Transmission shimming

[kmev]

I've been absent here for awhile, but my deployment orders soon end and I am thinking of finally getting my 250 NC assembled. In searching the topics here I haven't seen much on shimming the transmission or shift drum. From what I recall the last time I laid hands on my tranny, there was a lot of end play in the shift drum. Will shimming the shift drum produce more accurate shifting? If so, how does one go about that? I see shims available for bevel gears and crankshaft, but not for the transmission.

Semper Paratus,
keith

[Bevel bob]

I am advised by a well known Ducati expert to not shim the drum closely as the selector tracks ( 5 speed particularly) are a very poor design and need slack to work.Take the most care to get the crank shimming to nil and be wary that the shim sits against the crank face proper and not on the tiny shoulder of the shaft or it will wear loose fast . (file out the shim bore if needed).

[Jordan]

Funny, a Ducati mechanic taught me the opposite - that it's good to minimise as much free play as possible from the cam drum. Also, each gear can be individually shimmed, with the aim of getting as much engagement as possible via positive engagement during a shift, rather than relying on the undercut tapers on the gear dogs to pull them into full mesh. The shafts themselves should also be confirmed as being free from end play. Doing all that, it was claimed, will make for longer gear life, compensating somewhat for poor quality of the material they are made from. I can vouch for that statement, having seen many badly rounded dogs on singles' gears. Shimming to get best engagement gives the dogs easier life. Having been showed how, I did the gearbox shimming once, and can report that it is a tedious and time-consuming job.

[Bevel bob]

When you consider how little work the drum shims have to do one can only assume that the factory left them loose on purpose . It does make sense to get the gear engagement as good as poss.I did look at mine but found little to do.

[ericpritchard]

I've been told to shim the gearbox as well! Who really knows?
Actually I've found that getting the selector box setup correctly is the most important ingredient to a good gear change.
Compliments of the season to all
Eric Pritchard

[Eldert]

its not only the shimming that should be checked . measure the selectorforks new ones are 4.35 to 4.4 mm thick
this is very importend on the inputshaft to properly engage 2nd and 4th
the selectorforks should move very easy on the shafts and the selectordrum has to rotate easy in the cases

and like Eric says the selectorbox has to work perfect . i have a guy that reworks the selectorboxes and adds a positive
stop so no more missed gears

new selectorforks are available from Amici Dello Scrambler

Eldert

[ericpritchard]

I can testify to the modified selector box...great shifting and no need to do other gearbox mods (like under cutting teeth).
Eric Pritchard

[wcorey]

I'm presently going through a w-c transmission, I'd like to hear more details on this positive stop mod. Could someone elaborate?

Bill

[Jordan]

its not only the shimming that should be checked . measure the selectorforks new ones are 4.35 to 4.4 mm thick
this is very importend on the inputshaft to properly engage 2nd and 4th
the selectorforks should move very easy on the shafts and the selectordrum has to rotate easy in the cases

and like Eric says the selectorbox has to work perfect . i have a guy that reworks the selectorboxes and adds a positive
stop so no more missed gears

new selectorforks are available from Amici Dello Scrambler

Eldert

Good point Eldert. Shimming the gears is not to get a better shift (although it helps), it's mainly for getting maximum life of the gear parts. These things are expensive! If you can get better engagement during shift movement, it can compensate for worn selector forks too. Every fixed gear is theoretically able to be adjusted, if you can get hold of suitable shims. The factory shims have a splined shape. I'm not sure if plain round ones can be used in their stead (but can't be next to the circlips of course). I think most people find it too daunting to mess with the factory shimming. On the other hand, I know someone who bought many new Ducatis. He would immediately upon getting one home, disassemble the engine so he could improve on the factory shimming, which he reckoned to be too sloppy. I have a photo of the positive stop mod, from Eldert I think. Looks like it has obstructions that prevent the selector shaft from being able to rotate more than needed. Other ways to avoid missed shifts would be to check the detent spring in the selector box is strong and working as it should, or to lighten the selector cam drum by drilling or machining, so as to lessen its momentum during a fast shift.

[Bevel bob]

My splined shims were missplaced or missing! and drum really loose but gear selection was ok, clutch drag is a problem especially since i renewed the springs and fitted new thicker plain plates/ I think i will have to pack or grind the springs to get even lift,also considering a change to multigrade oil for winter use .