Pelican Parts Forums - Crack in 2.7 magnesium case

Pelican Parts Forums (http://forums.pelicanparts.com/)

- 911 Engine Rebuilding Forum (http://forums.pelicanparts.com/911-engine-rebuilding-forum/)

- - Crack in 2.7 magnesium case (http://forums.pelicanparts.com/911-engine-rebuilding-forum/913811-crack-2-7-magnesium-case.html)

Crack in 2.7 magnesium case

I'm in the process of stripping down a 1976 2.7 engine with unknown history. So far I've found a lot of bodge repairs, all of them possible to solve by getting new parts. The exhaust was patched up, the cooling fan ruined by running a generator with bad threads, 901 timing chain tensioner on one side and 930 on the other and so on.

http://forums.pelicanparts.com/uploa...1462965575.jpg

This last problem looks more complicated. There is a crack around one of the studs where the driver's side timing chain housing attaches to the case. There is obviously not a leak from this area but I don't like the thought of leaving the crack as it is either.

http://forums.pelicanparts.com/uploa...1462965472.jpg

http://forums.pelicanparts.com/uploa...1462965663.jpg

Does anybody have expericene with welding a magnesium case? Would it require the entire case to be pre-heated to minimize heat-stress from the weld? I assume it would be better to have the case halves bolted together during the welding process, for increased support against heat deformation.

If it doesn't leak I would consider drilling a small hole at the end of the crack to blunt it out and fill this with something like Belzona 1111.

If you drill into the case to about 50% of its depth it will probably be enough to arrest the crack

If you carefully removed the stud then re-tap so the thread fit is slightly loose and fit a new stud this will also reduce the 'bursting force'. The loose thread will reduce the hoop stress in this area.

The case has probably burst due to corrosion products in the thread expanding as the bolting forces in this area are quite low.

Even a loose thread should be OK for the low preload you need in this stud.

Quote:

Originally Posted by chris_seven (Post 9115495)

Chris, do you think installing a helicoil could make the thread stronger?

I don't think you need to make this thread stronger . In fact a loose thread with some red lock tight would be all you need.

All this does is hold the chainbox and compress a gasket. no stresses on it other than thermal expansion.

The use of an insert would make the thread stronger in tension but would maybe increase the hoop stress while it was being inserted.

I would agree that a loose thread with Loctite will be OK.