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Re: Torque Sequence

To: mgs@autox.team.net
Subject: Re: Torque Sequence
From: Chris K <ckotting@wideopenwest.com>
Date: Sun, 16 May 2004 07:25:43 -0400
>Date: Fri, 14 May 2004 13:38:20 -0500
>From: "Larry Daniels" <ladaniels@sbcglobal.net>
>Subject: Torque Sequence
>
>Hi gang,
>
>It's Friday afternoon and I'm sitting at my desk thinking about MG's instead
>of working.  (Shhh)
>  
>
Good for you!  Everyone needs a "brain break" now and again.

>I have often wondered why the sequence for unbolting the head is the same as
>for bolting it down.  If the idea of starting at the center and working your
>way out is to prevent putting unnatural stresses on the head, why wouldn't one
>do the exact reverse sequence when releasing the pressure on the head?  It
>would make sense in my feeble little mind that, in either case, one would not
>want the ends to be tightened while the middle of the head is not.  Where am I
>going astray here?
>  
>
I think because you're thinking that each nut gets tightened/loosened 
all the way in one pass.  Quoting the Haynes manual for the MG Midget 
(the closest one on the shelf at the moment):

"Following the sequence...tighten down the nuts  >> a part of a turn at 
a time << to the torque wrench setting quoted in the Specifications."

I usually tighten each nut about 5 ft./lbs at a time, and make as many 
as 9 passses or so around the tightening sequence after I've spun the 
nuts down to "finger tight".  I'm a bit anal about it, but the point is 
that it's gradual, and by working repeatedly around the order, none of 
the head studs has significantly more tension on it than any of the 
others, keeping things flat as they get pulled together.

>Also, I read that when retorquing the head one should -- again in sequence --
>loosen each nut a quarter turn and then retorque them.  I'm not clear on
>whether all of the nuts should be loosened and then all retorqued or each
>should be loosened and then retorqued and then move on to the next nut in
>succession.
>
One at a time.  The previous answer about breaking the "stiction" 
between nut and washer (and nut and stud, for that matter) is correct.  
However, sometimes you will see a specification for torquing something 
down with lubricant on the threads.  Here, you may have a lower torque 
setting, because you aren't fighting the friction. 

The really high-tech way (and more acurate/relaible) to do it is to 
measure bolt stretch, but that's even more anal than I am.

>  I wouldn't want to upset the MG gods by torquing my nuts wrong.
>That's as close to an obvious joke as I will go.
>
>  
>
Hoo boy.  Talk about your "target rich environments".  I'm not going 
there either.

Chris K.





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