9mm? 8mm? or both?

[abgwin]

Recently had some work done on one of my other cars to fix an exhaust leak and mentioned my Beta was very noisy in the exhaust. To my surprise, the shop owner not only knew what a Beta was but was eager to take a look for me.

They quickly replaced the gasket between header and downpipe but then found 2 missing studs on the manifold itself. I ordered the 9mm studs from Midwest Bayless but the shop said they wouldn't fit.

Here's the weird part: they claim that only an 8mm would thread into the inner head but the outer thread was 9mm. Their head technician is a really odd guy who's hard to understand, so I'm not entirely clear on all his comments, but something about a stepped stud was the old school fix. He fabbed up something from 8mm studs and all sounds great now, but of course I'm left with very expensive worthless-to-me 9mm studs and a very puzzled look on my face.

Does anyone have any experience with anything similar?

I've looked through the reams of service receipts on my car that date back to the early 90s and while there's two incidents of exhaust work, no mention of anything other than 9mm studs, though there was a valve job done at one point. Is there any other head, Fiat perhaps, that fits the 2.0 but has different fittings that might have been installed in place of the original?

[Ammy]

Studs used on a number of cars have two different threads,  coarse into the block and fine for the nuts.

[abgwin]

Yes, the studs Midwest Bayless supplied are those - fine and coarse - but 9mm. And don't thread. Weird, eh?

[HFStuart]

The only reason I can think to have 9mm studs is if the originals have stripped the threads in the head as the pitch for M8 and M9 is the same, making the repair easier. Beta manifolds do sometimes pull the studs from the head if the downpipe isn't secured correctly.

The only cars I know with M9 (and M7) threads as standard are older French models.

[lukasdeopalenica]

Originally the studs have M8 threads.

[abgwin]

As far as I know, and along with documentation of work done previously, US spec fuel injected 2.0 exhausts used 9mm.

http://www.midwest-bayless.com/p-7210-82378949.aspx

It's possible that a mechanic back in the 90s didn't actually do a valve job but put on a different head and used stepped studs, 8 AND 9mm. That would explain the situation, if earlier heads actually used 8mm, as the difficult to understand guy at the muffler shop mentioned "stepped studs" but didn't elaborate.