65air_coupe

I was taking an excursion up to 5 grand or so the other day and after coming to a stop, noticed smoke rising from under hood! Never a good thing, I pulled over and shut it down. Had the fire extinquisher handy in case it decided to flare up when I opened the hood but all I found was an engine compartment covered in hot engine oil. I determined the PCV valve had blown our of it's grommet so I checked the level and since it was down less than a full quart, continued home where I cleaned it the mess.

Next step was to determine why. I quickly found that I had a bit of crankcase pressure...also rarely a good thing! A quick compression check found a cylinder at 140psi, about 100 down from the others. That usually means a broken ring so I ran a leakdown test that pretty much confirmed my suspicion by hearing the sound of air hissing in the oil fill tube.

I started tearing down that side all the while hoping that I'd find little damage to the cylinder wall. A quick inspection before pulling that pistion showed two visible lines but no edge to them - so far so good. As I pulled the pistion out, I expected to have pieces of ring fall out of it only to find all rings were still intact! Strange!

Then I noticed something....

That subtle change in color separated by a line between the top and middle rings....what is that anyway?

It's this....


After removing the top two rings...


My question now is what caused this failure? Defective part? Improper assembly? Excessive cylinder pressure (260+psi)? Or perhaps even detonation? I'm running 10.7:1 CR with aluminum heads and 93 octance fuel and have never heard any pinging. Total advance is about 34 degrees.

They're Speed Pro hypereutectic pistons in a 406 sbc running 10.7:1 compression.

IGO200

The damaged portion of the piston with the broken lands looks like classic detonation...
BUT, what looks like zero damage to the piston crown and top land makes me think that's not possible. Truly a puzzle. Take a photo of the crown.

pewter-FRC

260 cranking compression?? you need to be running race gas. my race 11-1 385 sbc only cranks 200.

cardo0

Sorry to read of your loss. Well AFAIK 220psi is max for pump gas. And even forged piston can break from detonation.
To get an accurate c.r. measurement u really need to use liquid. Calculating c.r. with mfr spec numbers is guessing and not used for blueprinting at all.

DansYellow66

How the heck are you getting 260 psi cranking pressure with what look to be flat top pistons?

For pressures like that you should be running forged pistons.

I agree with detonation as a cause.

Pop Chevy

Hypereutetic pistons do NOT like detonation as you've found out.

Frankie the Fink

I was wondering that myself

427Hotrod

What are ring gaps? As mentioned...Hyper's don't like detonation at all....that's why I don't use them in anything I'm going to lean on.

It's possible rings stuck and then got ring land...you have to figure all that pressure is pushing on bottom of ring land.

260 PSI is huge...even for a "compression" motor. What cam is in it? Awul hard to get that with 10.7 ratio......the cam would have to be tiny or severely advanced.

JIM

65air_coupe

Here is the piston in question...


And here's the rest of them....

mrg

Another possibility:
https://www.hughesengines.com/TechArticles/2clearancerequirementsforhypereutecticpi stons.php

65air_coupe

End gaps were 0.016", checked each and every one. Re-checked that cylinder now and still the same.

As for those cranking pressures, no one was more surprised than me at those numbers. I refused to believe them at first and tried two other gauges besides my own but even now I still don't quite accept them as completely accurate. First off, my original, never rebuilt 327 starter would spin the motor cold without a problem. And secondly, getting to a final number was different than other motors I've compression tested. IIRC most reach their peak reading in about 3-4 compression cycles but this engine continues to pump up the gauge for many more cycles than that. It acts more like a air compressor in that regard so when do you stop?

As for static compression ratio, (10.68:1) the only two numbers not verified was the head volume and the valve reliefs in the pistons. Heads are listed as 64cc and pistons as 7cc which seem reasonable but they can be verified if necessary. And I suppose I could mic the head gaskets now that they've been compressed.

I listened very carefully for pinging/detonation in the first few thousand miles, even lugging the engine slightly and never heard a thing. Perhaps I need a knock sensor?

427Hotrod

.004" per inch of bore sounds tight for hypers. Especially top ring. Look at ends of rings and see if they are shiny.

Jim

Mike Smith

Is it possible for a broken exhaust valve spring or damage to the corresponding push rod / cam lobe to cause excessive cylinder pressure?

cardo0

Not making a debate of what u had. I know myself how a setback like that hurts and would like to help. Just think u need more accurate method of measuring cyl volumes for blueprinting. It looks like your in for a full rebuild. Now that engine is out its easy to cut a piece of plexiglass and drill a hole in it to add measured liguid (alcohol works well), just make sure the cyl is at TDC and seal the piston creavace volume sealed w/grease. BTW u need an accurate TDC for this also.

FYI a lot of compression stokes (w/carb throttles blades tied open) can mean leaky vlvs - 5 strokes should be well enough and not more than 10 strokes. And a serious overbore can bump compression too.

65air_coupe

Bingo! Both first and second rings have matching shiny spots on their ends. And crap too! Now i have tear the engine down completely and re-gap them all I guess I'm pulling the motor now rather than attempt this with the engine in place.

Thanks Jim!

65air_coupe

New motor, all cyls have equal pressures.

Pop Chevy

Can't tell by the pics , but is there any chance the piston hit the head ? What is your deck height ?

mrg

Similar to Speed Pros, I used KB hypers in my 327. The KB instruction sheet general clearance guidelines show ring end gap factors that vary, depending on engine application. For a street engine, it says to use a clearance factor of .0065 thou per inch of bore for the top ring. Second ring shows .004 end gap factor per inch.

https://www.uempistons.com/installat...stallation.pdf


The top rings were filed to achieve a .026 end gap clearance. The engine now has about 14K miles on it since rebuild with the hypers. ..
John

65air_coupe

Good info, thanks!

65air_coupe

0.025 -verified.