I jumped on this thread a little late, but it looks like you are trying to plug
up the O2 sensor hole on your cat.  I recently had an O2 sensor bung installed
on the crossover pipe on my air cooled 82 Westy so that I could set the CO.  I
didn't tighten the plug well enough and it worked its way out.  I just went to
an auto repair shop and asked them for a junk O2 sensor.  The mechanic pulled
one out of trash and off I went.

Quoting Brian Keese <brian.keese@STILTMART.COM>:

> So...I tried a spark plug in the hole and it was too small.  Does anyone
> know the size/thread pattern for the O2 sensor? I�d like to go buy a plug
> for it and don�t want to keep running in and out of the store trying
> different plugs if I don�t have to.
>
> Ron, maybe a note on the cat's product description about tightening the
> plug would help less experienced folks like me avoid this problem.
>
> Thanks again,
> Brian
>
> On Mon, 25 Apr 2005 14:23:37 -0400, The Bus Depot <vanagon@BUSDEPOT.COM>
> wrote:
> ...
> >exhaust bolts, can attest to that!)  The important part is to make sure to
> >tighten it well upon installation. The manufacturers generally ship them
> >only hand-tightened, since many people will be removing them to install an
> >02 sensor....
>