COLLAPSED PISTON

[82Waggy]

The piston does not appear to be galled above the oil ring, and the wrist pin end of the rod looks to be discolored from heat.

What does the top of the piston look like?

I'd bet it got hot and expanded too much for the bore clearance. The top of the piston probably stayed a little cooler from the intake charge.

[tufcj]

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

[1980_Cj7]

The top face of the piston looks fine, and just like the rest of the pistons.

Engine was definitely not overheated, and was shut down when the knock started. We paid particular attention to cooling and upgraded to a FlowCooler water pump, used a 165 degree thermostat, and installed an electric fan with an adjustable controller.

Yes, the detailed multi-page instruction and inspection booklet that came with the pistons went along with them to the machine shop. Looks like they looked at them because they came back with some grease on them. I was quite impressed with the Ross pistons when we got them by the amount of inspections they are subjected to. I'm no piston expert or anything, but they appeared to be good quality.

A question for you more experienced and knowledgeable engine builders, just how do Ross pistons rank among other brands? There isn't a whole lot of choices out there for AMC's, especially in the compression ratio range we were shooting for.

[82Waggy]

You can get locallized overheating on just one piston for a short time without overheating the whole engine.

Ross pistons should be fine. Other custom makers include JE and Venolia, among others. Tolerances should have been checked for bore clearance at assembly - matching each pistons to its' designated bore.

I'd say this one either had a bore that was a bit too tight, was dirty, or overheated due to a localized problem.

Your previous posts talked about the amount of advance you were running - this may have been too much for ole #7. Not all cylinders fire exactly the same so you must set the maximum advance not to exceed the most detonation prone cylinder under worst case loading conditions.

For the compression you are running and the use of pump gas, I would say you were in excess on timing. A 14deg base setting is usually plenty for most street engines, assuming a proper mechanical curve and vacuum properly limited so as not to cause detonation at part throttle loads.

With vacuum advance reconnected on a 20degree base setting, advance at cruise loads (off idle) may well have been too high.

I think you said you had the vac advance hooked to ported vacuum on the carb. If the carb was adjusted properly, you should have had no additional vac advance added to the base setting at idle. As soon as you touch the pedal advance would go as high as the can would allow and then only decrease as vacuum decreased to wide open throttle.

Of course, at low vac wide open throttle conditions vac advance is at its' lowest and mechanical advance is all there is.

10:1 static compression is usually consider high for pump gas and iron heads as well, but detonation is more dependent upon dynamic compression (when the intake valve closes on the upstroke and compression actually starts building). If you happen to be running a cam that is intended to increase dynamic compression in engines with low static compression (intake valve closes earlier), cylinder pressure could go through the roof.

[1980_Cj7]

We first set the timing at 10 degrees, and the old tired 258 would have run circles around it at that setting, so we advanced it to 15 degrees, and that is the highest base advance the engine was run at. At 15 degrees base, we were getting 35 total at 3000 RPM's.

We still didn't feel we were getting the power we should out of the engine, and talked to the machine shop/engine builder. They told us to set the timing with a vacuum gauge. They said to advance the distributor til we got the maximum vacuum reading, then back it off 2". They said this was the optimum timing for an engine.

When we tried that method. We got 17" of vacuum, and backed it off to 15", then checked what we had with the timing light and got 20 degrees. The engine wasn't run at that setting though other than idling long enough to check the timing. It just seemed like way too much timing.

The engine did turn over normally though at 20 degrees, and started right up, whereas at even 15, it struggled to turn over, but did start quickly.

[82Waggy]

Well,

Maybe timing was not a factor, but it seems the total advance with vac connected was not known and may have been too much at part throttle high load conditions. I think the stock distributors were curved for a base timing of around 10 (+-2) degrees.

Anyway, will be interested to hear what the piston to wall clearance was on this cylinder.

Are the other rods discolored at the wrist pin like this one, and are they pressed pins or full floating?

[1980_Cj7]

I'm not sure No. 7 is even discolored. I'll have to look at it closer tonite. Might just be the picture. Didn't really look at any of the others either.

The pins are "pressed in", not floated, but the shop has a rod oven and heats the rods so the pins "drop in". Maybe that caused the discoloration if it is truly discolored, but I wouldn't think they'd have to heat them that much to expand them to get the pins in.

I did notice that the pin on No. 7 isn't exactly centered side to side in the rod, but don't know that that would make any difference. The pin is still well recessed within the piston with the rod centered on the piston.

[jeepsr4ever]

Yes the discoloration is from the heat they used for getting the pin in there. Was their 4 areas on the piston that showed scoring? Do you have any pics?

[tufcj]

What does the top of the piston look like? You could have had some serious detonation causing the piston to bounce around in the bore.

Bob
tufcj

[1980_Cj7]

Looked at the small end of the No. 7 rod again. It is slightly discolored, but I think I see why. Looks like the shop did some grinding on it, probably as part of the balancing.

Here's some more pics. The top of the piston didn't look any different from any of the others when we pulled the head. It looks different now because we were wiping at it. The blackness comes right off and the piston is perfectly smooth underneath, not pitted or anything. See that one real shiny spot where I picked some away with my nail.

I don't know if the pistons are carboned up from the shop running it with the plug wires pulled, or from us experimenting with the carb tuning. Somebody mentioned it was maybe leaning out, but the tailpipe is all sooted up, so I think to the contrary it was running rich if anything. We had just switched to some colder plugs too right before this happened, and when we pulled them they were all black and carboned up, so I think that was a move in the wrong direction.

And the piston is only scored on the upper skirt side. Here's a pic of the opposite skirt.