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

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?

http://forums.pelicanparts.com/uploa...cked Case2.jpg

I'd appreciate the input. SmileWavy

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,

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

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.

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

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

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!!!

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

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.

My vote is to leave it alone...

-Wayne

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?

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

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.

Thanks everyone for the input. SmileWavy

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

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

What would you recommend for a lesson in fracture mechanics? The classic Dieter or the more recent Zahavi and Torbilo:rolleyes: 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

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

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.

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.

PS this is also a very big crack, not some small mirco thing that could be left alone, something caused this crack.

Hi Jack;
Yes -- something did. The stud that is in that hole. Since it looks like it won't be needed in my future configuration, the stud won't be going back in.

Thats if your SURE that its that stud. It may very well be, but then again... Even it its a stud thats no longer there, a crack, once started may propagate via other stimulus. The only way CERTAIN is to not have a crack.

Just as an FYI; When I backed the stud out, it brought a whole lot of threads with it. It sure looks like it was either cross threaded or over-torqued which caused the treads to strip.

I also found that at least two of the head studs had the same problem. As far as I can tell the engine (a 2.4 TK with iron cylinders and steel studs) has never been apart. I can only assume that it was built that way at the factory. For a while I was entertaining the thought of skipping the installation of case savers/time certs since I'm keeping the iron cylinders and the engine will be a 2.2, but now I guess I'll just have that done too.

Jim Sims is right, although Al's and mag's don't have a well defined fatigue limit as do most steel alloys. Very big subject!

John, you should be fine. If your not going to use the stude, just grind it off for your own piece of mind.
I would definitely use case savers and somedowel pins in that application your planning.

Snowman, you do realize that all materials that are processed start out with imperfections in their crystal structure, some of which are micro-cracks. something as simple as a grain boundary can restrict the growth of said micro-cracks. This is one of the reasons we heat treat metals, faster cooling results in smaller grain size and hence more boundaries in the way of crack growth. This makes that material both stronger and harder.

Very interesting subject and very deep....

Yeh but again this is no micro crack is it

To say it again and another way, if you can see the crack, it ain't a micro crack. It if ain't a micro crack you must do something with it, or be certain in some definite way that it cannot spread, ie drill and pin it , weld it , grind it off, something.

Being an engineer for 30 years has taught me one thing, if it can bite you in the butt it will.

Jack, Jack, Jack.......just remember, your not the only engineer on this board, there might even be some here that have completed some amount of research on the subject of crack instigation, growth, maybe even some damage accumulation. Thats ok, your welcome to the last word, I'm done

John, let us know what you decide, and I'm sorry your good thread deteriorated into a crack filled shouting match! BTW what will you do with your present engine?

electrical engineer. just another post where jack will get the last word in, regardless of anyone's expertise.

John Walker, say it like it is!

Snowman, a crack that is unstressed will NOT grow. Crack growth rate in most metals that I know is a function of applied stress. Zero applied stress = zero crack growth rate. For Pete's sake, the stud is removed from the hole, there is nothing that stresses the hole or the boss!!

So, jluetjen, proceed with abandon.

Just a humble opinion from a PhD physicist / metallurgical engineer type............

LeRoux

Yeh, I can remember several events where something shouldn't do what it ended up doing anyway. Just because what I thought I really understood was short a bit of extreemly obscure info that changed everything. So based on those professional experiences (and Mr. Murphy is a master of all professions, not just EEs), and over 40 years of taking engines apart (usually after I blew them up trying to see what they could do) I have found that if you see something that isn't right, no matter how inocuous if might first seem, it is in your best interest to take care of it, whatever it is. Thats why my engines no longer break. Knock on wood.

So there it is, the last word. But I didn't think the thread really degenerated. Just because I was expanding on my opinion with what I had personally experienced. I did not resort to name calling which some others have appeared to do. Knowing that a crack cannot propagate because of lack of stimulus is nice to know and informative. Knowing that there may be something that you didn't observe still contributing to the cracks expansion, based on past observations in the real world, is humbling. That humility adds a conservative bias to fixing possible problems, ie if under some condition, any condition, an unforseen condition it might somehow break, then don't give it even the most remote chance. Thats not scientific but experience saying take the precaution anyway. There are so many real problems you don't need things that might, even in the most remote of circumstances, bite you, you fix them because you can. This way they cannot become a problem. For example you forget about the crack and end up mounting something on the unused boss. Then what?

Against my fellow pelicanite's opinions, I might lean toward conservatism as well. But that's MHO, not a decree. Everyone is entitled to an opinion. As in all things here, read the suggestions and reasonings offered by contributors, then take it or leave it.

On another thread, a seemingly innocuous, straight-ahead (IMO) fact shared by the majority (akin to talking about the effect of gravity) was rejected/ignored, for whatever reason, by someone. Rather than argue the "direction of falling objects", you gotta say your piece, then move on. On matters not so cut and dry, let's discuss the hell of it. I'm here to learn too.

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

724DoorE; It's going to stay in my 911 and I'm going to continue to enjoy driving it until my new drivetrain is complete.

Only once the race engine and transaxle are complete will I take the car off the road and start the chassis prep. I figure that if I take the car off the road in September of whatever year I finish the drivetrain, I should have the cage, fuel cell etc. installed, everything painted and put back together by the following spring.

It's a plan. But if it take longer then anticpated -- that's OK. It's not the finished product that's important but the journey! I can live with that mantra as long as I can still drive the car.

After considering everyone's opinion, I'm torn between the old medical credo of "Do no harm" and wanting to ensure that the crack doesn't grow under the vibrational stresses of a hi-CR, 8000 RPM engine. I'm no engineer (marketing actually), but I could picture the two pieces of the boss vibrating like a tuning fork at high rev's and having the crack propegate further. I'm leaning towards just grinding the whole thing off since I want the bottom end to be solid for 2-3 seasons. But on the other hand, I dread introducing new problems with the grinder.

Since we've discussed the pros and cons of the crack at length, IF I were to grind it off -- any suggestions or gotchas? Grit? Speed?

Who is to say that crack is not under stress?

I have seen plenty of new/unused castings crack. I'm an inspector and see a lot of stuff that engineers didn't design in.

This particular casting will be under thermal stresses.

Using a die grinder and a carbide rotary file, it's a 10 minute job to eliminate the boss..or the major portion of it.

BTW, when removing studs from mag cases it is (unfortunately) quite common for the case threads to come with it...oops.
Heat is your ally here.

Carbide bit on a $10 Harrbor Freight die grinder works real well on Al and the like.

Old studs may have loctite on them so the heat may not only be a good idea but almost necessary.