Crank snapped in two. Oil starvation or crank flaw?

[71ghiavert]

So here's wassup:

Last year I built my first engine, a 1968cc, 74x92, 9:1 compression, Web 218 cam, L3s, dual Weber IDF 40s. Ran well and made 90 hp on the dyno.

Two weeks ago I upgraded a bit, adding Panchitos, 1.25 rockers, a 1 1/2 exhaust, and 32mm venturis. Re-jetted and tuned. Much fun. Lotsa pop. It was the engine I wanted. Maybe boosted to 110-120hp? Cool!

Two days ago I was driving it up a local mountain road--not lead-footed by any means, but having fun around the curves--when I heard a sudden clunk-clunk-clunk. I pulled over immediately and had it towed. My mechanic checked the dipstick and, yikes, the oil was dead low--just at the bottom of the dipstick, well below the "low" line. Looking under the valve covers, he found that one of the silicone seals had split, as tho someone had cut it with scissors along the bottom edge. I had driven maybe 60 miles since last checking the oil; the assumption is that in that time the engine pumped out a lot of oil. The flywheel now had a LOT of endplay, and his guess was that oil starvation had destroyed rod bearings, main bearings or both.

Today I took the engine apart, expecting to find a gruesome mess. Instead, all the bearings looked perfect. Not a sign of internal distress. But when I lifted the crankshaft, it came away in two pieces. It had snapped right next to the center bearing.

So now I'm wondering what happened. Was it the oil loss? Or was there a defect in the crank, revealed when it had to deal with the engine's increased torque? On the one hand, the oil was very low, and it's easy to imagine a quick, severe heat problem. On the other hand, the bearings are still perfect, and I would have thought they would have been destroyed before the crank itself gave way. Also, tho the oil was really low on the dipstick, there was still some oil in the case, plus the 1 1/2 quart deep sump, and the spin on oil-filter. Because the engine is "new", I watch the gauges pretty closely; before the clunk, the oil pressure remained high and the temp never went past 200. If the pressure remained high, that makes me think that there was still oil flowing to the bearing where the crank broke. But maybe I'm wrong about that.

Anyway, trying to figure out what happened. Killed by oil starvation? Or a defect in the crank? Or something else? FYI, it was a high quality crank, built on a German core, by a good supplier.

Thoughts? Thanks. I don't know enough draw a strong conclusion.

[glutamodo]

I don't think oil starvation would cause this, especially since you say you don't see signs of it on the bearings.

So, it was a German Forged crank that got built up then. Not one I would expect to suffer this. I had a replacement cast stock crank break on me once before, but it was not a quality crank like this one seems to be.

If you checked end-play at the pulley end, it means nothing when the crank is broken. The thrust bearing is where end-play is determined and is out of the picture once the crankshaft snaps. Since your thrust bearing was intact to the flywheel, I wouldn't expect much endplay there. Not like when this happened to me:

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Andy T.


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[anthax]

Oh thats a sad sight. :/
Is it me or does the rotating assembly looks like it overheated? Both crank and rods looks blueish/black from heat. It could be my monitor not being 100% in reproducing colour.

[Igpoe]

I'm thinking the crank let go and a sliver of metal from it traveled to the seal and split it. You pumped the oil out AFTER the crank broke. That crank could have had a crack in it for years or even always. Of course you'll never be sure and I'm just guessing.
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'73 super.

[oprn]

I once bought a '73 Beetle that was still running with a stock crankshaft broken just like this. The case bearing saddles were pounded out beyond repair too. Not knowing the history on the car and being a stock non counter weighted crank I just assumed it was from high rpm abuse.
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We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[gt1953]

Had a stock crank break in the middle of Kansas. Oh boy no fun no cell phones back then but hitch hiking was safe then.
Not sure why it snapped. Well maybe driving it for like ten hours straight.
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Volkswagen: We tune what we drive.
Numbers Matching VW's are getting harder to find. Source out the most Stock vehicle and keep that way. You will be glad you did.

