What do we like for fasteners on turbo hot sides?

I put this turbo setup together about 3 years ago, and put 8k street miles and a HPDE on it. This is a low-budget turbo LS--twin T28s on an L33. Anyway, the passenger manifold cracked (probably unrelated to the fasteners, pretty sure the head and/or flange wasn't flat) and while pulling it apart, I got to thinking about the different studs and nuts I used, and how they held up, so I decided to submit my results to the GRM hive and ask for opinions.

Manifold-to-head: Black oxide "alloy steel" studs with BMW copper-plated lock nuts (CRP brand, I think). These worked awesome. They seemed to stay nice and tight on the car and never needed to be re-torqued. The studs spun out of the head easily, and the nuts were all easy to remove from the studs with jam nuts. Nothing was seized and the nuts still have a nice locking action, so I'm actually planning to re-use everything, with a dab of copper anti-seize this time to replace any of the lost copper plating.

Turbo-to-manifold: Same setup as above, totally different results. The studs spun out of the cast iron manifold easily enough, but the nuts were wickedly seized on the studs. I eventually got them off with PB Blaster and an impact, but I mangled one of the studs and one of the nuts in the process. The copper plating on the nuts turned dark bluish-gray from the heat--or maybe the copper cooked off and that's the steel underneath? Anyway, these stayed tight while on the car, but the copper-plated nuts don't seem to tolerate turbine housing temperatures, so I think I should use something else here next time.

Downpipe-to-turbo: Cheapo eBay (304-ish?) stainless studs and nuts. These were left over from a generic LS header stud kit; I used them on this downpipe because I had run out of other hardware and I just wanted to get the car back together. They are almost mirror polished, and the threads are so smooth and loose-fitting that when I was using them to hold headers on, they would loosen up over time, and I had to keep re-torquing them. They seemed to work just fine on the downpipe, though. The internet is full of dire warnings about using stainless on stainless, especially with heat, everything will gall terribly and it will be a disaster! But nah, these were fine. They still seemed to be just as tight as when I installed them, and man, did they ever come apart nicely. The stainless is all rainbow colors now, but the threads are all perfect, and as a bonus, the studs have nice broached hex ends. I'll probably re-use these too.

So, what does the hive think? I was pretty happy with 2/3 of my choices, but I'd like to find something a little better to attach the turbo to the manifold with. Maybe I should try some of the stainless stuff there based on its success on the downpipe? Visual aid below (mangled stuff not pictured, threw it away already):

The head has coolant in it, so the manifold-to-head studs don't see anything close to the temperatures that manifold-to-turbo or turbo-to-downpipe do.

For track use on a turbo Miatas people have mostly settled on inconel studs for the manifold-to-trubo hardware, with stainless nuts and either stage 8 locking hardware or Nord-Lock washers.

Some people need the inconel on the turbo-to-downpipe studs, others find that works OK with normal stainless ones.

As far as the cracked manifold goes, was it cast or fabricated/tubular?  Making fabricated turbo manifolds last on track is hard, cast tend to be much more robust.

Turbo to manifold is the tough one. We used to use stainless studs, now we use Inconel and problems are very unusual these days. IIRC there are some factory Inconel options (Nissan?) where you can get good studs for cheap. 

Also, any gasket that touches the turbo seems more likely to cause a leak than prevent it. We haven't used them for decades, the only time customers report leaks is when they decided they needed a gasket...

Keith Tanner said:

Turbo to manifold is the tough one. We used to use stainless studs, now we use Inconel and problems are very unusual these days. IIRC there are some factory Inconel options (Nissan?) where you can get good studs for cheap. 

Also, any gasket that touches the turbo seems more likely to cause a leak than prevent it. We haven't used them for decades, the only time customers report leaks is when they decided they needed a gasket...

Ooooohhhh.... that's good to know.

Google says RX7 turbo studs for 10mm and 300zx stuff for 8mm. Does that jive with what you remember?

As others have said: Inconel is the way to go. Flyin' Miata sells great stuff for Miatas:

https://www.flyinmiata.com/default/stage-8-hardware-with-inconel-studs.html

Which solved our endurance race car's turbo-fall-off problem:

https://grassrootsmotorsports.com/project-cars/1996-mazda-miata/project-turbo-miata-post-race-maintenance/

codrus (Forum Supporter) said:

The head has coolant in it, so the manifold-to-head studs don't see anything close to the temperatures that manifold-to-turbo or turbo-to-downpipe do.

For track use on a turbo Miatas people have mostly settled on inconel studs for the manifold-to-trubo hardware, with stainless nuts and either stage 8 locking hardware or Nord-Lock washers.

Some people need the inconel on the turbo-to-downpipe studs, others find that works OK with normal stainless ones.

That makes sense. Thanks.

As far as the cracked manifold goes, was it cast or fabricated/tubular?  Making fabricated turbo manifolds last on track is hard, cast tend to be much more robust.

Cast iron. I bought it used and I suspect it wasn't totally flat. It cracked at the last bolt at the end of the flange--same place that Chevy trucks usually develop leaks, except the failure mode there is that the heads pop off the bolts. I had a pro weld it back together, he said he used a powder spray welding process with some kind of magical alloy. I had the flange resurfaced and hopefully it holds up this time.

I have an Audi A4 race race with the 1.8t engine.  Turbo to manifold bolts are stock Audi bolts and I've had no issue with them.  Factory setup for mounting the downpipe to the turbo was studs and special nuts.  Nothing I did would keep the nuts tight so I switched to standard black oxide coated allen head bolts with lock wire.  They had to be trimmed for length so they would be tight and not bottom out in the turbo.  I've never had an issue getting them out and with the lockwire they've never come loose in use.

Keith Tanner said:

Turbo to manifold is the tough one. We used to use stainless studs, now we use Inconel and problems are very unusual these days. IIRC there are some factory Inconel options (Nissan?) where you can get good studs for cheap. 

Thanks! I'll be looking into those OEM options to see if I can find anything that will work for me.

Also, any gasket that touches the turbo seems more likely to cause a leak than prevent it. We haven't used them for decades, the only time customers report leaks is when they decided they needed a gasket...

I am slowly climbing aboard the "copper high-temp RTV instead of turbo gaskets" bandwagon. The instructions for this turbo kit actually specify that's all you should use.

Thanks for pimping the FM studs, Tom, but they are custom-made for our application and may or may not suit something else. We have the measurements on our site, I would recommend confirming they'll work before picking some up. I don't remember the specific applications for the OE Inconels, sorry.

We don't even use RTV. Just go metal to metal. If your flanges are flat, you're good to go. If your flanges aren't flat, the solution is not RTV, you just need a Bridgeport like Tom's.

In reply to ColoradoBob :

ColoradoBob, would your Audi happen to be a silver '99 you bought from a guy in Ohio??

As in, my old car?

No specific knowledge but avoid stainless fasteners in general if you can. Galling is very real, and I suspect yours were plated or coated in something. Antiseize works well if you use it judiciously.