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Sat Jun 06, 2020 2:11 pm

Driving my 65 beetle with stock 1200 motor and the engine seized due to crank breaking. Machine shop asked if I’d been running big bore pistons and I had. He said he had seen it before. Anybody heard that big bore pistons can break the crank?

Sat Jun 06, 2020 2:12 pm

Stock crank?

If not was it cast or forged?

Sat Jun 06, 2020 2:54 pm

Big bore alone isn't necessarily the cause; 40hp cranks are known to not hold up to high revs well, a problem more likely to occur when the engine power has been increased. Another common cause is if the crank was reground incorrectly where the radius between the journal and cheek was not maintained.
https://www.thesamba.com/vw/forum/viewtopic.php?t=261915

Sat Jun 06, 2020 3:34 pm

It's kind of unusual to see it happen now, it was certainly an issue in years past, but my opinion is that most of the ones that were going to fail, did so a long time ago and the ones around today are the survivors.

I'm surprised that it seized as a result though. The original engine from my 62 Beetle... got transplanted into a 61 that I sold and later the crank broke. I helped her rebuild the engine after this happened, but the engine was still drivable, it just made a hell of a racket.

Many years later, I had a crank break on me, this one a 1600, while I was driving... again, the engine still ran but did not sound at all well!

Sat Jun 06, 2020 4:22 pm

any pictures of the teardown?

Sat Jun 06, 2020 4:49 pm

Here’s my broken crank.

Sat Jun 06, 2020 6:09 pm

Yes I've heard about broken cranks when running the big bore pistons. What is hypothesized is that the big bore kit makes the engine feel more powerful and the driver tends to rev it out farther, like past the stock shift points. This pounds out the #2 center main eventually causing the crank to whip around and break at the #2 journal.

Sun Jun 07, 2020 7:48 am

Been there before in July 1975. Driving from Mesa Verde to Kansas City.
It let go near Salina Kansas middle of now where on I-70. Engine still ran but um not to well.

Sun Jun 07, 2020 12:49 pm

Check that the crankshaft has proper radius where the bearing journals meet the webs. Had two cranks break due to no radiuses there, just a sharp 90 degree angle that put all the flex of the crank in those narrow areas, causing cracks.....

The below from the "Without Guesswork" technical booklets you can access thru the "Technical" button above at right.

Left click on image below to bring it up in its own window. Click on image again then to blow it up to easily read it.

Sun Jun 07, 2020 2:51 pm

glutamodo wrote: It's kind of unusual to see it happen now, it was certainly an issue in years past, but my opinion is that most of the ones that were going to fail, did so a long time ago and the ones around today are the survivors.

I'm surprised that it seized as a result though. The original engine from my 62 Beetle... got transplanted into a 61 that I sold and later the crank broke. I helped her rebuild the engine after this happened, but the engine was still drivable, it just made a hell of a racket.

Many years later, I had a crank break on me, this one a 1600, while I was driving... again, the engine still ran but did not sound at all well!

Andy, I'm not surprised. You should have seen the difference in weight on my pistons in my big bore kit. And I'll bet most people think they can install this stuff without equaling the weight and then not taking the rotating assy. parts to the machinist to balance the whole rotating assy.

Sun Jun 07, 2020 8:36 pm

You know, I've never come across new sets of pistons that were really that far off in weight. Usually pretty easy to correct. But oh boy, have I seen some "sets" of connecting rods that were not even close!

Sun Jun 07, 2020 8:51 pm

Nice car! Are you wanting to keep it Original 40 horse engine? It may have messed up the case. Note The 65 40 horse engines had better square boss heads! Something to hang on to if you want to keep it a 40 horse.

Sun Jun 07, 2020 8:55 pm

The change to square boss heads was MIDYEAR in 1965, so they were not all created equal. The change happened near the end of November 64, so nearly a quarter of the production year.