Magnesium trans case corrosion - can I use this?

[j_mancini]

I’m rebuilding my 915, which happens to be a mag case 1976 (915/44 716 0282). I got the cases cleaned at a machine shop, and upon close examination I am finding a bunch of corrosion. The exterior is fairly rough:







I am also finding some pitting on the case mating surfaces:





And also in the shift shaft hole (main case, 3-4 rod):



So, before I spend time and $ putting this back together – can I use this case? Or am I just asking for trouble? Bearing races are tight and I am not finding any cracks, just this pitting. Thanks for your input....

[ischmitz]

I have seen worse. I would use a straight blade to scrape and clean the mating surfaces. Keep the blade at nearly 90 degrees to the surface and work carefully until you got a nice plane shiny surface. That'll make sure the thin paper gasket will properly seal and the tranny will be leak-free.

Ingo

[j_mancini]

Thanks Ingo, will do. Do you think the pitting in the shift rod hole is an issue? There seems to be a fair amount of shift rod play anyway, so maybe it's not that big a deal?

[dvkk]

Lay a sheet of 220 grit sandpaper on a piece of flat glass then sand your gasket surfaces. Scraping them will just make paths for leaks.

Looking at the bore for your shift rod, you can see how the chamfer is completely gone on the worn side. What does the diameter measure at? I bet it's got nearly 1mm of wear on that one side.
The cure is to bore it out oversize and install a bronze bushing. Ream to fit. This will last much longer than the original part did.

[sc_rufctr]

Most of what you've shown us is cosmetic only... But the shift rod bore definitely needs attention.
The fixes suggested above would resolve your issues...

How are the bearing races in the final drive? Are they secure? Is the area around the races flat/undamaged? Are the studs secure?
Take a photo of this area and post here please.

[E Sully]

I'm curious. Did the shop know the case was magnesium and use a cleaner that was safe for it. It seems strange that the mating surfaces would show corrosion unless they were separated for a while.

[j_mancini]

Races are all good and tight, will post photo evidence tonight!

[euro911sc]

You can smear a little Loctite 571 on the paper gaskets when you put it back together. That will help it stay very dry and does not add any noticeable dimensional stack. It will seal up any pitting or scratching.

I would not be too concerned w/ the shift shaft hole. The clearances on the whole shift shaft and brass follower will make the little corroding of that hole a non-issue, IMHO. How much play do you have compared to the other shaft?

-Michael

[jcge]

Suggest you give it a coat (after assy) of a sealer like Tectyl 506

[j_mancini]

"How are the bearing races in the final drive? Are they secure? Is the area around the races flat/undamaged? Are the studs secure?
Take a photo of this area and post here please."

Here are some photos of the races:







The input bearing seemed to "walk out" during cleaning, my guess due to heat. It's quite tight like this, but obviously not seated.





I installed the shift rods and detents - to see how it would work:




My guesstimate of the play is about 1-2mm at end of rod. The 1-2 rod seems to have about the same play. With the intermediate case in place, everything seems to move as it should. My conclusion is that the corrosion is not affecting the rod movement much. All the studs seem tight, but I haven't torqued any nuts on them yet.

[euro911sc]

Are you saying that the input bearing came out of the bore on hot tanking? I would be concerned it would do that when the trans gets nice and hot... Even with mild driving it may walk a little each time and next year or two you may be back in there fixing it and the collateral damage. You may be able to pean it just a little tighter, but I would want comentarty from an expert on just how much to do it.

Tectyl is great but looks like crap! Someone suggested some kind of special oil for the Mg fans and carriers to keep them from corroding... I forget what it was called though. Anyone? Bueller?

Best regards,

Michael

[JMH82SC]

Originally Posted by euro911sc

Tectyl is great but looks like crap! Someone suggested some kind of special oil for the Mg fans and carriers to keep them from corroding... I forget what it was called though. Anyone? Bueller?


Michael

http://www.roadsters.com/gibbs/#intro

[dvkk]

Originally Posted by j_mancini

I installed the shift rods and detents - to see how it would work:



My guesstimate of the play is about 1-2mm at end of rod. The 1-2 rod seems to have about the same play.

That doesn't look like 1-2mm in the pic. Can you measure it?
If it is indeed 1-2mm, that's pretty bad. I'd rebush them both. When you ream the bushing, shoot for .1-.2mm of clearance max.

[pwd72s]

Originally Posted by jcge

Suggest you give it a coat (after assy) of a sealer like Tectyl 506

Good advice! (edit) You might contact RoninLB...he may have some material left after coating his transaxle.