I'm no machinist - need help turning a steel part to remove air gauge

I'd like to replace the gauge on my Cricket 2 with a Wika that actually shows the pressure level rather than a few pretty color bars. I was reading another thread where someone said to get a 27mm ratchet head and turn the outside down until it fits to unscrew the air gauge. 

I've only got a HF mini lathe, and have never had much luck on steel with it, only softer metals. So I manage to take a little bit off, but the hardened steel is just eating the bits and I've made very little progress.

Any help on how I can turn this 35mm outer diameter down to ~31mm from someone who actually has a clue what they're doing? 
 
Annealing it first is a good idea, as long as the wall doesn’t have to be made so thin that its strength becomes a problem. I doubt that would be an issue for removing a gauge

Otherwise use a grinding wheel to reduce it. Cover everything but the chuck so the grit doesn’t get into the carriage and such. Freehand will suffice…you don’t need precision down to a thousandth of an inch. Just stop from time to time and take a measurement with calipers and keep going until you reach the needed diameter.

 
Annealing it first is a good idea, as long as the wall doesn’t have to be made so thin that its strength becomes a problem. I doubt that would be an issue for removing a gauge

Otherwise use a grinding wheel to reduce it. Cover everything but the chuck so the grit doesn’t get into the carriage and such. Freehand will suffice…you don’t need precision down to a thousandth of an inch. Just stop from time to time and take a measurement with calipers and keep going until you reach the needed diameter.

I was thinking the grinding wheel as well. Just wanted to hear someone confirm it before I went that route.
 
@ ctshooter. If you need this tool shoot me a pm and i will drop it in the mail. 
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