TR4/4A Performance Cam=Broken Rings???

[karls59tr]

Some Club members here disassembled a TR4 motor that was running but was down on power and blowing some smoke. They found that the all the top compression rings were broken on all four pistons. The engine was stock except for an M766 performance cam. I don't know how hard the car was driven by the PO but I was wondering if the race cam, without any other modifications to the engine. may have contributed to the broken rings and if so what was going on there that would cause the damage ie. valves misadjusted or timing wrong etc. or not related to cam?

 

[CJD]

There are 2 possibilities.

1) Engine sat for an extended time, so the rings rusted to the cylinder and broke when somebody broke them loose.

2) Severe detonation...although there are normally signs of damage to the piston crowns if this is the case.

A radical performance cam has more overlap between intake and exhaust strokes, which tends to reduce the effective compression ratio. Therefore high perf cams tend to reduce detonation...so I would say "no", the cam had nothing to do with the broken rings. Advanced timing or low octane gas would.

Edit...just a thought, there is a 3rd possibility. The rings could have broken when they were removed from the cylinder and caught on the ridge at the top. Or...they could have been broken during installation. Both of these would imply mis-handling during assembly/disassembly.

 

[bnw]

Curious. How about excessive taper in the bore?

 

[CJD]

Curious. How about excessive taper in the bore?

Then it was thouroughy worn out and needs a new piston/sleeve set anyway. Just curious...have you seen a tapered bore break rings? I've rebuilt really worn out POS's, but have never pulled out broken rings, even though some had .050" ridges in the bores.

My old man's Chevy is the only engine I've rebuilt with broken rings across the board. He would use crap gas and lug it at low speed so the poor thing rattled like an SOB for minutes at a time. When he gave it to me it burned 8 qts of oil per tank.

 

[trrdster2000]

One thought here. If the installer did not cut the ridge off the top of the sleeve, the first time he turned the engine over is was finished. The old rings had worn along with the sleeve and were a little rounded out at the top. The new ones with sharp edges didn't take but a couple of times at piston speed to not being able to occupy a square peg in a round hole.


Wayne

 

[bnw]

I've seen broken rings on 2 stroke Yamahas that were bored out incorrectly (punched too big a hole). The piston would rock back and forth at the top of its stroke, rattle like crazy, and eventually break the rings. 8000 rpm red line. Totally different animal.

 

[sp53]

Could they break if someone ran the engine at high rpm then down shifted a lot and the vacuum pulled the rings away on an old engine with worn out sleeves?

 

[glemon]

My Healey 100 had extremely ridged bores, and many broken rings when I tore it down. Though it blew a little smoke, it started an ran remarkably well considering the condition. Head turned out to have multiple cracks too.

 

[karls59tr]

My Healey 100 had extremely ridged bores, and many broken rings when I tore it down. Though it blew a little smoke, it started an ran remarkably well considering the condition. Head turned out to have multiple cracks too.

When you tore the motor down do you remember how you removed the pistons? Do you think the rings were broken on removal or they were already broken? Apparently the motor I mentioned was blowing some smoke but was still very driveable as well before teardown.

 

[glemon]

Although some time has passed I know I had the motor out and on a stand, and pretty sure I pulled them out through the bottom of the block. I know the compression was pretty low across the board on the motor before the rebuild, under 100 psi in each cylinder.