[T3] Weird Idle Dips

[Jim Adney]

On 2 Mar 2012 at 12:01, Gary Forsmo wrote:

> A question has been rolling around in my head for several months.

A fresh subject line would have been good here.  ;-)

> IF (big IF, here) indeed the spark plug hole has been cross-threaded
> at some time in the past, what is the correct remedy?
> 
> While reading about HeliCoils, I came across other metal inserts to
> repair spark plug holes.
> E-Z Lok, http://www.ezlok.com/InsertsMetal/carbonSteel.html  &
> Time-Sert.  http://www.timesert.com/html/sparkplug.html

I prefer Heli-coils, and there are several reasons why. 

1) All of these choices are made of steel or stainless, which expands 
less with temperature than alum. That means that solid steel inserts 
always get loose in the head as the head warms up. Because they are 
made oversize, helical inserts will "slip" in their outside threads 
to accommodate the change in size. This will make them tight when 
cold, but that's not a problem. But solid inserts get loose and will 
often come out with the spark plug, and you'll have trouble getting 
it off the SP to reuse, or you'll need to replace it every time you 
change plugs.  

2) The spacing between the SP hole and the valve seats, inside the 
combustion chamber, is very small, and this is where most of our 
heads eventually crack. For this reason you want to use the thinnest 
insert you can get. Helicoils and Timeserts are both good choices 
here.  

3) The most common style of solid insert has a flange on the face 
that prevents it from being installed too deep. This is what the SP 
seats against when you tighten it. It also means that when the head 
warms up and the insert gets loose, the SP is still tight to the 
insert, but the insert is really LOOSE in the head. This doesn't 
cause any noticable difference in performance, but that loose stuff 
moving around will eventually wear out the threads and require an 
oversize insert. That's why those inserts are made in oversize and 
double oversize.  

4) When an insert comes loose, the heat transfer path from the SP to 
the head is interrupted. This makes the SP run hotter than it should. 
This can be a problem. Helicoils don't get loose in the head as the 
head warms up.

5) I don't have a lot of experience with inserts, but I've had 
several Helicoils and never had a Helicoil come out, and I don't see 
how it could possibly get pushed in. I have one car with a solid 
insert, and that cylinder is a real annoyance any time I have to deal 
with that SP. Of course there are people who can screw up anything, 
but you should be careful when starting ANY SP, regardless of the 
nature of the threads. If you just assume that the threads are OE 
aluminum and use proper care, you'll never have any problem.   

6) The need for special tools is common to ALL of these approaches. 
Regardless of what you decide to use, you'll need the right tap and 
you'll need to do it carefully. Yes, Helicoils require a special 
insertion tool, but that's because they are oversize before 
installation. The fact that they are trying to expand after 
installation is what keeps them in place and makes them the right 
choice.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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