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Head stud stretching problems (Read 30 times)
dcarbone
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Head stud stretching problems
09/18/20 at 08:07:01
 
Hey all,

I am attempting to replace the head gasket on a 2007 S40 and having a problem getting the head to tighten down.

It seems like the head studs themselves are endlessly stretching.  I am using a progressive tightening approach:

20ft/lb -> 40ft/lb -> 55ft/lb

All of the nuts tighten to 20ft/lb ok.  When I try to go to 40ft/lb, the OPEN top nut always tightens just fine, but twice now the other three never break the wrench.

When I first attempted this, I purchased new copper washers but re-used the old nuts and studs. During the 40ft/lb phase, one of the studs actually snapped off near the bottom of the nut-end threads, and it looked like it was significantly skinnier than the rest of the stud, leading me to believe it was either twisting or was somehow structurally compromised.

The 2nd attempt with a new set of 4 head studs, but re-using the new copper washers and original nuts, again the open top nut tightened down just fine.  The other three, however, exhibited similar behavior to before where they simply never get tight.  This time, one of them even started popping THROUGH the top of one of the closed nuts!

I'm at a bit of a loss here.  I've never encountered head studs that stretch this much.

I must be doing something wrong here.  Any ideas?
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DragBikeMike
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Re: Head stud stretching problems
Reply #1 - 09/18/20 at 11:27:02
 
"I must be doing something wrong here.  Any ideas?"

Yes, you are doing something wrong.  You are grossly over-tightening the fasteners.  The torque for the 9mm studs is 21 - 24 ft-lbs.  The specified torque for the 8mm studs is 16.5 to 19.5 ft-lbs.  Where did you get the 55 ft-lb value?

This old post provides a study I did on the 9mm studs.  It might be helpful.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1576269113/12#12

Make sure you do a thorough inspection of the internal threads in the aluminum crankcase.  You may have damaged them.  At least you proved that the fastening system is designed correctly.  The stud should break before the internal threads fail.

I imagine you have also galled the aluminum spot faces under each acorn nut.  You probably should try to clean those spot faces up but you must keep them flat and perpendicular to the stud.  The copper washers are toast, and the nuts are now suspect also.  Replace them all.

The studs are suspect too.  In all likelihood you took them well past yield strength.  All must be replaced.

Please share where you got the 55 ft-lb torque spec.  It will help all the other forum members if we know where that bogus value came from.  Pictures would also be most appreciated.

Good luck, Mike
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