Thoughts about root cause?

[Walt Fricke]

For the IS gear mesh - on reflection, maybe it was the steel crank gear I flipped to move it more into mesh. In any event, the chain gears should be centered in the openings into the housings, and the IS shaft does that no matter what - or should.

[AlfonsoR]

Originally Posted by Walt Fricke

The 70.4 crank is not, I think, quite as stout as the 66. A friend had the crank break on his '74 911S some years back. They track the car, but there was nothing special one could point to as the cause. I think they, and those of us kibitzing, thought a chain tensioner had failed, so suggested they take it easy driving home from a track. They got home all right, broken crank and all.

I once drove a VW with a broken crank for three hundred or so miles. After the first appearance of some problem I saw that the crank pulley was broken. A welding shop along the way fixed that. A couple of hundred miles later it broke again and it was time to stop.

I'd be inclined to blame the chain failing due to improper installation for this, though you can probably tell from the fracture surfaces whether a crack had developed and was getting bigger, or whether it all cracked at once. The older surfaces would be a bit smoother (unlikely to be rusted or discolored given that they live in an oil bath), and would have a semi circular tree ring like pattern as the crack grew larger. That all ends with the final fracture, which is of a more uniform appearance.

Cranks breaking aren't all that common, but that is not unheard of either.

Walt, one of these days, I will go to Colorado and buy you a beer to gladly hear more of this VW broken crank road trip.

Safe, at this point, in what direction is your friend heading with regards to getting this 911 back on the road where it belongs? Just curious.

[Flat6pac]

Have you removed the cams, as I have seen that type of damage when a cam freezes in the cam carrier
Bruce

[safe]

Originally Posted by Flat6pac

Have you removed the cams, as I have seen that type of damage when a cam freezes in the cam carrier
Bruce

Yes, they were just fine.
In fact all things have been properly lubricated, but the crank seemed to bind, but that could be because of it being broken, maybe.

[safe]

Originally Posted by AlfonsoR

Safe, at this point, in what direction is your friend heading with regards to getting this 911 back on the road where it belongs? Just curious.

We are getting the case back to the builder for a check up, he said it was fine when he assembled it.
If it checks out we'll look for a new crank and "chain parts". Otherwise I thinkk the best thing is to look for an SC engine and move over the MFI, S-cams and SSI.

[safe]

Originally Posted by Walt Fricke

The 70.4 crank is not, I think, quite as stout as the 66. A friend had the crank break on his '74 911S some years back. They track the car, but there was nothing special one could point to as the cause. I think they, and those of us kibitzing, thought a chain tensioner had failed, so suggested they take it easy driving home from a track. They got home all right, broken crank and all.

I once drove a VW with a broken crank for three hundred or so miles. After the first appearance of some problem I saw that the crank pulley was broken. A welding shop along the way fixed that. A couple of hundred miles later it broke again and it was time to stop.

I'd be inclined to blame the chain failing due to improper installation for this, though you can probably tell from the fracture surfaces whether a crack had developed and was getting bigger, or whether it all cracked at once. The older surfaces would be a bit smoother (unlikely to be rusted or discolored given that they live in an oil bath), and would have a semi circular tree ring like pattern as the crack grew larger. That all ends with the final fracture, which is of a more uniform appearance.

Cranks breaking aren't all that common, but that is not unheard of either.

Good input Walt!
I don't have a good picrure on the fracture, but it looks a bit like you assume.

[sc_rufctr]

Originally Posted by safe

More pieces of the pussle that we missed until we lifted the crank....



Never seen a broken crank before. Might explain the wear on the bearings. Hard to tell but the case looks tweaked, will be sending it out to be measured.

Not sure if this is related to the chains, can't imagine how.

I've seen that before on a 964 engine (same location on the crank ~ Porsche 911 world magazine). The timing belt in the distributor had failed and according to the article they thought that may have caused the crank to break.
It happened on the highway. The case was junk but the rods and pistons/cylinders were OK. Valves etc also all OK.

Weird. I was never completely sold on the idea of an ignition fault could cause a crank to break.

[thumbdoctor]

Is it possible when the crankshaft broke it walked rearward and the I S shaft gear hit the chain.

[safe]

Originally Posted by thumbdoctor

Is it possible when the crankshaft broke it walked rearward and the I S shaft gear hit the chain.

Well, is has walked backwards because the IS gear on the crank has begun to mill into the case near the distributor. But the crank and the IS are separate and can move in relation to each other, if allowed, but that should not happen. IS is held axially by the IS cover.

[Lapkritis]

I would think you would get some ignition advance jitter if the IS can move axially from the crank while the engine is operating. Might be a couple degrees fluttering about which would make for strange surging in output.