72 type 1
72 Squareback
({59 Euro bug, 62, 63, 67, 68, 69, 73 type ones 68 & 69 type two, 68 Ghia all sold})

[oprn]

[Dale M.]

Fault is with crank shaft, could have had stress riser in it for years and the day it broke was the day it decided to break.... If bearings look good it was probably not oil starvation problem, because there would have been galling on bearings if they ran dry....Some have had engines that actually ran decent with broken crank other that it hammers case to a point it is not useable....

Had a engine in an crap BAJA for a while that had about 1/4 inch of endplay at pulley end, always suspect it had broken crank but never tore it down to see (whole project was a disaster) Sold it to a fellow with understanding engine was bad, but never heard from him what he found out about engine...
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All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[BIGMIKEY]

So, crank is toast. How much else is still usable? Case? Rods? Definitely following your thread.

Mike T
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BIGMIKEY

Deserter Series 1 project.
https://www.thesamba.com/vw/forum/viewtopic.php?t=787047&highlight=

1973 Beetle Driver, Marina Blue.

[Clatter]

This happens a lot, actually.
Just like yours, right next to the center main.
Aftermarket, factory, counterwieght, stock, stroker, doesn't matter.
When your number comes up, it comes up.

It's not something you did wrong or anything else.

It just happens.


Hopefully your line-bore is still good..
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Bus Motor Build

What’s That Noise?!?

[Zundfolge1432]

Here’s another disaster scene. https://www.thesamba.com/vw/gallery/pix/1416609.jpg

[74 Thing]

Was it a 4140 or 4340 crank?

At least you got to drive it a bit and find out it was the engine set up you wanted. Now you will have to go through the case etc and get a new crank.

It happens-machines break. Don't be hard on yourself and get to building a new engine.

Who do you use for machine shop work in your area, Jansen?

[Pruneman99]

It's an OG crank with welded counter weights. If it's 74mm, it was offset ground. Maybe to a smaller rod diameter (Buick)?

Who knows how much abuse it had on it before it got worked over. I would assume that the shop that did the work magnafluxed it before welding. But maybe not. Maybe it just broke..

Doesn't really matter. The results are the same. Hopefully since you shut it down fast, the case and everything is still ok.

[74 Thing]

My bad, I did not look close enough to see the welds on the crankshaft!

[liquidrush]

Can you provide close ups of the break on both sides? Thanks.

[bugguy1967]

Yep. It just broke because it was its' time. The area where it broke doesn't get touched by the crank grinder, except to turn-down the journal during a regrind, so it woulda happened regardless if it was modified or not.
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"A petrol engine can start readily, run smoothly and give every appearance of being in good order, without necessarily being in good tune." - Colin Campbell, "The Sportscar Engine"

[71ghiavert]

Thanks for the input, everyone. In the end, yeah, I think it broke because its number was up. There was no damage to the bearings or the case, so I came away lucky. The engine felt a lot snappier with the new heads and 1.25 rockers, so there's even a chance that it was just the little increase in torque that proved too much for a weak spot on the crank.

Also, I liked the engine a lot with its new modifications, so will rebuild it exactly the same way. New bearings everywhere, have everything balanced again and re-check deck height, and a general cleanup. If it performs the same, I'll be happy.

Not sure what steel, just that it was built on a German core by a reputable supplier. Supposed to have been magnafluxed. I guess this now falls into the immortal category of: "Stuff happens."

Liquidrush: I'll post some close-ups when I have a moment.

[rosevillain]

Contact the supplier and let them know that you had a problem. Have pictures ready and be cool. Maybe they provide a better product in the future, maybe they help you out with the next one.

[modok]

It's a bit out of warranty, being some 30-40 years old

Offset grinding it "could" have brought a flaw to the surface that wasn't a problem before, something like that, but, I'd just say it's bad luck.


On a long enough time line.... every crank will ultimately break,
just in MOST cases we never run them long enough to find out.

[71ghiavert]

By popular demand, broken-crank close-ups!!!

I think I'm going AA next time . . .