Looking up the part by the GM part # (and thank you for posting it), I see...
Buy ACDelco 12F31L Professional Locking Fuel Tank Cap: Fuel System - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
(warning - pic is a bit grainy)
(and it lists as a match for my '03 Envoy XL, btw)
- If you look at the shape of the cap, it looks very much like the Stant caps (of which I have one, just sitting around... more on this later). So GM uses them for a supplier, most likely. And they're a different supplier than the OEM non-locking caps. I know that the Stant caps have no provision for a tether (b/c this bothered me, too, as I recall)
- If one were to take a dremel-style tool with the cutting wheel attachment, they could scribe a ring around the circumference of the cap, and perhaps fit the tether, that way. I have no suggestions about maintaining depth uniformity of that ridge.
- As for why it doesn't have the ridge / tether to begin with... perhaps it has something to do with the key likely being attached to the ignition keyfob / ring (weight, potential key scratches, etc.) Or perhaps the logic is, "since the owner will need the key to restart the vehicle, they don't need a tether, b/c the keys will still be in the cap" (although as I recall, the key is supposed to be turned back to the 'lock' position after unlocking, in order to remove the key (and the cap). And it's also a bit unwieldy to remove (and reinsert) the cap with the keys still in the lock.
Oh, and as for why I have a locking cap and don't use it?
I figured out some sort of 'issue' (forget what it was, but it's described in one of my posts)... and I received response that the truck has an anti-siphon valve. So... you don't NEED a locking cap (at least for purposes of fuel theft; it *will* help prevent adulteration of the fuel supply)
