Cracked rings and piston lands Why?

[Young1]

After performing a compression test cylinder 7 had 45 pounds and cylinder 6 145 pounds. All others are 185 to 195 pounds. Thus the reason for excessive blow by. Pulled one head, pulled 2 pistons and discovered cracked piston lands and rings. The head gasket was fine. On cylinder seven I could pull off two pieces of ring land if I tried. What could be the cause? The only thing I can think of is some fuel boil over at heat soak. This engine has probably only revved to 6000 RPMs 10 times in it's 14,000 mile life. 38° of timing all in at 2600 RPM.

[Tom Cimino]

Maybe the end gap on the rings was too tight and didn't allow for expansion when hot.

[scottj]

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

[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?

[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.

[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.

[blykins]

Detonation....

195 lbs pressure is quite a bit. What gas are you running?

[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.

[Wbulk]

X2 with blykins. Too high compression ratio for gas used. Advance to high for compression. It does not take high rpms to damage the engine under these conditions. With loud exhaust it makes it hard to hear it knocking. Since you have some pistons out measure or find the CCs of the heads and find or calculate piston valve relief CCs and piston height. Do some reading on Dynamic Compression ratio. If you have the cams specs you can calculate it here.
https://www.uempistons.com/index.php...bc6726aec853da
Generally, you want dynamic compression between 8 to 8.5.
Static compression does not take into consideration your cam, which is critical.

I have a friend that can't drive his Cobra on the street without race gas. Big mistake on the engine design. Good luck.

[Tommy]

Some years ago I also lost some pistons to suspected detonation. Following a rebuild with new pistons and to new specs, I installed a knock sensor that provides a visual indication of detonation, because its impossible for me to hear it. As I recall, it was an MSD product.

[CHANMADD]

Hey ....this is not a new problem. When they shifted the top ring to a higher ...hotter ...position on the piston, the ring gap needs to be around .035 inch. If you do not have enough gap the ring ends bump and the crack the top land. New pistons and gap them properly . Don't listen to any of the other crap.

[blykins]

Unless it's a Keith Black hypereutectic, that data is not correct.....

I buy pistons by the truck load, gap the rings at .0045/.005 per inch of bore and don't have this trouble.

To the OP:

If you pull the piston out and all the ring gaps are lined up, that's another sign of detonation. Also check out the upper rod bearings and lower main bearings. You can also have some piston/head clearance issues which will hammer on the ring lands.

[DanEC]

[quote=blykins;1358658]

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.

[blykins]

Young1, if you make no other changes, I would at least pull some timing out. However, maybe you can take some measurements and use a little thicker head gasket, or make some other corrections to help out.

Dan,

Not sure of the science behind it, but it's one of those things that has proven to be a tell-tale....

[Young1]

Just checked. Ring gaps are not lined up.

[DanEC]

Originally Posted by blykins

Dan,

Not sure of the science behind it, but it's one of those things that has proven to be a tell-tale....

Thanks - some things defy logic.

[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.

[Young1]

Looks like some tanks of 89 out on the road in the boonies got me. And probably some of the 91 that I put in is not 91 either. Shall I back timing off to 36 when I get it back together? Btw I have a 4150 holley with 72 primaries and a 65 power valve at sea level . Malloy Diz with no vac advance. Hey ya pay to play sometimes. Summer is the best time for cobra work here. Way too hot and too Rainey now.