DM 350 Transmission Help

[DucRedux]

I've assembled several Ducati top ends but never a bottom end. The motor in question is a 66-67' Sebring unit disassembled by me several years back and being put back together now. When taken apart, I took precautions to remove component assemblies in order, so that the mainshaft and countershaft gears, shims, and washers were wrapped in the same orientation as originally present. Ditto for the shift forks and dowels.

The problem I've run into is finding neutral in the shift pattern when manually operating the mainshaft and countershaft. Everything is currently assembled in the left case. The gears are sound, but move clumsily in the vertical position and the drum has some stiffness and snatch, but this in part due to the fork dowels not being fixed to both cases.

The drum channels are clear and the edges have been linished lightly to assure that no burrs are present. The fork dogs are smooth and round and the fork recesses are uniform. The thrust washers on both shafts are 2 mm thick (although the Ducati maintenance manual also shows 5mm washers). The assemblies are lubricated. The shift drum has also been tested in positions 180 degrees apart to check for full movement through all gears.

Before I mate the cases, some of you please comment with your experience with transmission assembly, including pitfalls and solutions to problems. I look forward to your guidance. A photo is attached.

[Jordan]

I would not necessarily worry that the gears don't change smoothly when the fork shafts are floating at one end.
All should be well with regards shimming, if it was working well beforehand, shims and washers are placed back to original positions, and the paper gasket at the crankcase halves is the same thickness as before.
I once built up a widecase gearbox from parts. Shimming it all from scratch took me days to complete.

[Bevel bob]

I have read that the 5 speed drum tracks are not as well designed as they could be and that the drum works better if it has a bit of axial clearance to allow it to compensate .

[Jordan]

That's not been my experience, Bob.
Sounds like a quick bodge?

[Ducadini]

I shimmed the drum to a standard play, fitted the gearbox in the cases (leave the crank out) closed and tightened cases and fitted some parts to pull the primary shaft towards the clutchside and the secondary towards the sprocket.
Then checking in each gear how much the dogs interact WITHOUT putting stress on the forks (visible through the cylinderhole) and adjust primary, secondary and/or drum accordingly. I did have to look for some extra shims.
Worked for our 500cc racer (untill the valves let go )

ciao
ducadini

[Bevel bob]

Hi Jordan, My drum had a bit more axial movement than I thought it should have . I went looking for shims , was advised by an authority that loose is better than minimal clearance. Perhaps there is some inconsistency in the drum machining. Some good,some bad, I think its more a problem with the 5 speed , mine selects freely but the selector box needs new bushes at least.

[DucRedux]

Thank you for the replies, gentlemen. I'm feeling more encouraged for the assembly. As for a trial fitting without the crankshaft, I was following the the process in the manual for putting it in the case first. I did wonder if the transmission shafts would then fit properly between cases with the original washer and shims, so I estimated equivalent lengths for fit with a metal straight edge and feeler gauges. I do have time for this project, so removing the crankshaft and working through the gears in the closed cases while in their normal position makes absolute sense.

[DucRedux]

More confident that all might be right with the transmission after your comments, I removed the crankshaft and tested the shifting with both cases closed. An external drive was set up using a power drill, a bit coupling with stepped vinyl tubing to fit over the mainshaft end on the drive side. I liberally sprayed lubricant on the gears through the cylinder opening and my wife operated the drill while I shifted and watched the components move.
The added lubricant provided smoother changes and, by varying drill speeds, the output shaft could be handled safely to sense power transmission. Neutral was finally found.

The cases are now buttoned up with internals present and no endplay. The rest of the build can go forward.

[Jordan]

A dentist's inspection mirror is handy for this job - good for gear teeth too.

Just to mention, I had access to all the needed shims from a helpful Ducati mechanic.
He showed me the theory of minimising play as a way of improving dog engagement, which leads to increased life of the gearbox.
All gears and the cam drum is able to be individually shimmed, which takes time. Of course, some play is needed but it can be almost zero.
With excess play, the fork cannot shift the sliding gear as far into full engagement, and results in premature rounding of the dogs.
Fuller engagement means not relying on just the detent mechanism and the back-angle of the dogs to pull them towards each other.

As a matter of interest, I tried the same principle when rebuilding a Burman gearbox, UK made and fitted to many makes of bike.
These have no shims as standard. The result of my work was a horrible noisy and unreliable change - so all the shims were removed and normality restored!