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

[snowman]

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

[jluetjen]

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.

[snowman]

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.

[jluetjen]

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.

[724doorE]

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....

[snowman]

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.

[724doorE]

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?

[john walker's workshop]

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

[LeRoux Strydom]

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

[snowman]

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?

[911pcars]

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

[jluetjen]

BTW what will you do with your present engine?

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?

[J P Stein]

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.

[snowman]

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.