Causes of Broken Crankshafts

79SuperVert · Posted: Mon Oct 29, 2007 7:04 am Post subject: Causes of Broken Crankshafts

I suspect my crankshaft is broken. My 64 40hp was rebuilt with oversize pistons, so I'm told.

If the crankshaft is broken, what precautions can I take when it gets rebuilt to prevent it from happening again?

keifernet · Posted: Mon Oct 29, 2007 7:43 am Post subject:

It was probably a fatal flaw in the crank that did it. Just having bigger bore on a 40hp is not what broke the cranks IMHO... Just an old part that either had a flaw in the original forging that took years to show or it was damaged at some point in the life of the engine(s) it was in.

A forged crankshaft will have a "ring" to it when tapped lightly with a hammer or wrench. If it does not ring it is cracked.

Other causes may be improperly/poor balance of parts and running with flywheel loose.

There are still good 40 hp cranks out there. I have a couple of .010/.010 I was saving for some builds if you need one Joe. And some NOS rods too.

79SuperVert · Posted: Mon Oct 29, 2007 7:59 am Post subject:

Thanks, I'll keep that in mind. If I need a new crankshaft I don't want to skimp on the quality. I'll be stripping the engine down to the long block and taking it to an old VW mechanic who does engine overhauls for his son's VW shop.

keifernet · Posted: Mon Oct 29, 2007 8:08 am Post subject:

79SuperVert · Posted: Wed Nov 14, 2007 2:42 pm Post subject:

The verdict is in. Broken crankshaft. No other apparent damage (isn't that lucky!) Mechanic is going to poke around to see what he has in stock and will get back to me.

He said (if I heard him right) that the broken crankshaft was machined to .025. He is going to look for a stock one and will replace the bearings to match.

I really don't understand much about bearing sizes, so why would a crankshaft be machined to that size? Are the .010 ones you have the stock size? And what does that dimension refer to?

keifernet · Posted: Wed Nov 14, 2007 2:49 pm Post subject:

.025 is the MM measurement = approx .010" of an inch.

So that means the crank that broke had been reground/ "turned" from standard size.

79SuperVert · Posted: Wed Nov 14, 2007 2:57 pm Post subject:

So why does someone decide to regrind a crankshaft? To re-use it after it's worn?

Is splitting the case a normal part of fitting oversize pistons?

tim240 · Samba Member Joined: November 14, 2006 Posts: 176

the 40 hp crank is weaker then later versions according to the vw engine rebuilding book I have. If they have been ground and are being used with the 83mm(big bore) pistons instead of the 77mm the cranks can break(if you have a lead foot it dosn't help). Get a good german crank thats std/std or 10/10. One vw mech. told me to watch out for cranks with rod journals that have been ground to much. He said there are reliefs on the sides of the journals that get ground away and that they tend to break if that relief is gone. Anyone got an opinion on this? You might think about dynamic balancing when you rebuild.

Dale M. · Posted: Wed Nov 14, 2007 5:24 pm Post subject:

Splitdog · Posted: Wed Nov 14, 2007 5:29 pm Post subject:

Build a 1600.
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keifernet · Posted: Wed Nov 14, 2007 5:31 pm Post subject:

KTPhil · Posted: Wed Nov 14, 2007 7:59 pm Post subject:

I had a 40hp crank crack, and upon inspection, it could be seen that the machinist removed the radius (stress relief) at the edge of the throw when it was machined oversize, cause a local stress point. Inspect yours carefully before you install it, better yet before you take it from the shop.

tim240 · Samba Member Joined: November 14, 2006 Posts: 176

at 10/10 the reliefs in the rod journals are probably still there. when the rod journals have been ground to say 30 the reliefs are gone. I have one that is ground at 20/30 and I bought some other cranks because I didn't want to use it.

79SuperVert · Posted: Thu Nov 15, 2007 7:02 am Post subject:

Thanks for all the comments. I will be talking to the mechanic this week.

79SuperVert

Pics of the broken crankshaft:

Image may have been reduced in size. Click image to view fullscreen.

wcfvw69 · Posted: Wed Nov 28, 2007 8:59 pm Post subject:

I am just curious as to what you were doing with the car/engine when it broke? Just crusing? Hard take off? Etc.
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KTPhil · Posted: Wed Nov 28, 2007 9:04 pm Post subject:

When my friend's broke, it was cold and he noticed a noise. Louder than a valve, but the engin ran FINE. We didn't know what was broken as we slowly disassembled it. Actually had the case split and lifted the crank, I mean lifted HALF the crank out of one half before we knew what was wrong! The parts had stayed keyed together (thanks to nice tight end play from the 500 mile old rebuild) and it ran fine, just that damn noise!

aitor11 · Posted: Thu Nov 29, 2007 5:22 am Post subject:

Rowroy

[quote="79SuperVert"]Pics of the broken crankshaft:

quote]

Time to get out the JB Weld! Laughing

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79SuperVert · Posted: Thu Nov 29, 2007 7:03 am Post subject:


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