contact spot. New and flat head will have the same clearance outside the chamber as the head on it. If the block was never decked the pistons were probably not made exactly right or the head gasket is thinner than originals.

I measured the gasket to be around 0.057-0.060 thick after I removed it. The pistons protrude 0.023 above the block so I have about 0.034" clearance now. Before the "front" was filed off I'd estimate 0.015-0.020 clearance is all.

Why not have that area of the pistons machined and remove like 100 thous in that area rest of the piston is OK correct

I vote to double gasket and enjoy or if not happy with that then mill the top of the pistons some. I think as the engine wears you will be back to the same problem if you don't get a bit more clearance than you have by filing the labeling off.

(quoted from post at 04:36:26 10/04/16) Why not have that area of the pistons machined and remove like 100 thous in that area rest of the piston is OK correct

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I would not recommend that, you would probably not be happy with how it runs after taking that much squish out the engine.

You should be ok at .030", I'll bet the top of the letters were less than .005" to the head when cold. You meantioned the crank was reground, I suppose there's a small chance the grinder may have slightly offset ground the journals a few thousandths. And then there's the chance that the pin height in the piston is off a little. Mind you everything could be in tolerance, but depending on which side of the tolerance it could add up and touch the head when it gets hot.

When I was in college we were tuning a little .61 cubic in two stroke glow plug engine to max power, we would check the squish or piston to head clearance and then lap the sleeve down till the clearance was 0". They ran really good after that, always picked up more trust.

I would agree with you, head should make no difference.

I would remove the front from the pistons and find a gasket that is at least .020 but not more than .030 thicker when compressed.

I am with the regrind of the crank shaft being the reason for your piston protrusion problem. A grinder will sometimes go with offsetting the throw a bit due to the wear pattern. No two head gaskets for me though. Rework the top of piston . I personally would dial indicator each piston to check for total length of stroke just for my own satisfaction. Might surprise you.

A lot of good suggestions here.

I would make sure the letters did not stick up past the top of pistons.
I would not take any off the pistons.
I would crank the pistons over center by hand, measure depth with dial, then push piston down, then re-measure to make sure you do not have slop in rod bearings.

More squish the better, so keep it as close as possible.

yes , i agree, i must have had a blonde moment. was thinking differently as i see where the piston was hitting.

Measured a used victor type gasket, metal on both sides at between .060 and .065 thick. Probably be close to what you have when torqued.

Ok to push piston down when checking but with new grind, new bearing I would surely hope clearances were checked and good. If you are hitting the head you have too much squish and taking some off piston to make them all alike and acceptable clearance to head is the way I would go. If you order a special head gasket, next time it comes apart, same problem. Two gaskets of that style with fire rings don't work well at all. Still would want to measure total stroke of each cylinder. I had an 806 diesel pull some crap like that after a reground crank shaft was installed. Considerable difference in stroke between cylinder. Had no trouble hitting head as they were not high enough. Had to have rods worked over to bring center to center holes on rods to as much as we could get. Timing marks were even wrong due to the regrind after a rod failure. Runs good now.

Why not remove a little of that side of the piston for clearence were not running a mega thous RPM. Tractor pullers did all kinds of tricks for pulling