4 out of 6 Piston Rings Broken
 

A few weeks ago we put the 911SC on a chassis dyno, and observed 158 HP at the wheels at 6,000 RPM. At 80,000 miles, and a life of mostly track events and getting to and from the track, this seemed good news.

But, since the car has been smoking through the pipes upon shut-down at the end of the straight (and at all major deceleration events, including on the dyno), and since there was minor oil leakage onto the heat exchangers, we decided to tear down the engine and investigate.

Upon removing the cylinders, we discovered that pistons 1,2,3, and 6 had broken top rings, mostly in three or more pieces. On piston #2, the land between top and middle rings had also broken, into a 2" long piece. There was also some carbon build-up on the cylinder side of the rings, indicating that the rings had broken some time ago.

Other observations:
1. There was medium carbon build-up on the piston tops, as well as inside the head combustion chamber, and on the valve faces. There was also some carbon on the bottom of the valves.
2. There was no scoring of the cylinder walls, at all, indicating that the broken pieces were just along for the ride.
3. The cam tower/head interface showed evidence of some blow-by on a few cylinders, as well as corrosion evidence of there not being a good seal between these two surfaces.
4. The studs were all in good shape, with no corrosion on any of them. All the studs were tight, none were broken.
5. The upper studs were magnetic, indicating steel. The lower studs were non-magnetic, indicating Dilivar.
6. All the parts, except for the broken rings, were in fine shape. No scoring anywhere - rocker arms, cams, cylinder walls, valve stems, etc were all in unscored shape.
7. The valve guides were worn; we were able to rock the valves in their guides.
8. The cylinders have 10 fins, and KS stampings, indicating Kobleschmidt, we think.

Questions (for a continuing stock engine):

1. How can an engine with 4 broken rings produce 158 HP ?
2. How can 4 broken rings not show any wear on the cylinder walls?
3. Are the cylinders reuseable, or should we get new Mahle 3.0 or 3.2 cylinders?
4. If the steel studs show no corrosion, should only the lower ones be replaced with 993 studs?
5. Obviously piston #2 needs to be replaced. Should all the others be replaced with new Mahle pistons?
6. What is causing the carbon build-up inside the combustion chamber, and on the backs of the valves? Have we been running too rich (the muffler tips show black, not brown).

Thanks for any help with these questions. Wayne, your answers will help us decide how to proceed. We've read your book cover to cover, but still have these questions.

Lots of questions. I'll defer to the experts. Reading Wayne's book will also answer most of them.

However, my 80K 3.0 had a broken oil ring on cyl. #5 when I took it down for head stud work. The motor also had excellent power, compression and used almost no oil. The cylinder wall was not marked in any way by the broken ring.

All I know is some of the causes. Mabe not yours...wrong install procedure,wrong rings, tapered cyl.s, wrong piston size (rock)and I am sure others will add a few to the list

Let me think about this - I'll write back tonite...

-Wayne

It's difficult to answer all, but if the engine is factory stock (not touched by German hands), the broken rings and piston land could be a symptom of past detonation damage.

You may not have sustained any cylinder wall damage because there was no longer any tension from the rings - as you say, after breaking they were just along for the ride. Carbon buildup is normal; depends on type of driiving.

MHO,
Sherwood

Indeed.
I have seen that kind of damage on m/c engines used for racing.
In fact, somewhere in the "Mechanical Mayhem Files", I have a nice shot of a Ducati 888 motor with severe damage to the pistons, due to detonation.

Tapered cylinders, bell shaped wear, intoducing a ridged formation at top of cylinder catching ring causing breaks?

My 87 930 (50 K miles) had broken compression rings on 4 of the cylinders and still ran very well. It just burned a lot of oil and also had a number of oil leaks. The cylinders were in excellent shape. This was caused by a failed wastegate, resulting in too much boost, which caused DETONATION.


By the way, the motor went back into the car tonight. Hook up a few oil lines tomorrow and test run!! :D

Andras. I'd agree that detonation is a common cause of cylinder top rings breaking.

Carbon buildup is highly normal.

Be sure to measure the heck out of the parts you intend to reuse, particularly pistons and cylinders. There is a long and excellent discussion of Alusil cylinder reconditioning and re-using on this Engine Rebuild Forum. Find it and spend the time to look it over. Yes, there is way too much technical information there, but your decision should be made WITH that information rather than WITHOUT it. I re-ringed my Alusil cylinders and the engine uses no oil and has beastly power. They had been run through a parts washer and were cleaned to a dull finish. DO NOT hone Alusil cylinders. Do not reuse pistons if the ring lands are not tight.

You're probably going to want to replace valve guides, seals and valves. Yes, split the case. It's quite easy to just pop new bearings in there. Replace the wrist pin bushings.

Again, measure each and every cylinder, about 1" from the top, and about 3" from the top. The most wear I found was around .0015" (miniscule). If your wear is not greater than this, then you have some good hope of successfully re-ringing them.

Re: 4 out of 6 Piston Rings Broken
 

1. Obviously, the other rings were holding compression, at least to achieve some HP. The 911SC engine is a medium compression engine - at higher compression you would have seen more losses

2. They could have just broken recently, or, more commonly, the rings did not disintegrate and break apart. That would have caused bits to scrape the walls. Instead it was like multiple ring gaps? When the engine is running too, the compression has a tendency to push the rings out against the walls and hold them in place. If you broke an oil ring (like the photo in the book), you would have seen a big smokescreen.

