Installing exhaust studs

[andreic]

Hello all,

I am in the middle of a rebuild project on a 1.8L engine that came out of a 1974 P914. The engine was dismantled by the previous owner, and the heads were rebuilt by someone I do not know. When this was done the exhaust studs were removed from the heads. I am now trying to put new ones in. (Unfortunately I did not do this while the heads were out of the engine, so it would be quite a pain now to remove the heads from the rebuilt engine...)

The first problem is that I was not able to clearly determine the size of the studs I need. Looking at the old studs (which I still have) they appear to be M8 on both ends, which is in contradiction to what the parts listing from Porsche calls for (for 1974 1.8L it calls for M9 at the head end, and M8 at the end which holds the exhaust). Can it be that Porsche made some 1.8L heads with M8 exhaust studs? Or maybe M8/9 means something else (maybe an M8 with funny thread pitch)?

So I ordered M8/M8 studs, but today when I tried getting them installed they go in for a few turns, but then they get stuck, as if the thread pitch was wrong or the threads in the head are stripped. Can it be the case that the threads got messed up when the old studs were removed? In this case, should I chase the threads with an M8 tap? Or are the studs supposed to go in with great effort (so that they lock in somehow), and I should not tap the threads?

Any help will be highly appreciated.

Thanks,
Andrei.

[Emo993]

Andrei, I'm not an expert.....but 25 years ago on my 1.8 I had replace the exhaust studs. I just looked at what I had and the head portion is 5/16 and the portion to the exhaust manifold is same size in diameter but much finer thread. Don't know the history of my engine, so it could have been modified......But, try a 5/16 coarse thread to see if it works. No, they should not go in with great effort. Mark

[Emo993]

Could have a Heli-coil or Time-Sert....That would explain why I have some studs with 5/16 on the head side still setting in the nut and bolt box....... Mark

[andreic]

Well, I tried chasing the threads with an M8 tap, and it worked beautifully, with the exception of one stud hole which seems to have been enlarged and retapped to 9mm at some point in the past. I now need to order one 9/8mm stud.

(Like everything in this project, it is 2 steps forward one step back... But at least it goes forward.)

Question: do I thread the studs so that they bottom out (but this may mean they don't in fact bottom out, but only run out of threads), or do I just screw them in until no more threads are left visible out side the head? I decided to do the latter, using blue Loctite. I expect that it will be more the total tension on the stud after the exhaust is installed and tightened with the nut than holds the stud in place, rather than the initial torque in the head.

Suggestions?

Andrei.

[Dave at Pelican Parts]

I believe you're doing it right.

--DD

[John Rogers]

From what I read, the engine is out of the car.......at least for your sake I hope so! If so here is how I used to do mine back in the day.

- Make sure the exhaust openings are taped or plugged or covered well.
- Look at each exhaust stud hole with a strong flashlight and magnifying glass to see if there are any inserts and to make sure the bottom of the hole is clear of stuff.
- Use a "bottom tap" and tap lube to chase each hole's threads. The fit should be snug and if one is loose when taping you probably have M9 threads. Get a M9 bottom tap before proceeding if needed.
- Use hi temp anti seize and screw in the studs to correct torque.
- Measure each pair to insure they are equal. If not, investigate why?
- Use new exhaust gaskets and uncover the ports. Make sure an old gasket was not left in the hole. I have seen heads come back from a shop after full overhaul and a gasket was still in the hole!
- Insert the gasket, install the exhaust header and use your strong light to make sure there are enough threads on the studs to tighten the nuts. If the nuts run out of threads you'll have exhaust leaks for sure.
- Use hi temp anti seize and install the exhaust nuts (Porsche Exhaust Nuts), tighten finger tight and then tighten to the correct torque.
- Go relax, have a beer, ETC, ETC and after a few hours, retorque the nuts.

Note, my mentor always used "Porsche Exhaust nuts" NEVER plain steel nuts although he did like to use a washer under the nuts because of the flat steel piece the nut would tighten against which would never be smooth.

Good luck