OK, so today I thought I'd ask a question for a change... but not before setting the stage:

I completed the disassembly my 97 Northstar engine today with the hopes of getting by with a head rebuild. Things looked pretty good even before I started to take it apart... the cylinders passed compression testing with a consistent 200 - 195 psi. After getting the heads off, I found the cylinders were within 0.001" of being out-of-round and tapered, and this with only 0.001" wear on the cylinder liners... well within the 0.004" margin. I wasn't convinced I should tear into any further until I read a few threads on smoking, oil sucking warmed-over Northstars here on Pennocks. Anxiety over "not-having-gone-far-enough-when-I-had-the-chance" got the better of me so I decided to complete the teardown. Good thing I did. Check out what awaited me on pistons 2, 4, and 6... all three the same! Anyone seen this before? Is this caused by detonation or by ring seizure?

how carboned up where the rings?

There's no noticeable build up on the rings. I should point out the other five pistons weren't anywhere near that fouled up. I suspect that the pistons with the broken lands are carboned up from the blow-by. It doesn't appear as though the engine was run very long in this state, otherwise the fracture sites would probably have been more smudged.

I haven't seen that before, but I do have a set of coated stock pistons (actually, one early set and one late set).

Email me if you're interested.

I haven't either and I have taken a few N*s apart with failed ( broken ) or carboned up rings ..I am wondering if its just a bad batch but it is weird they would all break in the same spot

I would lean toward not enough end gap in the rings during assembly.

Steve

I suppose it's a possibility, but bear in mind this is a '97 engine with over 100K miles on it. It didn't look like it had ever been apart and so I doubt it would've lasted that long with improper ring gaps from the factory. Tight ring gaps would've also likely scored the cylinder walls, but all the cylinders are evenly worn.

I would say cast piston + to many RPM by by ring lands i have seen it before not on N* but SBC you are a lucky man when i did it it took the whole engine with it .Oh look its got an oil fire in the Headers

It may have been detonation although from my experience in a 3.4L I would have expected the top of the piston to have been chipped from it. The significant clue to me is that the damage is consistant across one bank which was mainly the case in my engine although four out of six pistons were affected the bank closest to the firewall (even numbered pistons) were affected most showing the greatest amount of damage to the cylinder bore from broken rings and also subject to the greatest amount of heat due to their location in the engine compartment. No ring lands were broken though.

[This message has been edited by Joseph Upson (edited 09-04-2009).]