Date:         Fri, 31 Jul 1998 22:28:57 -0700
Reply-To:     Austin <austins@IX.NETCOM.COM>
Sender:       Vanagon Mailing List <Vanagon@vanagon.com>
From:         Austin <austins@IX.NETCOM.COM>
Subject:      Re: Have I screwed up this head?
Comments: To: Tom Young <young@SHERLOCK.SIMS.BERKELEY.EDU>, vanagon@vanagon.com
In-Reply-To:  <Pine.OSF.3.91.980731210140.2009A-100000@sherlock.SIMS.Berk
              eley.EDU>
Content-Type: text/plain; charset="us-ascii"

At 09:03 PM 7/31/98 -0700, you wrote:

> I decided I'd "chase" the threads in
> the head with the appropriate tap.
>
> Turning in the tap took considerably more effort than I would have
> expected (the threads in the head *looked* fine) but now the stud
> turns in easily by hand.  In fact, the stud now "rocks" very slightly
> when it's at the correct depth.
>
>Tom Young               young@sherlock.SIMS.Berkeley.EDU

This situation has popped up twice now (recently it was ref spark plug
threads).
Chasing threads with standard cutting style taps is a real losing
proposition - losing metal, that is.

There is a type of tap generically called a 'roll' tap; it works by
force/forming a thread rather than by cutting the parent material.

Special considerations apply when making *new* threads with a roll tap, but
as a buggered thread chaser it can't be beat; it will start more easily
than a cutting tap (using a 'plug' style tip) and when coated with a *dry*
film of anti-seize will do as good a job as possible re-forming the metal
as opposed to removing it.
Won't ever be as good as the original thread, but is far better than what a
cutting tap gives you.

You won't find them at OSH or Sears, but a true machine shop supply
distributer (Shop Tools, Western, Advanced, etc) will know what you want
(get an 'H3' class fit - sorta yields a 2B thread);  they *are* pricey
compared to a Vermont/Sears/etc (junk-mho) tap, but if you salvage that
thread w/o having to remove the head....??

Austin

(ps - in your situation Tom, it's too late; gotta Heli-Coil it...believe it
or not, there are roll taps sized for helical inserts - the *ultimat*
answer in aluminum & other soft materials <properly done, cold-flowed
threads are much stronger than cut threads>).