Urgh! My 7R Case is Cracked -- Big issue or not?

[jluetjen]

While taking apart my project engine, I noticed that the case is cracked on the boss that I marked below. (BTW - I don't have access to a digital camera, so I've marked up one of Wayne's pictures.) It looks like either the hole was undersized or the stud was marginally oversized. I know that this stud is used for the CIS's throttle linkage, but I'll be building this motor with carbs. Will I be using that stud and/or boss with the carbs? Should I just leave it as is? Have it welded? Drill the bottom of the crack? Just grind the whole boss off?



I'd appreciate the input.

[Thomas Owen]

I would get it fixed, but I am not sure the best way to do this - why don't you call John Walker?
Good luck,

[911pcars]

It looks non-structural to me. Cracks tend to grow, so I'd consult a good welder to repair it (TIG). However, there is a chance welding heat might distort the case. I suggest bolt and torque the case together (empty) and evenly heat it (oven?) before welding.

Sherwood Lee
http://members.rennlist.org/911pcars

[J P Stein]

That's the side opposite the thermostat, right?
I didn't use it for anything.

I'd drill stop it or grind off the lug. Welding on a mag case would be something of an art form, me thinks. Just leaving it alone would be an option, but it is under the shroud and you can't keep an eye on it.

[kwikt 911]

I have the same crack on my 2.4mfi case. The cracked area is where the mfi pump is bolted to. Mine is still cracked and my mfi pump is still attached and holding fine.



Kevin
72 911T/E

[john walker's workshop]

i'd leave it alone, being nothing will be mounted there.

[snowman]

Someone told me that cases were a dime a dozen in a post fairly recently, so what me worry? Get a new case for a couple of bucks, NOT!!!

[Jim Sims]

I received a used 7R case with a similar crack. I discussed it with the machinist working on it and we considered all the usual repair options: drilling the tip, welding and doing nothing. Since the crack appeared headed towards a cylinder stud hole (as is yours) I sent the case back to the salvage operation I bought it from and they sent me a better one which was essentially pristine. Jim

[island911]

Grind it off.

It looks like the bolt (or stud) mounted in the hole put a bit too much hoop-stress on the boss, causing the crack.

With out anything in the hole, I doubt the crack will go any farther. . . but if you grind it off, nothing will have a chance to push the crack wider.

[Wayne 962]

My vote is to leave it alone...

-Wayne

[jluetjen]

Thanks everyone. I'm leaning towards either having a Pro-welder weld it or grinding the whole thing off. The only reason is that the engine is planned to have an 8000 RPM redline (I forgot to mention that) and so I can picture a lot of vibration going on and I'd hate to see it progress further down to the stud hole. John and Wayne, would the 8K redline change your suggestion?

[john walker's workshop]

not really. it's just an appendage, not the actual case wall.

[snowman]

Note one thing. Cracks DO NOT STOP by themselfs. It will continue to some end, likely undesireable. TO stop it it MUST be drilled and pinned, welded. If anyone ever says leave a crack alone, ignore all of their advise.

[jluetjen]

Thanks everyone for the input.

[Jim Sims]

Cracks do stop by themselves; all real material is riddled with cracks. The issue is crack size, the energy required to extend it (depends on the material and crack geometry) and the possible sources of extension energy such as cyclic mechanical or thermal stress. If you want to learn more, read up on the topic of "critical crack length". Cheers, Jim

[snowman]

Jim,
Read ANY book on metalurgy, cracks DO NOT STOP without help.

[Jim Sims]

What would you recommend for a lesson in fracture mechanics? The classic Dieter or the more recent Zahavi and Torbilo No, cracks do not grow without a source of energy "sufficient" for their propagation. Specimens of steel may be stressed essentially endlessy without fracture if the stress level is below the fatigue limit (endurance limit) for the particular steel, the geometry, surface conditions, notches, etc. You can bet that any real metal part contains cracks; the questions are how big they are and will they grow under operating conditions and if so, how fast. Cheers, Jim

[Wayne 962]

If there is no stud in the hole, there should be no force on the boss, and thus there shouldn't be any reason for the crack to propagate. It's just a boss on the case - I wouldn't worry about it...

-Wayne

[john walker's workshop]

it's good to have an opinion, but not good to think that your opinion is the "last word", and anyone who thinks different than you should be ignored, jack. it's like thinking that everyone but me is out of step in this marching band.

[snowman]

There is a reason that the crack is there to begin with, the same "reason" will see that the crack continues to propagate, until something says stop.

you all are just guessing as to the origin of the crack. Seeing that this will become a high bucks engine, highly stressed to boot, I cannot see any reason to leave ANY cracks unattented.