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RE: heli-coil or ?

To: <shop-talk@autox.team.net>
Subject: RE: heli-coil or ?
From: "Randall Young" <ryoung@navcomtech.com>
Date: Sat, 25 Jan 2003 07:04:11 -0800
> I have an after-market intake manifold for my 51 dodge truck.
> I just got the exhaust manifolds back from the guy who split
> them and it into a dual-exhaust.  So far so good, however,
> the intake manifold bolts to the exhaust manifold with four
> 5/16 x 18 bolts.  The intake manifold however has 3/8 x 16
> holes tapped.

I assume you're saying the bolts go through the exhaust and thread into the
intake; and also that you cannot drill the exhaust to clear a 3/8 thread.

One option might be to use a stepped stud, with 3/8 threads into the intake
and a 5/16 shank sticking through the exhaust, with a nut where the bolt
head would be.  I'm not sure where to buy these, but I know I've seen them.
Check out ARP's P/N 100-1401, it might be the right thing
http://www.arp-bolts.com but you'll have to ask them.
Or find someone with a lathe that can make them.

> My question is, would a heli-coil insert for 5/16 x 18 thread
> into a 3/8 x 16 tapped hole?  Or do they use there own tap
> sizes?

A Heli-coil won't help here, they can't change the pitch of the threads.
Normally you use a special tap for them, that is oversize but has the same
thread pitch.

There are "hard wall" inserts that have standard threads on the outside,
however going 3/8 to 5/16 doesn't leave room for the insert wall.  You would
have to drill and tap the intake manifold out to 7/16 to use these inserts.
This is probably what I would do, if there is enough meat in the manifold to
hold the insert.

See for example P/N 93340A415 at
http://www.mcmaster.com/

If the intake is aluminum,
Randall

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