Shimming the Transmission

[Phil-UK]

Hi

One for the experts .. I've read in various magazines, books etc., about shimming
motorcycle gears, so anybody out there any experience of shimming up the transmission
on a CB72/77 ? ( or any other bike really ).
I'm wondering if it might get rid of the difference in slack in the clutch cable depending
what gear you are in and more importantly improve gear selection.

So a number of questions :-

1) Is it possible ?
2) Would it be of any benefit ?
3) How do you do it .. is it taking up play only at the ends of the gear shafts within
the cranckases.. ?
4) Where do you get the shims from .. I guess not Ebay . ?

thanks in advance

Phil

[dkwilfert]

Back in my high school days in the 1960's I worked in a motorcycle shop, and we did shim a few transmissions. A constant mesh transmission transferrs power between the primary/secondary shafts by the shift fork sliding gears with dogs on their sides into/out of adjacent fixed gears with holes in their sides. Ideally these dogs fully engage when that gear is selected and fully disengage when it is not. Shimming involves fine displacement using shims of the fixed gear to optimize that engage/disengage operation. The result is better shifts, fewer missed gears, and no jumping out of gear. This has nothing to do with clutch operation or behavior.

[jensen]

Hi Phil,

I have experience with shimming my transmission (and crank), but I'm sure that there more ways then one to look at it, so I don't call myself an expert.

The engines split horizontal, so from that point of view is much easyer then vertical splittted engines

a) yes, it's possible.
b) benefits are improved gear selection, smoother operation, fewer missed gears, less yumping out of gears, less noise and less damage on shift forks and other parts over the years
d) I make these myself, so I can't really answer that question.

dkwilfert mentioned already the reasons to shim the gears, honda has ofcourse shims (gear cotters in different sizes) for these jobs for a CB72 / CB77 to make sure that the dogs fully engage. Next to that shimming can be used to have as much surface area of both gears to engage (lower pressure per square mm). Also it's important to make sure that the shafts stay on there ideal position.

My starting point is always the main shaft on the clutch side, that should be your reference point.

Make sure that the main bearing behind the clutch is fixed properly in it's position in the crankcase and use a new circlip on the clutch side before measuring the actual play.
Start with shimming the main shaft on the clutch side and make sure that the clutch can't slide more then 0.05 mm and less that 0.03 mm (I use a shim between the clutch outer and circlip).

Then shim the counter shaft, and after that individual gears, etc.

Never use shims smaller than 0.4 mm because these will collapse. If you have a play of let's say 0.3 mm between clutch and circlip, and you need 0.05 mm play than you have to make the clutch outer 0.14 mm smaller by machining. Be carefull witch side to machine because the primary chain has to be properly alined to the crank primary gear (you shim that gear to if needed by grinding / shimming).

It seemes to be a lot of work, and it is, but it's fun to do.

Jensen

[Phil-UK]

Hi Guys

Thanks for the replies, and the detailed information.
I have several sets of cases and gear clusters so when I get a chance I'll be giving this
a go as a dummy run.

cheers

Phil

[e3steve]

Hi

One for the experts .. I've read in various magazines, books etc., about shimming
motorcycle gears, so anybody out there any experience of shimming up the transmission
on a CB72/77 ? ( or any other bike really ).
I'm wondering if it might get rid of the difference in slack in the clutch cable depending
what gear you are in and more importantly improve gear selection.

So a number of questions :-

1) Is it possible ?
2) Would it be of any benefit ?
3) How do you do it .. is it taking up play only at the ends of the gear shafts within
the cranckases.. ?
4) Where do you get the shims from .. I guess not Ebay . ?

thanks in advance

Phil

Phil, there's some info in Bill's HERG (the red-covered manual), p100, on shimming.
1) Yes
2) Definitely. Improves dog-engagement, particularly 2nd gear.
3) You'll need a manual.
4) Try emailing FAST FRED or LM for shims (cotters); I think there are 3 thicknesses: standard (23941-250-010), +20-thou' (23941-259-305) & +40-thou' (23943-259-305).

Rgds,

Steve

[jensen]

Hi Phil,

Using the gear cotters from Honda will only let you shimming a few gears (second gear main shaft and third gear counter shaft. It's a good way to start, but that isn't the complete story.

Before starting shimming individual gears you have to start shimming the shafts first , to make it impoosible for them to move around the engine cases.

It's a pitty that I didn't stop by in Brighton last weeks on my holiday in the UK,


Jensen

[nander]

Is there anything specific that you use to shim the shafts of the transmission? Off the shelf washers?
thanks,
nander