Cyl. Head Torque question???

[Volky]

Good day,

I just finished re-installing the heads due to a compression leak on the 1/2 side. The case I have is an AS41 with 8mm studs which should be torqued at 18 ft-lbs. as per Haynes book. A friend of mine stopped by to help me torque the flywheel and said that I should torque the heads to 25 ft-lbs.

Anyone torquing that high? Or just 18 ft-lbs.

Thanks,

[Clonebug]

I torqued mine to stock specs on my 1679 turbo.

I wouldn't think you would want to torque the 8 mm studs tighter..........

[Steve Arndt]

25 is for 10mm studs
18 is for 8mm

[ONEBADBUG]

When I use Chromoly 8mm studs, I go to 24 ft. lbs.
18 for stock.

[rs58rag]

Bugpack 8mm chromoly. I used 21 lbs. torque. Stock 8mm I use 18 lbs.

[Marc]

For OEM German 8mm studs I've found that 21 lb-ft is the maximum safe torque (assuming clean, dry threads)...and that's only with a torque wrench that you trust
Assuming you're not in a rush, take 'em to 18-19 and come back the next day to recheck at 21...beyond that is risky business in my experience.
As for CrMo, I suppose that depends on whose. ONEBADBUG, what brand have you found to be good to 24?

More often that not, breakage occurs right at the end of the threads at the inboard end. Personally I believe that it's best to only run the studs into the inserts finger-tight so as to not overly stress them there.

[miniman82]

I left mine at 20, it's only 2 more than the recommended 18. No leaking at all on a 1915 turbo.

[Ol'fogasaurus]

An interesting question. Out of ingnorance on my part: what kind of torque will the threads in the block/case take. You are torking the heads down and raising the pressure in the cylinders which has to add more load in each cylinder. I would think the studs should be able to take more than the case.

Lee

[Marc]

8mm head studs screw into steel case-savers which are m12x1.5 on the outside (compare to factory 10mm studs, which screwed directly into the alloy)...the ultimate tensile strength of the studs is the limiting factor with 8mm studs.

The raison d'être for the small studs is that their thermal expansion rate more closely matches that of the heads & cylinders, which keeps the clamping pressure of the heads against the cylinders more consistent through thermal transients - even with the lower fastener torque they're less likely to develop compression leaks than the 10mm, at least on a mild engine. With high C.R. or boost pressure the small studs don't have a lot of safety margin left - just a little detonation is enough to make one fail, so one should avoid starting out too close to their plastic deformation zone.

[Chip Birks]

I took my raceware wannabe Bugpacks to 20 the last time I built it. Its seen 28psi and isn't leaking as far as I can tell.

[ONEBADBUG]

I buy most everything from CB Performance, I don't know the actual manufacturer. On my Bonneville turbo engine, I went out hooning around the autocross course once I got home, forgetting about the much lower altitude and ambient pressure, blew the 92 cylinders apart. The heads were still at the installed torque value, that tells me the studs are doing there job.

Oh, also, if you think about it, CV bolts are 8mm, and torque spec is 24, so I see no problem with that number for head studs, assuming quality material.

[Marc]

...CV bolts are 8mm, and torque spec is 24, so I see no problem with that number for head studs, assuming quality material.

Material quality is the question. The cheap "white-box" replacement studs that were coming from Mexico years ago were known to fail at 18 ;(

CV bolts don't have to deal with thermal cycling - the tensile stress on them remains constant...

[Ol'fogasaurus]

Question restate: I was aware of the inserts but isn't there some cases (year/serial number) you just want to stay away from for something like this?

[Marc]

`68/`69 H5/B5 cases are infamous for their inferior alloy (the "cottage-cheese" cases) which suffered torn-out head studs even in stock applications. They also were the worst for warpage & cracking. I've been assuming for purposes of this discussion that those door-stops are off-the-table; not likely that there are too many still around...even in marginally-serviceable condition, let alone the possibility that anyone would choose one as the foundation of a turbo motor. H5 cases probably served as the inspiration for "case-saver" inserts (as well as the Mickey-Mouse self-tapping repair studs)...about the only positive thing one can say for them