Mine is 1", its 62MM bore, but I am running a stock 4.0 TB on it. After you make the spacer, you need to bolt it on the manifold and mark the manifold for grinding/cutting in the bore. I used shop rags covered with grease stuffed in the manifold runners to block the shavings from going down in the motor and used a shop vac while I did the cutting with a 1/4 in electric die grinder and a carbide burr.
The spacer moves the power band it does not really add power. With mine my power comes on at 2500 and pulls to about 5000, without the spacer the power bad was about 3200 to about 6200. (My motor is built for revs so I can get away with this I don't recommend it on a stock engine)
I pull the spacer out if I'm headed down to play in the dunes. I'd like to try a 2" spacer for crawling..
Dave