Britbike forum Forums British Motorcycle specific Brands Vincent forum Comet Burman Transmission main shaft thrust?

Hey all
I've replaced the high gear bushings on a very worn Burman BAP
My routine check of the main shaft thrust after full assembly of the primary and the inner transmission cover revealed over .040" of thrust movement present
This is too much...I say this because the primary chain will grind it's way through the inner primary if I leave it this way.
In finding this I noticed 2 things...the old primary chain WAS rubbing and the new chain is a full .010" wider at the pins.

I feel a .020" shim will rectify this...a shim between the inner cover bearing and the gear. McMaster car sells a 20mm ID shim in .5 and 1mm thickness. Perfect....but I was thinking
When this transmission gets hot...will the case grow and lock up the main shaft if there is too little movement?
How much thrust is too much and how much is too little?

I tested my shim with a feeler gauge..20 thou will keep the chain from rubbing..but should I use more?

Anyone out there with experience on this?

The gearbox on a Comet will never get hot enough to worry anything. The end float on the mainshaft can be up to .032" Anything over .010" will cause no drama. A few points, though. The bushes inside the output gear (33 teeth) must be "OILITE" or similar self- lubricating material, as must the top hat bushes at either end of the layshaft, in the cases. If you use phosphor- bronze here you will lock the gearbox up, which can wreck the gearbox and throw you down the road. Are you sure you've got the right mainshaft and the clutch roller race is correctly assembled, i.e. .160" thrust plate innermost, .250" race, with 6mm. long rollers .110" thrust plate outside. Do up the mainshaft nuts tight before fitting outer cover and use the lock tab on the clutch end. Both mainshaft threads should be 5/8" x 20 t.p.i. A modern self- levelling grease is the best lubrication, 800ml. The mainshaft usually wears more than the bushes, and the ratchet sleeve on the kickstart end is sometimes incorrectly made too long, which can lock the gearbox on assembly. Any more questions please post 'em. I've never seen a primary chain rubbing myself. Early BAP gearboxes had the two layshaft free running gears (28 and 34 teeth respectively) running steel on steel but I think all Meteor and Comet ones, which are coded G97, have phosphor bronze bushes fixed on the layshaft for these gears. They are lubricated with 1/8" cross- drillings which must be clear of old hardened grease. I remember Phil. Irving saying that gearbox lubrication was a matter of two choices: oil, which lubricated until it leaked out, or grease, which didn't leak and didn't lubricate! I now use the thixatropic grease mentioned above, but used to use grease with an eggcup full of oil chucked in occasionally.

Thank you for all the detailed information hardarser. Very helpful
I purchased the parts from our friendly Vincent parts supplier. I do understand what you are saying about the type of bearing and I did make sure I was using the correct type. I am lucky to frequently make bearings here for veteran machines and was lucky to learn early on that bearing types are important without having to get chucked off a bike.
When I disassembled the transmission the clutch bearing plates were reversed...not to mention a few other poorly assembled components and a crack between the shift drum bore on the drive side and the output bearing bore. After correcting these and installing the missing blanking plug I was on the path to success. Most of the parts are badly worn...which does not match the current condition of the motorcycle (perfect looking bike with serious wear under the gloss)
I did have to rectify a mushroomed ratchet sleeve that was binding the Kickstarter intermittently and assumed I would need to take up the slack here..but after finding a new piece and measuring the difference I could not justify the total thrust with the <5 thou difference in the sleeve length. This is aside from the detail that the spring for the sleeve was pinched between the bearing and the sleeve itself upon disassembly.
The mainshaft is worn - to a taper. I did "micro" polish it as I have been with all bearing shafts. Micro seems to be the new fancy way of charging more for a 100+ year old service to a plain bearing. I was able to get the recommended clearance throughout the high gear bushings even with the taper in the shaft...I was happy with that bit of work.

Learing from mistakes in the past I made sure to take up all the hardware to full torque with gaskets in place and then perform my measurements. As an example I was able to find .008" more when making sure the transmission mounting bolts were fully tight. I did double check again after your suggestion. This detail is easily looked over.

And the controversial detail...Oil
I have been converting these to actual oil. Calling a number of oil makers and settling into Amsoil and BelRay here in the states I have found both make a product safe for the bearings and with enough EP additives to make for a safe long term operation. The work is in keeping the oil in the box as Irving mentions.
I put quite a few off road hard miles on my Burman/B34 and found the conversion to save me lots of grief.

Thank you again. Your details are excellent. I was not able to find them in any of the many Burman articles.

Hi Denis J,
On the subject of chain widths
A lot of replacement chains are as you found wider and heavier than those originally fitted (Renold?)
I have found that Regina (gold) is as close to the Renold width as makes no difference
I recently scored a box of Iwis Megalife on fleabay but have not fitted it to the open primary of my vintage (pre 30) bikes yet
It is the same width as the Regina chain

John

Possibly you've not come across this detail, but Panther 600/650 gearboxes, coded G42 have all the same internals as the Comet gearbox, except the mainshaft, and even that can be modified to suit. I've had Comets and Twins since 1970, when no-one wanted them and my preferred ride is a light, modified 665cc Comet nowadays. My email is: [email protected] I've no problem sharing any information on this Forum, but if you want pictures or any parts, please email me. Cheers, Charlie.

Good to know. Here I am removing a Reynolds and installing a new Reynolds that is much heavier. Things change I guess.