The Official 356Talk Forum

I would like to get some opinions on an issue I just discovered. I'm getting ready to put together a replacement engine. I have decided to use a 912 case I had laying around. Looking at the 3rd piece I noticed moderately sized cracks in the two lower bosses where the 912 rear motor mount mounts. However, this engine will be used in a 356, so there will be no mount there. In my uneducated opinion, I can see no reason why these cracks would cause a problem in my application. The third piece has numerous bolts holding it in place. And there is no mount to support any longer. The only stress on these damaged areas will be just pressure of the two (of many nuts) holding it on the 3rd piece on. Can anybody think of a reason why this is not a usable 3rd piece. (FYI, if it matters, it appears to be a replacement part with no number and a R after the stamping inside.)

Glen
64Cx2

Could we see a close-up picture?

Didn't know how to do this, and then poof, I figured it out. Here's some photos.

I would use it, maybe add a regular flat washer on first under the wave washer.

Personally, I'd weld it up.

jim nelson wrote:Personally, I'd weld it up.

Either that, or if they are shallow enough, machine down until there is no crack in the remaining material.
Cracks propagate; that's what they do.

Back in my flying days, I remember them drilling holes at the end of any crack they found on the air craft to stop the crack from propagating (I'm sure there were some cracks they were allowed to do that with and some they weren't. What I didn't know wouldn't hurt me.). My point or question is would drilling a hole at the end of the crack stop the crack? That is something that I can do in my garage. Thanks!
Glen
64Cx2

In a piece of sheetmetal, you can be sure that a well-oversized drill left none of the crack if you drill normal to the surface. I'm saying you can't be sure of that in a casting of some thickness where the crack may well have wondered out of plane to the visible end of the crack, and the 'length' of the crack may not be equal inside and outside. Suffice to say, I would not trust a drilling to "end" the crack in a casting.
It is not a high-stress location, but if you race with it, and it lets go, the guys following you will probably pay you a visit in the pits after you're black-flagged.

With the nuts only at 15 ft/lbs I would call it an extremely low stress application as long as it is not in a 912. Of course I am not a mechanical engineer.

The four holes in the 912 timing cover are where the rear engine mount brackets attach. Probably is a fair amount of stress in those four points in the 912 engine mount application. The holes are counter bored to take shouldered nuts, 616-115-171-00. See photo below. Welding and re-bore should be the fix.

In a 356 application, I would do NOTHING to those cracks. There is no stress on them in a 356; they will not propagate because there are no loads being applied. Don't waste any time, or money or worry about them. If you're still concerned, I'll send you another; do you want one with or without a number? I have one wall covered with about thirty of those things.

If there is no stress, why did the crack form?

The crack was caused by some incident in the past life in a 912. It did not FORM; it was CAUSED.

The 3rd piece of the engine was stressed in the previous 912 application with the rear engine mounting cross member. Why did Porsche have the 356 engine unsupported and mounted only on the bell housing of the gearbox and yet felt the need to have the rear of the engine supported in the 912? I feel that the engine must have been constantly trying to escape at that rear mounting, thus the stress cracks.
Jay

"the need to have the rear of the engine supported in the 912?"
Because there was no rear transmission hoop to support the transmission and engine.