Difficulty installing 99 twins onto manifold studs?
 

So this is unexpected. I’ve got an engine out of the car on a stand and I’m trying to install a very gently used set of 99 twins onto the stock exhaust manifold. Thing is, the studs don’t seem to quite line up. With all six studs in place they will not cleanly pass through the turbo manifold. It’s like it’s a mm or two off from the holes in a couple spots.

Do the manifolds ever warp enough over time to be out of spec? I wouldn’t expect that. I’m using used turbos and a used manifold that I’ve sourced from separate places. New studs. Is it possible that I just need to dremel out the housing in one or two of the stud passages, or does this mean something is screwed up somewhere else? I figured this would slide right on with at most some shimmying and cursing but it definitely does not fit.

I've run into this when putting together a good set of twins out of multiple used sets. A solution that worked for me was to remove just enough studs off the manifold to get the turbos to mount onto it and then reinstall the studs that were removed.

That’s really helpful to hear it’s not just me. I guess the tolerances just aren’t exact when mix-matching parts.

I already tried to put the turbo on first and mount the problem studs after, but the first one I tried wanted to start cross-threading on me. I think I might just need to make some really small enlargements in a few of the turbo manifold passages.

Try leaving a gap between the turbos and the manifold and see if that helps you get the studs on without cross-threading them.

Open up the holes a bit on the rubos on the ones that are binding. Thats what i ended up doing

Local guy just had a similar issue. Said he got a replacement manifold from Mazda (i think) and it did not line up. Also they stripped out a stud hole on their existing and were at a loss to try and repair.

I had/have a few spare manifolds and it seems the one I gave him lined up fine enough.

There were at least three twin turbo iterations: 255 hp, 265 hp and 280hp, but I have not heard of the manifolds being different.

My S8 parts manual has the manifold reference number as 13-450 and the part number as N3A7-13-450C.


The JDM S6 Version one original parts catalogue has the part number as N3A7-13-450. (i.e. no "C")

The version 4 catalogue has the part as N3A7-13-450B.

The version 2 and version 3 catalogues have the part as N3A7-13-450A but after Vin number 301812 the part switches to the "B" variant.

I have no idea of the significance of the number changes.

I believe some very late FC got twin turbos as well, and I wonder if the manifold might have been different on them.

Is there a part number anywhere on the manifold and or turbo?

At some point I plunked 1992 255 hp twin turbo onto my S8 engine's manifold with no issues (or perhaps the manifold had already been swapped out by a PO).

Definitely the turbo I took off was a 265hp.

I just reviewed the Japanese FD Model intro manual and the change manuals up to S8. I did not see any mention of a change of the manifold from when is was introduced in JDM Version 1. Changes of the turbos are mentioned, but the manifold does not figure in the diagrams.

The Oct '94 "final" USDM parts catalogue lists the part as N3A1-13-450A.

The significance of the "N3A1" versus JDM N3A7, I do not know.

Most likely the replacement turbos are clocked slightly off. Loosen all the bolts holding the turbine housings together and bolt them to a manifold. Then tighten what you can access. Remove them, tighten the remainder of the bolts and reinstall the turbos. Its easier to do this on a manifold out of the car in a vice or something.

I wonder if the wrong size studs might have something to do with it. The simply will not turn in.