COLLAPSED PISTON

[KJMac]

Call MadDog's 800 number, ask for extension 113 and ask to talk to D.W. He's their AMC guy.

Looks like the block was bored without enough piston/wall clearance. Looks like the piston expanded and galled itself on the cylinder wall. There should have been a spec sheet that came with the pistons that should have had the piston diameter specs, and the necessary clearances. Your machinist should have worked from that to determine final bore diameter, not just a standard .030" over stock.

I have Ross pistons from Mad Dog in my 69 390. They are a high quality piece.

Bob
tufcj

The piston to cylinder wall clearance was set up too tight!!!

[82Waggy]

Yup, I think it was too tight myself - maybe some dirt.

Most non-AMC builders do not have a torque plate. I had to custom order one from BHJ for my two projects.

I also have considerable experience with the cold seizure issue on small engines and your piston does look just like that problem. Skirts start to stick first as the piston head stays cooler from the intake charge.

Most people also do not realize that you need to let a forged piston engine warm up good before you lay on the power - otherwise the block does not expand as quickly as the piston.

It also looks like you were running rich to have that much soot build up already.

[1980_Cj7]

Yes, agreed that we were running rich. We were working on carb tuning and plug heat ranges, with Fuzz as a matter of fact, trying to get that straightened out, when we had to quit due to the knock showing up.

Hmmm, maybe I'm on to something here. I wonder if Fuzz sabotaged our engine so we wouldn't blow his doors off??? No, wait a minute, that couldn't be...he ain't go no doors! Some ricer already must have blown them off.

[Goose]

Well I hope ya get it figure out.. and poke the holes looser(Sorry couldnt resist) cause as we all know theres chevys and fords and even a few mopars out there but none of them sound like an AMC doin what its best at..(Sorry here you have to imagine a Trosley drawn AMX and a CJ smokin off into the sunset goin "WAAAAAAAH"




(To anyone who doesnt know who George Trosley is that last deal wont make a bit of sense).

[1980_Cj7]

Yes, ours did sound REALLY good, the short time it was running. Even Fuzz commented on how good it sounded, and I consider that a real compliment coming from him.

[AMX69PHATTY]

Yep, a torque plate insures cylinder roundness.
When the head bolts are torqued they cause some distortion in the cylinder walls.
So cylinders honed round without a torque plate can be out of round when heads are installed.
Using a torque plate on the block while honing prevents this by introducing
the same stresses in the block as will be there when the heads are torqued down.
I've been told machine shops hate AMC Blocks because they wear out there boring tools
due to the fact that AMC blocks have high nickel content and are very hard.

[1980_Cj7]

I have heard the same thing about the AMC blocks being hard and wearing real good. I hope it is true and our No. 7 cylinder isn't ruined.

I just came in from checking, and the oil holes in the rod caps are all to the inside.

[Holeshot]

why don't you take the rings off another piston & stick it in it's bore & see how much clearance ya have so we can all take another breath?????

[1980_Cj7]

I just went back and re-read all your posts and info..If that Galling is on the" top side" (I'm guessing toward the intake manifold) then the odds that this was caused by Detonation/crossed wires or anything to do with the power stroke is small. since that would normally appear on the bottom side of the skirt..(IF you think about it a minute the explosion forces the piston down.) so my guess would be it is a clearance problem or oiling..
Especially when you take into account it happened in 300 miles.

Good point. And yes, the wear is on the top or intake side.

Should be getting some measurements tonite.

[1980_Cj7]

OK, the engine builder made a house call tonite. Just before he arrived, we pulled piston No. 1. It was starting to show signs of wear in the same place as No. 7. The builder was really stumped. Said he never saw anything like this before. He said the wear is always on the bottom side of the cylinder, and at the very ends of the piston skirts. He said the pistons are tapered, and showed us that the pistons measure .004 bigger diameter at the ends of the skirts, than they do in the middle. He demonstrated this on the piston that wasn't worn.

He measured the cylinders, and said they are in spec at .004 over the piston size. Luckily the cylinders, even the worst one, No. 7, didn't measure out of round, even where the piston was rubbing.

He said he didn't use a torque plate, but that with engines this heavy it wasn't necessary. He claimed you only really need them on thin-walled engines like 302's.

So, while he was here, we pulled the other two pistons, and guess what, they were worn just like No. 7, but hadn't progressed as far. So, we thought there might be a pattern to the wear, getting worse the farther back the engine you went. That didn't hold out though, the order of best to worst was 1, 5, 3, 7.

The builder looked at everything, head, valves, lifters, pistons, rods, bearings, etc., and said that everything looked fine and normal except for it running rich and being carboned up. I mentioned that some of you noticed the discoloration on the small end of the rod. He looked at it and said that was normal and was from their rod oven, and that if it was from overheating, the pin would show discoloration too.

So, he, and we, are basically stumped. He recommended we pull the other side apart and see what we find there. I asked if he thought Ross might be able to provide some insight, and he said we should send them pics of the pistons and see what they think. Said we might even want to send them some pistons to check out.

The pistons do have offset pins by the way, but with the way the valve pockets are cut, it would appear they can only go in one way. Or, could they be switched from side to side??? I have to think about that one a little.

I'll get some more pics of the wear on all 4 pistons and post it, but right now I'm too ed to look at it any more tonite. The only good news is that the block is salvageable...at least so far. We haven't looked at the other half yet. There no sense putting anything back together though until we figure out what caused it, and what to do to keep it from happening again right away.

The builder didn't have anything along to measure the deck height. I tried my best to measure it with my digital caliper, and was getting less than .045. What effect would that have, other than valve and piston interference, which there is no indication of. The builder mentioned something about quench, and less deck height giving more power and allowing more timing advance, if I understood him right.