Turbo to Exhaust Manifold Nuts - and why we have to be careful when we replace them, with an extra section on general fastener information.

Post Date:    Fri, 20 Jul 2001
Subject:      Re: Turbo to manifold mounting problems Exhaust Question
List:         imon@lists.merkurowners.org




Andrew wrote:

> I am having a really hard time keeping the
> nuts from backing of the turbo mounting studs.

I suspect you replaced the nuts with a non-mechanical locking fastener when 
you put everything back together.  The original nuts where of the prevailing 
torque type with either stoverized or crownlock tops.  This means that the 
top of the nut has been slightly reshaped to deform the inner threaded 
section.  It works similar to the more obvious and familiar 'nylock' nut, 
with the nylon insert.

If you look at the top of the nut, you should see that the opening has a 
slightly oval shape and areas where the top of the nut have been squeezed or 
there are three stamping marks (typically) on the top of the nut.

We have some interesting fasteners on our XRs (if you're in to that kind of 
thing).  The exhaust manifold to head bolts are of locking design too.  The 
cross section of the minor diameter (the valley of the threads, so to speak) 
is not a circle.  It's sort of like a really rounded off triangle (often 
refered to as a 'tri-roll' in the fastener industry).  If you look at one of 
these bolts you'll see high points of the rounded off triangle if you notice 
where there is shiny metal in the minor diameter.

It's things like this that make it difficult to replace some of our 
fasteners.  It also requires that you pay a little attention sometimes.  For 
example, I almost ran a die up and down all the exhaust manifold bolts before 
I put things back together this week, until I noticed that the threads looked 
a little odd.