Exhaust manifold thread destroyed

[markintheair]

Hi y’all,

I’ve been driving around my restored 81 FI for a couple of hundred miles now, but a few weeks ago the car started leaking oil on the exhaust side. I’ve been checking the forum here, and took a good look. The oil was leaking out of the left upper exhaust thread, the right one was dry. Upon inspection I saw that the M8 manifold studs were way too long, and took them out. The one that was leaking was actually not even a stud + nut, but just an M8 bolt. Recipe for disaster it seemed.

I purchased 5 new exhaust manifold studs M8 with corresponding locking nuts and thought I would just replace the incorrect studs. When I started to remove the studs I noticed a lot of aluminium residue on the nuts… After trying to install the new studs I found out the thread is almost completely gone inside the head, the new studs won’t even stay in (except for the middle one).

Long story short: the thread in my head on the exhaust side is gone, do you have any tips on rethreading these holes? I know the two upper ones are not blind (hence the oil) so rethreading might be a bit risky, as the leftover material from rethreading might enter the engine.

Any help would be appreciated!

[ORFORD2004]

helicoil time

[markintheair]

Helicoil looks promising, but won't this be a problem as the two upper threads enter into an oil channel on the inside?

[wizard124]

I remember I had one stud which was leaking oil. Maybe the threads were a bit loose or stripped.

IIRC, I cleaned out the hole with a solvent then seated the stud with some JB Weld.

[otter]

I had a plug blow out on my 911 and I used a Threadsert and a special jig to rethread the head. Obviously a spark plug hole is not a blind hole, and it's a high-stakes one, too. The usual technique of loading up the cutting tool with heavy grease to catch the material, and going slowly in a few steps did me fine, and I think it'll do you fine here, too.