Crack in case-7R
 

While getting the 2.7 ready for the cylinders, I noticed (after this has been together for a year) that I have a bit of a crack. It is in an area where there are some stresses (the top right engine mount). Mitigating is that this is one of four mount studs. Application is in a 914 track only car. I cleaned it out a bit with a file to determine the severity. A really fine job of pre-assembly inspection I did here. The stud hole does not penetrate the oil galley, and the existing threads in the hole are good. The other three mount holes and studs are fine.

As a former aircraft machinist and mechanical engineer, the prudent side of me says "stop drill the crack and weld it". The more dice rolling side says "the stresses will be distributed".

I solicit your opinions!!!

http://forums.pelicanparts.com/uploa...1204247038.jpg


http://forums.pelicanparts.com/uploa...1204247065.jpg

That's a pretty common thing you see quite a few in service with a crack from the engine mount stud.

Are you sure it not just a gouge in the mag? this stuff is very brittle, If that is a crack and opened up that far you should see another in that mounting boss.

I wouldn't worry about it too much. It looks more like a defect or gouge. Just inspect it every once in a while.

-Andy

Looks like a gouge to me as well, and I have a case to sell!

Wouldn't worry too much since others take the stress of the mount as well.

No, it is definitely a crack. Kind of a weird spot.

Call the guys at Ollies.

Weld it up proper and don't look back. A proper repair coupled with new engine mounts and you'll be crusin' with no issues

At first it doesnt look like a crack. The first picture at the small end is where the crack starts to show good. If it were mine I would have a experienced machine shop drill the end of the crack and weld it. However there is risk. I was checking out a welding job that my machineist was working on the other day. The cracks were in some alumnium heads and while he got a couple of the cracks welded one of the cracks spread further. He said that he was not done with it yet and that he felt he would still be able to weld the crack and stops its progress. I wonder if metal stitching would also work in this case?

This area looks weldable. I'd keep the crankcase torqued to spec during the welding process. Maybe consult a FAA welder if you're especially nervous about any side effects.

Sherwood

I'd avoid a weld.

A stop drilling, possibly a deeper stud tapping, and good epoxy repair will be my choice ..

Kind regards
David

I don't really want to weld it. I can put a stud back in, and torque it too full torque, and see no spreading of the crack. So, maybe I'll run it for now like this, and keep an eye on it. I did clean it up, so I'll go with the stop drill and some Devcon. That way, I can easily see if the crack migrates any further.

Pat,
I'd suggest following your aircraft hunches. You describe this as "an area where there are some stresses (the top right engine mount). ". Plus, race car, probably with solid engine mounts = more vibration transmission.

I'm not sure how Devcon, epoxy or any filler is going to prevent this crack from increasing if it wants to. Seems like now is the most convenient time to repair it since it's out of the vehicle and it's clean. Or you can take your chances.

MHO,
Sherwood

I have never heard of anyone welding magnesium.

A simple click on google yields this:
Abstract:
"Magnesium can be welded by many of the arc and resistance welding processes, as well as by the oxyfuel gas welding process, and it can be brazed.Magnesium like aluminum is produced with different tempers. These are based on heat treatment and work hardening. The strength of a weld joint is lowered in base metal, in the work-hardened condition, as a result of recrystalization and grain growth in the heat-affected zone. This effect is minimized with gas metal arc welding because of the higher welding speed utilized. This is not a factor in the base metals that are welded in the soft condition."

source: www.key-to-metals.com/Article35.htm

Sherwood

Sherwood,
Thanks for your input. Mag can certainly easily be welded. I need to make inquiries about getting it done. I may not help in this situation, however. I will look into it further. If someone can weld it for me with the case together, I may consider it.
Pat

Here is my $0.02. I think the stop drilling and welding is the sure fire way to fix it however, I think there is another soloution. First, stop drill the gash/crack or what ever it is and fill it with epoxy (for preservation purposes). Then remove the stud, overdrill the hole, tap it and then install a case saver with red loctite (14 mm OD IIRC). The case saver will extend back beyond the length of the stopped gash/crack. Since the case saver is a steel sleeve it will not allow the crack to propagate into the threaded area. It having a larger diameter it should better distribute the load from the 10mm stud to the case.

I've had a mag case and a fan housing welded before. Worked great.

What I did was use a cone shaped grinding bit to grind the crack out. A rat tail file works fine also. Then the welder just filled the valley with the weld bead. Don't worry about going to deep. the welder can layer the bead if needed.

Find a welder who's welded magnesium before. At the very least they should be well versed in welding aluminum. Even expert welders will struggle with magnesium so finding someone who's done it before is a plus.