Top Hat Spacer Mystery (Two Questions)

My friend and I both have Norton Commandos, and we have a specific question regarding rear wheel "top hat" spacers. First things first: my friend has a 1971 Commando 750 Roadster; I have a 1974 Commando 850 Roadster.

When we disassembled the 1971 Commando rear wheel, we found (in order from left to right: (1) bearing spacer emerging from the wheel; (2) Smiths speedo drive: (3) top hat spacer slotted in the speedo with the "long" part pointing to the left; (4) big axle spacer; (5) axle washer; (6) axle.

QUESTION #1: Does the top hat spacer slot INTO the gearbox from the right/outside, OR does it rest INSIDE the gearbox and slot into the bearing spacer?

When we disassembled my 1974 Commando rear wheel, we found (in order from left to right): (1) bearing spacer emerging from rear wheel; (2) silver, possibly aftermarket (?) speedo drive; (3) big axle spacer; (5) axle washer; (6) axle.

QUESTION #2: Am I missing a top hat spacer and, if so, does it also slot INTO the gearbox from the outside (right of the gearbox, pointing to the left)? We tried to put a top hat spacer into the gearbox in this manner and it would not fit (close, but no cigar). When I torqued my axle to 80 ft/lbs, the wheel seemed to bind a little, though not quite as much as my friend's. Not sure if the 1974 850 wheel is different from the 1971 750 wheel, or if I need the spacer. As I noted, it does not seem to fit into the gearbox at all, and I don't want to pound on it.

Thanks for your help!

When I first got my 1972 750 there was no top hat spacer but after a while the speedo gearbox started to collapse. I replaced the gearbox and put a top hat spacer in (long side into the gearbox) and it has been fine ever since.

Just about every washer and spacer was either missing or in the wrong way round when I got it so I don't know if this is correct but it works.

As far as I'm aware, the right hand side spacer assembly shown in the link is correct for all rear wheels up to '74 (mine is '75 so is different).

The top hat spacer [34] as it is shown, would appear to fit into the drive box from the outside. http://www.oldbritts.com/1973_g20.html

I took another look around and it seems like my speedo (which is silver) may actually be the original correct unit. My friend's 1971 750 has a black unit stamped Smith's on the outside, and mine is smooth on the outside, but I've seen photos of other 1974 850s with a unit similar to mine.

The strange thing is that the new top hat spacer does not seem to fit into the speedo drive very smoothly. Looks like I will just have to tap it in with a hammer.

Here's broader question: What does the top hat spacer DO that a simple washer would not accomplish? I can understand that the top hat shape would be appropriate if it went INSIDE the gearbox unit (thereby standing it off of the wheel) but there seems to be no purpose to the shape of the spacer if it merely takes up some slack in the spindle assembly. Just curious, and thank you all!

nomadwarmachine said:

I took another look around and it seems like my speedo (which is silver) may actually be the original correct unit. My friend's 1971 750 has a black unit stamped Smith's on the outside, and mine is smooth on the outside, but I've seen photos of other 1974 850s with a unit similar to mine.

Click to expand...

The original Smiths drive units would normally be plain alloy finish, and usually have some raised cast-in lettering on the outside face, such as "Made in England" and a patent number etc...
Painted ones wthout any lettering are normally pattern.

nomadwarmachine said:

The strange thing is that the new top hat spacer does not seem to fit into the speedo drive very smoothly. Looks like I will just have to tap it in with a hammer.

Click to expand...

Originally, there would have been a whole range of different drive units available, all having different numbers, and a particular numbered unit may only differ from another type due to the size of the spindle/axle hole, for instance?

There should be a stamped number on the inside metal ring of the drive box (along with the drive ratio) normally starting with "BG", such as BG5330/171 which as far as I'm aware, would be the correct 15/12 ratio, 11/16" dia. axle hole unit for Commandos '71-'74.
Can you say if both yours and your friends units have the same BG number or not?

nomadwarmachine said:

Here's broader question: What does the top hat spacer DO that a simple washer would not accomplish? I can understand that the top hat shape would be appropriate if it went INSIDE the gearbox unit (thereby standing it off of the wheel) but there seems to be no purpose to the shape of the spacer if it merely takes up some slack in the spindle assembly.

Click to expand...

I'm not actually sure what the reason is (or even if the parts diagram shows the top hat in the correct position?) but what is important is that the drive box shouldn't be crushed or forced inwards against the wheel when the axle assembly is tightened.

The tophat spacer prevents the drive unit from being crushed into the aluminum wheel cover. The spacer transfers the torque from the axle spacer to the inner bearing spacer. A simple washer would not be long enough to prevent crushing the drive and if the tophat spacer is not inserted into the drive from the outside can allow the drive to move laterally on the axle and cause wear on the drive ring and gear in the housing.

You can usually tell if the tophat has been omitted or if it has been crushed by the marks on the wheel's alloy plate. The rotating wheel disc wears against the stationary speedo drive.

Thanks to everybody for their responses. I fitted a spacer into the speedo drive, torqued down the axle and the wheel spins perfectly without any binding at all.

Unfortunately, I am now having an issue with the bike dying after about 10 minutes and not wanting to start again. I have Mikuni 34 mm carbs on there and a Boyer. Since this is a completely different issue, I will post separately. Thanks again!

tophat spacer mistery

just wanted to say thanks for the post. I recently replace the speedo drive on my 73 MK1 and omitted the tophat spacer and left it in the old speedo drive as it was tightly pressed in the old unit. Anyway , thanks to the post I found the problem and corrected it before finding out the hard way.Cheers