1. Start with a cool engine!!!!
Remove the air intake tube. Set aside.
2. Grab a 1/2" socket and remove the two bolts holding the bell housing and nipple.
3. Make sure you have a 1-gallon heavy-duty freezer bag to catch spillage as the bolts come out. Don't worry if you have a spill though, because there is plenty extra in the overflow reservoir to tolerate spilling most, if not all of it. NOTE:
Coolant is toxic to all animals including dogs, cats, and humans. Be sure to clean up spills promptly and thoroughly.
4. Once you drain the coolant in the upper hose, push it aside to expose the OEM t-stat and red rubber gasket.
5. Use a medium flat screwdriver to gently pry the old unit out. Be careful to preserve the gasket. Also, be ready for a final gush a coolant from behind.
6. If you intend to grind the t-stat's metal flange down to 63mm exactly, then now is the time to do it. Next, transfer the rubber gasket to the new t-stat. Install the new t-stat taking care to orient the dog bone air bleeder to the top.
7. An unground Motorad t-stat is a tight fit, so a knife or thin screwdriver can help "shoe-horn" the gasket. Once the gasket is started, push with even pressure until the t-stat seats into the housing - it will not be flush and a little gasket will protrude beyond the machined face. This ensures a watertight seal.
8. Reverse the steps above, then pour the recovered coolant back into the overflow tank. Start the engine and let it warm up thoroughly. Rev the engine a few times to help dislodge any residual air. Note the reservoir level and add more coolant as needed.
credit: Meister (LXForums) for much of the technical data