Your comments on the cam tower/head interface - Hmm, this is a sign of a poor rebuild, as this interface almost never leaks from factory assembled engines. You did mean the head/cam tower interface, right? Wait a sec, your description doesn't make sense to me, as there is no way for blow-by to be in this area. Perhaps you really meant the cylinder to head interface?

3. Spec the cylinders - if they are in spec, then reuse them. If not, then replace them.

4. Yes, replace only the lowers...

5. Double-check the cylinder to see if there is any wear

6. Carbon buildup = normal, see the photos in the book for a comparison.

Regarding pistons and cylinders, if I were you, I would buy new ones. Too many questions on these current ones to take the risk.

As for what made this happen, I agree that detonation is probably the culprit. If it were on only one cylinder, then there might be something else, but four broken rings across four cylinders? That says detonation across the engine.

Okay, that begs the question, why did this engine see so much detonation? There could be a few reasons, and it's probably a combination of many of these:

- Really bad gas.

- Someone rebuilt the engine and bumped the compression

- Excessive carbon buildup bumped up the compression (not likely, but possible). Also excessive carbon buildup can pre-ignite the fuel in the cylinder causing a diesel effect and pre-ignition. I'd say the carbon build up was probably a contributor to this.

- Ignition DISTRIBUTOR - this needs to be checked. It could be that your total advance is WAY off, and causing the timing to be too advanced, which will cause ignition before the cylinder has past TDC. In fact, I'd lay my $$$ on this as the culprit. That is why it is so important to check the total advance (Most people DON'T do this...)

- Someone may have rebuilt or hot-rodded the motor by increasing the compression

- On Turbo cars, you see this with too much boost and no knock control. Also SuperCharged cars...

Hope this helps,

Wayne

- Improper mixture - sometimes an improper mixture will cause pre-ignition, or

-

Thanks Superman,

I'm reading and re-reading Wayne's book whenever I get a chance, and sometimes just before sleep.

One of the things I might do (am looking into it now for cost), is to have EBS re-plate the Alusil cylinders with Nikasil, and then use new JE or Mahle pistons.

I don't think I'll split the case, since the engine has only 80,000 miles on it, and there is no wear whatsoever on the cylinders, the cams, the rockers, the wrist pins, and other parts that indicate I've always had clean oil (I change every 3,000 miles).


Other than that, I will replace everything that Wayne recommends (including the sheet metal screws), have the sheet metal and other stuff powedercoated in satin black, and perhaps do the fan, housing and strap, and the tensioner covers in clear. I think I'll keep all this looking stock, and then I'll have the only stock engine and automobile in all of history (ha ha ha ha ha ha ha). Aren't we a sick bunch of guys!!!!!

I'm currently getting quotes from a number of sources, and hope to make decisions soon. In the meantime, the engine is all apart, the parts are in baggies large and small, and I'm getting nervous.

Sigh, there is no light at the end of the tunnel yet, but soon..........

Replace your rod bearings too. If your rings took a beating from detonation, I can almost guarantee you that the rod bearings are in trouble too.

-Wayne

Thanks Wayne,

Everything you say makes sense. I will check everything, of course, and proceed with your advice.

By the way, I've owned this car since new, ordered it for Euro delivery, which Paule and I took delivery in October 1982. So I know what's been done, and it's never been apart.

I believe you may be on to something with the ignition advance, etc.

Also, the sports muffler (which I use all the time, expecially at the track), shows very dark black, not brown or tan insides at the tips. Perhaps I've been running too rich. Could this account for the strength of the engine (158 HP on the chassis dyno just before tear-down)?

Interesting to note: I read somewhere that the Honda engineers who designed their 1.5 liter turbo F1 engines (900 hp) back in the 80's once experimented with pistons w/o rings. They reasoned with enough boost and a close-fitting piston, they could generate enough HP as well as reduce a major source of frictional loss. They discovered too much blowby and gave up on the idea. Still, always thinking. I haven't seen a current F1 piston, but I'm pretty sure they try to reduce as much friction in this area as much as possible to wring out 19,000 rpm.

Sherwood

While my money is on detonation one other cause that has not been mentioned here is severe overheating.

Its possible for a severely overheated motor to break the top rings when they get hot enough to close their endgap and bind.

Very interesting, Sherwood. I had never heard this but I am interested in those engines. Any idea where you might have read that?

Hi Andy,
Not sure. It might have been on one of the F1 forums on the net. There's only about 15 of them. The couple I frequent are:

http://f1.racing-live.com/en/index.html?
http://www.speedtv.com/index.php

BTW, if the engine suffered from overheating, I think there would be other symptoms of this (e.g. scored cylinder walls or piston skirts). Usually what happens is that at elevated temps, the piston expands much faster than the cylinder; this in conjunction with a borderline oil film between the sliding parts. But no amount of great oil can keep parts separated when they're essentially the same size (no clearance).

Hope this helps,
Sherwood

PS: fixed some spelling, 6/7

siezed piston (s) will do this of course

Thanks, Sherwood!

thanks for the compliment