Cracked rings and piston lands Why?

[Gaz64]

Originally Posted by Barnsnake

I'd guess detonation (knocking/pinging). It's the functional equivalent of hitting the piston crown with a hammer. I've seen it break top ring lands and knock piston skirts off.

Exactly correct.

You can't hear normal combustion because it's a controlled explosion with "burn rate".

If you can hear detonation, imagine how hard it is hitting the piston, and the valve heads, combustion chamber, cylinder walls, wrist pins, rods, and bottom end bearings.

[Topfuelhydro]

[quote=DanEC;1358708]

Originally Posted by blykins

To the OP:

If you pull the piston out and all the ring gaps are lined up, that's another sign of detonation. QUOTE]

Brent - curious, how does detonation cause the ring gaps to all line up? I know rings move around but trying to figure out in my mind the connection between the two.

My guess would be its not necessarily detonation lining up the ring gaps but excessively high cylinder pressures which you would get with detonation. I know on a blown nitro engine after a 3-4 second run almost all the ring gaps will be lined up when I pull the engine apart. I have even ran them on the trailer and had to take them back apart for another reason and several of the ring gaps would be lined up after just a few minutes of idle time. These engines have an insane amount of cylinder pressure and are on the verge of detonation all the time.

Steven

[DanEC]

Well, we've established before that piston rings do rotate around during engine operation. With a blown engine like that I can almost imagine that cylinder pressure bleeding through the ring gaps and seeking the path of least resistance, tends to stabilize the ring gaps in a row after they coincidently line up.

Not sure I explained that very clearly.

[zrayr]

Originally Posted by scottj

Mild detonation over a long period of time can cause this.

My vote too. It can be hard to hear when slight.

Z

[Young1]

All is back together now. Southern Automotive was a blessing to work with. They do business the old way. Send you parts then send you an invoice later. Nobody operates on trust like any more. But can anyone answer me why I had 15 Hg vacuum with two very weak cylinders and now 10 Hg vacuum with all eight hitting correctly?

[RET_COP]

If all else was the same as before, are you taking the reading from the same port and is the idle speed the same as before?
Maybe you have a slight vacuum leak now.

[DanEC]

Besides a vacuum leak it could be due to slower idle speed or less initial timing. I assume you don't have vacuum advance on the distributor? Is the cam advanced or retarded differently than before?

However 5 inches less vacuum is quite a lot of difference.

[olddog]

Originally Posted by Young1

All is back together now. Southern Automotive was a blessing to work with. They do business the old way. Send you parts then send you an invoice later. Nobody operates on trust like any more. But can anyone answer me why I had 15 Hg vacuum with two very weak cylinders and now 10 Hg vacuum with all eight hitting correctly?

Work with the builder. You need to understand why, before operating further. It may be for a reason that is ok, but it could also be an indicator of a problem that could kill your engine again.

[CHANMADD]

Those Keith Blacks did the same to me years ago. I trusted a shop to do the short block.( never again).He never gapped the rings properly which cracked the top of the piston. Did it myself with proper gaps.....I thrash this motor it seas 8000rpm often.....

[Young1]

I had forgotten I had taken 2° of initial timing out to resolve my hot start problem. Probably the idle was a little slow too since the oil was not hot. Made some adjustments and now it is 14 ish Hg All good. Thanks for the advice. I am ready to cruise on Friday with the NPD gang here in Ocala this weekend

[DanEC]

Originally Posted by Young1

I had forgotten I had taken 2° of initial timing out to resolve my hot start problem. Probably the idle was a little slow too since the oil was not hot. Made some adjustments and now it is 14 ish Hg All good. Thanks for the advice. I am ready to cruise on Friday with the NPD gang here in Ocala this weekend

Good deal - I wouldn't e surprised that with a little more fiddling with the idle mixture screws and so forth that it comes back up to 15.

[RET_COP]

It's not the idle mixture screes that directly effect the vacuum reading it's the idle speed. Clean up the idle with the screws and then set your idle to therpm you want. Vacuum reading should be taken at the same rpm to compare otherwise you'll have different readings