924Board.org

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

For the events leading up to this, see:

https://www.924board.org/viewtopic.php?t=44805&postdays=0&postorder=asc&start=0

So I have my head now and some spare parts as well. I've ordered most all of the gaskets, bolts, etc but I can't seem to find the exhaust manifold studs anywhere. Advice in the earlier thread was just to get something from the hardware store.

Can someone confirm the length of the exhaust manifold studs? I had written 25mm M8 w/ 1.25 pitch, but that 25mm can't be right...it's way shorter than what is currently in mine. I measure about 20mm worth of threads at each end and maybe 7-8mm of unthreaded space. None of them are free of either the head or manifold at this point though, so I may be off a bit, or I may have a non-stock part.

Is stainless preferable to grade 8 steel? Also someone in another thread mentioned copper nuts, is that correct?

Thanks...
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1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Any reason I can't use 944 studs? They appear to be readily available and they're M8x46mm. Trying to measure mine in place I got 47mm so they have to be pretty similar? The two threaded halves may not be the same length, but has anyone used these?

My local recommended "bolt shop" said the only thing they have in M8 is 50mm. That definitely seems a bit too long, not sure if it's too long to matter but it might be, although I suppose they could be cut down.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

I found this thread but that certainly does not seem to match what's on my car:

http://924board.org/viewtopic.php?t=14514&sid=0b1a863b493bdef6e91a2db55f91fac7

It does however match the 25mm that I had previously read...
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

Cedric · Joined: 27 Aug 2004 Posts: 2608 Location: Sweden

I can see if i can find my latest purchases. But I've ordered general VAG stuff both online and from the local vw shop before, both m8 studs and copper plated nuts, just be sure that they are suitable for exhaust components.
_________________
1980 924 Turbo
www.instagram.com/garagecedric/

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Does anyone know how long the threaded side of the stud that goes in the head is supposed to be? I bought some 40mm studs that are 10mm on one side and about 20mm on the other. The ones I took out are overall about 42mm but closer to symmetrical, so whichever side I screw into the head will be either longer or shorter than what came out. I want to avoid both too short to hold as well as long enough to do damage when being torqued down.

I still can't imagine how 25mm could be long enough...
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Regarding manifold studs, the closest I could find locally are 10mm on the short side and 40mm overall. So, shorter on the head side, longer on the outside, but I checked and it seems to fit OK, so that's what I'm going with. Any thoughts on copper antiseize? I'm planning on using it on the studs, stop me if you disagree.

Planning on putting the MLS head gasket on dry...what about the pile of paper gaskets I have...do those go on dry? Or do you use some sort of sealant usually? Thinking distributor housing, intake manifold, tensioner/thermostat housing, etc etc....
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

One other thing...the ARP head studs came with their own lubricant, which I'm using, but they mention "if any of the studs penetrate the water jacket, use ARP sealant." Now...I don't think any of ours do? Please correct me if I'm wrong....but I do seem to have oil in two of the stud holes. Is that normal?
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Just an update: There's no update.

That's not entirely true...I have the head, I've been putting the bits on the head that I had taken off (e.g. thermostat/tensioner housing, exhaust manifold, the stuff on the back like oil pressure switch, distributor housing, that thing with the coolant temp sensor (?) not sure what to call it) and a good part of that time was waiting for some gaskets that apparently took the long way around the world to get here.

Right now, I'm trying to figure out the best way to clean the head gasket surface on the block. The gasket seemed to come off pretty easily in one piece, there are no chunks of gasket left, but there is some sort of residue. It just feels a bit tacky, and from what I've read the Cometic gasket needs that surface to be SMOOTH. So I'm thinking of getting one of those Roloc bristle brushes, if it will attach to a grinder, and using that. I just...do not want to do this twice!
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

anthonimartini · Joined: 17 Feb 2020 Posts: 240 Location: portland oregon

honestly since you dont have a freshly decked block i would just use a victor reinze head gasket or at the very least use copper spray on the cometic. as for the head studs, they do not go into any coolant or oil passages.
_________________
ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
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formerly known as turbellion, lost password.

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

Thanks for that info. I already have the cometic gasket so I'd like to use it...given that they say not to use the spray (of course they also say it has to be smooth to a certain standard that I can't measure) is there any downside to the spray?

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Try not to round off edges. If it were me, I would carefully scrape preferably with something plastic then follow with a flat object wrapped in 1500-2000 grit sandpaper. Piece of ground steel would be best.

I just went through this with an assembled block out of the car. The PO had used some sort of motorized brush which scratched and rounded corners. Based on the same things you read I didn't think that would work with a standard cometic.
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

MOTANUL · Joined: 19 Nov 2018 Posts: 214 Location: ROMANIA BUCHAREST

bureau13 · Joined: 07 Sep 2017 Posts: 480 Location: South Florida

I have a head gasket question. If I look at the block, I see numerous "passages" that look like coolant or oil passages (mostly coolant, I would think). For instance, there are four such openings surrounding each cylinder bore. I see this on the head as well. So, given that these appear to be matching coolant passages...how is it that, for most of them, there is no opening in the head gasket? Am I misinterpreting what these openings are for?
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

924RACR · Posted: Sun Oct 24, 2021 8:41 pm Post subject:

That's a common question about the head gasket; the reduced size passages are to control flow, and are correct.

Regarding paper gaskets: I learned a long time ago to soak those with oil before installing. Opens them up some to make them softer and seal better, plus replacement later is much easier, little or no clean-up. Great tactic on a racecar, and often can re-use the gaskets without issue.

Head surface finish for Cometic gasket: yes, that's a surface finish spec. I actually did this for my 931 rebuild one prior (not the one I'm just finishing up now), since I did get a Cometic HG. In my case, I took it to my machine shop sponsor and manually honed the surface of the deck using a honing plate, grinding paste, and muscles. Bit of a pain, but now it has a nicer head surface. The original top/deck surface was quite poor and probably would not have sealed with the Cometic.

And yes, they showed me the surface finish measurement device; not complex, but such a thing does exist.

For contrast, my GSXR blocks (in my other racecar) use an MLS head gasket standard from the factory; they have basically a mirror-polished head mating surface. Not that we needed to go that far.

So if you're dying to use a Cometic, it'll be a lot happier if you have a machine shop refinish the deck surface of the block... but as long as your block isn't notably pitted, you should be OK. If it is, I'd stick with the stock HG if you're not going to have it refinished.

For my current rebuild, I'm just using a stock HG, for cost.

On the studs - yeah, I found the same oddity; Porsche specs the stud length by threaded exposed end, but everywhere you buy them they're specced by overall length. But if they're too long overall, for the head/manifold flange you may struggle to get the nuts on and off... and on the turbo flange you may struggle to even get the turbo into place (as I do, I've got one that's a bit too long in the exhaust manifold and I need to swap in a shorter one next time, can barely even get the damn thing in at all, thanks to the stupid insertion path with studs on different axes)...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Cedric · Joined: 27 Aug 2004 Posts: 2608 Location: Sweden

After my experience with the HG leaking again(machined head+arp+cometic) , i wouldn't trust the cometic on a non machined and perfect engine block, i wish id just put a standard gasket in there. Now i have to build a new engine instead, and this time i will try to get the block perfect. Ra says nothing about flatness and only a little bit about surface roughness, at least it shows their intentions of a smooth surface.
_________________
1980 924 Turbo
www.instagram.com/garagecedric/