The mountain versions came with 8 tooth drivers -- that provides an additional 0.4" of clearance compared with 9 tooth drivers.
So figure it this way... if a 1.5" will fit with 9 tooth drivers, then a 1.9" will fit with 8 tooth drivers. So how tight does a 1.5" lug fit? I would say that a 1.6 should fit without problem using the 9 tooth drivers, which means that a 2.0" will fit with 8 tooth drivers. RX1M originally shipped with a 2" track.
The 03 RX1 and RX1M have the same part number for the front heat exchanger, which means that they BOTH came with front heat exchanger protectors.
In 04, RX1M front heat exchanger part number was no longer the same as RX1. This part number stayed with the M version right up to at least 08. The 08 shorty shows the same heat exchanger part number as 03RX1.
With the M change to 3" pitch, they went with 7 tooth drivers. These drivers are larger diameter than the 8 tooth 2.52" pitch used in older M's, which means less clearance. In fact, with the 03RX1M, you could not get a 2.25" track into it without trimming the heat exchanger protectors, but even with the larger drivers, then now DO use a 2.25" track. That means that the current model M (in fact, anything 04+) has at LEAST an additional 1/2" of clearance... thanks to a different heat exchanger -- lacking the protectors.
That all means that with the STOCK 9 tooth drivers on a shorty, that you can fit up to a 2" track, but that it WILL require removal of the front heat exchanger protectors.