[safe]

More closeup pictures of fun...

[Lapkritis]

Definitely looks like it cracked some time ago except for the middle ~95 degrees. When it completely let go it was such a break, however, that the parts fit together in a manner across the broken surface that the part still functioned before you took it apart.

[Walt Fricke]

This kind of "broken but still sort of locked together" seems to be the common break for cranks. The thrust bearing is at the flywheel, so that half won't move axially much. I can't think of a suitable thrust surface rearward (away from flywheel) for the rear half of the crank, but doubtless the crank itself and the bearing webs will do some of that.

Based on a belief that while the motor can run for a fair amount of time with a broken crank, you know something is quite wrong, I am a bit dubious of the theory that it was crank failure, then chain failure, since there is no report that the crank failure was proceeded by a period of rough running, was there?

Though, on the other hand, my understanding is that if you have chain wheel parallelism off by more than a couple of shims (which isn't much), you will know it if you listen, and also when you rotate the motor with the covers off during setup. But I have no experience with the "off by a whole row of the teeth" chain setup. That just seems more likely to push the IS shaft against the cover plate.

Coincidence bedevils forensics. What are the chances that an unusual failure (the chain) is the final straw for another unusual failure (a crank with a developing crack)?

[thumbdoctor]

I have seen many similar crank failures (mostly Scat cranks) caused by regrind and the fillet (small radii at the edge of the journal) not properly ground causing a stress riser.

[thumbdoctor]

Originally Posted by safe

Well, is has walked backwards because the IS gear on the crank has begun to mill into the case near the distributor. But the crank and the IS are separate and can move in relation to each other, if allowed, but that should not happen. IS is held axially by the IS cover.

I meant the IS drive gear on the crank.

[safe]

Originally Posted by Walt Fricke

This kind of "broken but still sort of locked together" seems to be the common break for cranks. The thrust bearing is at the flywheel, so that half won't move axially much. I can't think of a suitable thrust surface rearward (away from flywheel) for the rear half of the crank, but doubtless the crank itself and the bearing webs will do some of that.

Based on a belief that while the motor can run for a fair amount of time with a broken crank, you know something is quite wrong, I am a bit dubious of the theory that it was crank failure, then chain failure, since there is no report that the crank failure was proceeded by a period of rough running, was there?

Though, on the other hand, my understanding is that if you have chain wheel parallelism off by more than a couple of shims (which isn't much), you will know it if you listen, and also when you rotate the motor with the covers off during setup. But I have no experience with the "off by a whole row of the teeth" chain setup. That just seems more likely to push the IS shaft against the cover plate.

Coincidence bedevils forensics. What are the chances that an unusual failure (the chain) is the final straw for another unusual failure (a crank with a developing crack)?

The owner who drove the car during the failure, he said the engine started sounding a little rough at the end of the straight and got two corners before the "bang".

[TimT]

The owner who drove the car during the failure, he said the engine started sounding a little rough at the end of the straight and got two corners before the "bang"

We prepped a car for a guy who said a similar thing, he drove the car for two laps at Pocono Long course before the engine died completely.. Another crank break I have seen locked the engine up almost instantly, resulting the the car spinning....

Another instance where I have witnessed a crank breaking was quite interesting...

The crank broken either side of a rod throw..pics...

remains of broken rod with crank throw still in rod end



even the portion of the crank that still had thrust bearing walked a bit... look at the webs, you can see where the counterweights did a little bit of machining







The owner of the car Cupcar with the broken crank was going to sue us, and had his hired experts check out the build we did on the engine...The guys he hired have serious cred...They rebuild F1 and Uber high dollar machinery....

Seeing the car owners look when they basically told him we were not at fault was "priceless"

It was never disclosed to us that the car had run off track and hit a wall rear first prior to us doing the engine....

We think the that the crank had a violent counter-rotation previously, contributed to the failure...

[safe]

Tim, the bearing wear (shiny parts) looks similar to our engine. Is that a result from the broken crank?

Originally Posted by TimT

[HawgRyder]

This is one reason I always "ring" a crank before installing it.
Ringing the crank consists of hanging it on your fingertips in one hand...and striking the counterweight gently with a metal object (box wrench?) with the other.
If the crank rings like a bell (nice long sound)...then it probably has no cracks.
But...if you get a "boinc" sound like a sour note...you had better get a dye penetrant or ultrasonic test before using.
It's quick...easy to do...and a good practice to get in to. (cheap too...LOL).
Bob

[Lapkritis]

Magnaflux test is always on my list for not only the crank but the block and heads when doing a thorough build.