> On this down pipe there is a little probe fitting that
> has a metal screw on cap.  Well.. my little screw on cap has fallen off.

Just get a metric cap at the hardware store.  Or get a nipple and
cap and have a muffler shop weld them in place of what's there.

I have one of Frank Condelli's stainless J-pipes without the
port.  I had a local shop weld in one--but not stainless.  It
rotted away after 3-4 years, so I got a stainless nipple and
cap (at McGuckin's) and had them weld it in.  I wasn't smart
enough to ask for the J-pipe with the port already installed....

                                Richard A Jones
                                Boulder, Colorado