Symptoms of an upside down ring?

[shoedoos]

Can anybody explain the symptoms of a ring installed upside down?

BBC is low on compression in one cylinder. 180/185 on the others, 70 on the low one. Squirted oil down low cylinder and came up with the same result. Pumped 50 first hit then climbed to 70 over the next 4 gulps.

Leakdown done. Valves are good, hissing madly from the crankcase. Don't have a percentage - just plugged the air hose straight into the cylinder at 60psi.

Engine runs fine, idles cleanly (284 duration cam/850 rpm)......followed the car and watched for any oil smoke....nothing. It certainly hazes under the car from blowby when under the throttle (PCV in one cover/breather in the other). It looked more like fumes and when wiping residue from the valve cover there wasn't a lot of oil, in fact it was hardly wet. Hood was off and there was no oil on the windshield after taking it for a run. Give the engine a rev in neutral and it breathes (not smoke - more like fumes), but that goes away when back at idle.

Plug was not oiled up, it did look like it had been fuel washed though (sorry no pic)......idle was clean and the thing revved without a misfire.

So? it it were a broken ring I would think the thing would smoke? If it were a scratched bore - I would think it would smoke? So what else could this be? Upside down ring? Stuck ring?

Any ideas would be welcome....

[modok]

If a tapered face second ring is wrong it will burn oil, but I'd not expect much blow-by
The other rings it may make very little difference, or maybe some, again depends on the design of the ring.

[Kevin Johnson]

Borescope would let you examine the cylinder bore surface through the spark plug hole.

[shoedoos]

I had one of those ebay cell phone cameras down a couple of bores earlier (not the affected cylinder) and it still showed a cross hatch. Those cheapie borescopes aren't the best for detail unfortunately. Will get it back to see if there are any scratches....

I find it strange that just one cylinder is wounded...and it doesn't blow smoke.....

[user-23911]

You're not going to fix it without pulling the piston.
Then you'll know.

[shoedoos]

so I can rule out a gummed up ring that could unstick with a bit of magic juice down the bore?

[user-23911]

You haven't said whether it's a new build or it's got 500,000 miles on it?

[shoedoos]

car was recently purchased without knowledge of its history. Judging from the grime buildup on the oil pan, it certainly isn't a fresh engine.....

[bigmike]

You really need to look at cylinder with a bore scope. Many years ago one of the local racers had a wrist pin leave two deep grooves in the cylinder wall on a BBC 4 bolt. The piston floating pin clip came out at some point. That one cylinder was really down on compression and did not respond to adding oil in that cylinder.

[user-17438]

car was recently purchased without knowledge of its history. Judging from the grime buildup on the oil pan, it certainly isn't a fresh engine.....

Well then, you don't know if at some time the engine lost a rod bearing and smacked the head, pinching the bearing. And they just threw a rod bearing in it with a pinch ring.

If it bothers you, just pull it apart and inspect it, if you are content, drive it.

[tresi]

I had one of those ebay cell phone cameras down a couple of bores earlier (not the affected cylinder) and it still showed a cross hatch. Those cheapie borescopes aren't the best for detail unfortunately. Will get it back to see if there are any scratches....

I find it strange that just one cylinder is wounded...and it doesn't blow smoke.....

To save anybody else from getting ripped off the E-bay borescope I bought kinda works with a laptop but not at all with a phone

[engineguyBill]

car was recently purchased without knowledge of its history. Judging from the grime buildup on the oil pan, it certainly isn't a fresh engine.....

That makes it very hard to diagnose the problem, looks like it is time to disassemble the engine for examination. As far as the ring being upside down, most high performance ring sets use a barrel-faced top ring therefore there is no installation direction necessary. However, most ring sets use a taper-face second ring and if this ring is installed upside down it will pump oil up into the combustion chamber and should be very noticeable when you read the spark plugs.

[shoedoos]

From what I saw with the borescope on that earlier investigation, the pistons look like old TRWs with the smaller dome. Pretty sure that would mean they have 5/64 compression rings. Looking at other engine componentry, I doubt the original builder spent money on a set of "flash" rings....

If the top ring was installed bevel-down would it allow compression past on the way up but stop oil getting past on the way down?

[engineguyBill]

Yes, if the top rings have a bevel machined in the ID of the ring, the bevel should go towards the top of the cylinder. The bevel allows some combustion pressure behind the ring, mainly to enhance the seal between the lateral surface of the ring and the sides of the ring groove.

[pamotorman]

my guess the compression ring is broken.