Exhaust Stud Fitting

[JJ 911SC]

All my holes have been clean and I'm ready to put the new studs in.

The CAT call for M8x30 (999-062-239-02) so I order from PP 999-062-239-02/58 by OEM.

Just using finger power, the Head end (short) go in about 5mm of the 16mm.

If I use the Manifold end (long), it goes in 16mm no problem.

This is the same in all 12 holes (including the Time-Sert one). I can get the old ones going all the way with no problem.

We measure the new studs and the manifold side is 7.87mm at the end & shoulder while the head side is 8.00mm at the end but 8.05 at the shoulder.

A generic M8 stud (gently use) from the hardware store is straight at 7.83 across.

So the new ones seem to be tapering out by .05mm.

Any concern or explanation?

Long End in (left one) / Short End in (right side)



Old / New



Side by side

[Gunter]

Better tight than wobbly.

I would just double-nut the long side and screw the short side in making sure the studs extend out the same length as the old ones.

Use Loctite if called for.

[JJ 911SC]

I agree and can drive it down no problem...

I just wonder why they are made like that.

I'll get one from Porsche and compare the mesurment.

[James Brown]

This is common in roll thread manufacturing vs. cut threads. I think the Porsche studs were cut threads back in the day. Now, it's cheaper to make roll threads and there stronger. As long as they don't bind up, you should be fine. Test fit them first prior to putting the loc-tite on them.

[Gunter]

Amen.

[Canuck_Targa]

I always believed that the short end that goes into the jug was always an interference thread fit because even though you want them removable, you don't want them screwing out every time you are removing the manifold nuts. In my experience it is like that on all makes, Porsche, Chevy, Yamaha...........
The only time I think loctite is required is if you do have a problem with the studs backing out. Using loctite just makes them that much tougher to get out when you go to replace them again.
But thats just my opinion.

[JJ 911SC]

Originally Posted by Canuck_Targa

I always believed that the short end that goes into the jug was always an interference thread fit because even though you want them removable, you don't want them screwing out every time you are removing the manifold nuts. In my experience it is like that on all makes, Porsche, Chevy, Yamaha...

Yeah, it has to be done on purpose as the long end goes all the way down the hole using finger power

[James Brown]

I thought you had to seal them so the coolant wouldn't leak out.

[cgarr]

Do I see something different? It looks like the old stud is rolled, larger thread dia than the rod and the new one looks cut? When we roll threads you end up with a larger thread than the rod you start with but then I have not rolled threads since I built my plane years ago.....

[JJ 911SC]

Originally Posted by James Brown

I thought you had to seal them so the coolant wouldn't leak out.

I'm starting the water cool conversion next week.

[JJ 911SC]

Originally Posted by cgarr

Do I see something different? It looks like the old stud is rolled, larger thread dia than the rod and the new one looks cut? When we roll threads you end up with a larger thread than the rod you start with but then I have not rolled threads since I built my plane years ago.....

If so why it apply to only one side of the stud?

[Canuck_Targa]

The fit of the thread is not determined by how the threads are machined (either rolled or cut) they are defined in classes. Class 1 having the most clearance and class 5 being an interference fit, a stud would have a class 2 or 3 on the end that you would be attaching or removing the manifolds and a class 5 on the end that threads into the cylinder head. You could have all these classes in either formed or cut threads, it is merely 2 different methods of machining threads. In the case of the picture of the studs, I think it is just two different manufacturers way of producing them (one formed and one cut).

You are correct James, if the holes were through holes into cooling jackets, you would need to use thread sealer so they didn't leak, but I haven't seen many exhaust studs that go into the cooling jackets (but that certainly doesn't mean they it doesn't happen).

[James Brown]

Lol

[JJ 911SC]

Originally Posted by James Brown

This is common in roll thread manufacturing vs. cut threads. I think the Porsche studs were cut threads back in the day. Now, it's cheaper to make roll threads and there stronger. As long as they don't bind up, you should be fine. Test fit them first prior to putting the loc-tite on them.

They went in easy.

Did a search for the torque and came across a posting from Warren (RIP) and he suggest snug to 10Nm...

So that is where they are now.

Can't find my exhaust gasket!!! Now I have to check all my PP order to see if I even order some (suffering from Craft decease which is a precursor to Alzheimer).

[James Brown]

[JJ 911SC]

Short for... or about the gaskets?

[James Brown]

yes, it translates: What the ****, DOUH, i'm screwed!!

[JJ 911SC]

No wonder I can not find them... Never order them

Order is going in tomorrow morning. If I find another $50 to spend I'll get Free delivery