The answer : Oil level (air gap) without spring or spacer: 151mm (5.96″)
If you’ve ever tried to change the fork oil on one of these bikes with the 41mm forks, you’ve probably run into the same problem I did with the confusing directions on oil level. The oil level air gap is supposed to be 100mm with the forks fully compressed, spring removed and the spacer installed. The problem is the spacer goes on top of the spring, so it will be nearly impossible to get out once you’ve installed it without the spring and covered it with oil.
Here’s the solution. I’ve verified that it works because I measured the oil level as it came from the factory without the spring and spacer. It was just a little below the calculated level. This is expected as a little bit of oil always gets past the seal as the fork moves in and out, and there was probably a bit of oil still on the spring and spacer when I removed them.
Here’s how I computed the new level.
The spacer is 200mm long (L) with an outside diameter of 35 mm (DOD) and an inside diameter of 30 mm (DID)
The fork has an inside diameter of 36.6 mm (DF)
(The volume of a cylinder is π r² h. With a little bit of algebra you can show that the amount the oil level drops without the spacer is:
L*(DOD² – DID²)/DF² = 51 mm
Adding this to the 100 mm spec gives the final answer. Without the spacer, the air gap above the oil is 151 mm, or 5.96 inches.
BTW: The suggestion that others have made about removing the plastic caps and knocking the plug loose with a hammer and socket before removing the forks is a good one. I didn’t expect to find corrosion on the plug as the bike has always been garaged, but there was just enough corrosion to freeze the plug in place.
When I reinstalled the plug, I covered the top of the plug in axle grease to help keep moisture away from the clip and sides of the plug.