250Desmo - Big Issue!

[mattwint2]

Hi folks

Firstly, what a pleasure it is to discover this forum and such an enthusiastic band of fellow Singles fans! Hoping I can get some useful input for my issue.

The Bike is a 1974 250D that we have owned since 1976 (37 years!). During that time it has lived variously in sheds and garages during house moves, but mainly in the lounge. It comes out occasionally for a run, but last year I rebuilt a lovely warm, dry garage where it now lives very happily alongside a few other bikes. I decided to do a full restoration that I completed 3 months ago. It ran beautifully for 100 miles until this week when a suspicious noise started up from low down in the crankcases. Luckily I was near home, so rode slowly back and started to investigate. What I found was horrifying and (probably) expensive damage to the timing side. I can't find any out of place object that has caused this so I guess I have somehow got the shimming of the bottom bevels wrong....?

So questions please folks as follows:

1. Ideas as to what has caused this tragedy?
2. Where do I find accurate and complete information on how to correctly rebuild the bottom end / timing side?
3. Where can I find replacement bottom bevels of suitable quality....?

Thanks so much in keen anticipation...

Cheers

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Matt

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[blethermaskite]

Oh dear!.....firstly I would try Nigel Lacey for bottom bevels, You need either brand new ones or else a matched pair of used gears, why has it done this??? ....either the mesh was too loose or too tight, or is your desmo valve gear set up correctly if the valve gear has any mechanical binding it will load up the bevels, or are all the bearings in the bevel train ok? or is your crankshaft end float in spec? Quite a few things to investigate, and then you have to get all that shrapnel out.
Cheers,
George

[ranton_rambler]

I’m nowhere near as expert as some on here, but first suspicion would be bevel mesh too tight, either by setting, or due to crankshaft endfloat.
I also wonder whether anything “upstairs” might be the problem. Too much torque to rotate the bevel train.
Have you taken the cover off too see what’s in the cam tunnel?
Ian

[Duccout]

What a shame! It is a bummer. As the others have said, it is due to the bevel mesh being too tight or end float on the crankshaft. Luckily, new bottom bevels are available, and hopefully the damage will be limited to what you can see.


Good luck,


Colin

[veloduke]

Ouch!

Widecase.com have some gears at £228, I got a set from Ducati Vintage Store, current price E185, but you'll have to add VAT, postage and potentially import duty to that.

I destroyed a set through meshing the gears too tightly when I did not appreciate how the lower bearing assembly affects the engagement when fully tightened up

[themoudie]

Commiserations Matt,

1. Ideas as to what has caused this tragedy?

Whilst I do not own a desmo, binding anywhere in the valve train from "tight" valve clearances, to excessive crankshaft endfloat, could be the cause of your debacle.

Bearing in mind the age of the machine and the length of time that you have owned it, how many miles has it covered and have you ever made any adjustments to any related part of the engine, before the catastrophe? If no adjustments have been made it could be "wear and tear"?

I too, have owned a 1974 450 MKIII since 1976, but it now has had 3 engine rebuilds, 2 bodywork rebuilds (after mishaps!) and getting on for 90,000 miles under the wheels, in all weathers. It's part of the family, along with some other singles and I understand your angst. However, it is a machine and "Made by man, we can rebuild it!" applies, it may take time, ingenuity and cash, but it can be done.

2. Where do I find accurate and complete information on how to correctly rebuild the bottom end / timing side?

Personally, I would be stripping whole of the engine, because of the amount of shrapnel that has been generated and your having ridden the wee beastie home. There could be steel slivers in any of the bearings, or the crankshaft sludge trap, plus the gears in the oil pump will need a looksee. Also, the additional strain placed on the upper bevels, due to the mayhem below, may have caused damage, and will need examination. Quite how it got you home is something of a miracle!

Both the Clymer and Haynes manuals go into the rebuild procedure, there is also the Ducati workshop manual, all are readily available. However, there is a sequence of shimming to follow, starting with the crankshaft endfloat in the crankcases. Time, patience and accuracy are required to assemble, measure, disassemble, re-assemble, re-measure the components and assemblies. That's why Ducati stopped making the bevels, a rubber band could do the job and was easier to manufacture and set up than a train of gears. Use the 'Search' facility on this website to read previous Q&A threads on shimming the bevels and if all that doesn't answer your specific question, open another thread with a specific 'Subject' heading, so that we can all find it later if we come across the same problem.

3. Where can I find replacement bottom bevels of suitable quality....?

As George says, an email or telephone call to Nigel Lacey first (there are other suppliers, BUT!), to enquire about supply of gears and possible causes of the catastrophe.
N.B. you may not get a reply to a telephone call on Friday! Leave a short message.

Be aware that price isn't everything and some of the materials used in replacement gear sets, along with the accuracy of the machining maybe wayward! Mixed/odd /unmatched gears put together as a pair are not the route I would take.

Good health, Bill

[ranton_rambler]

Both the Clymer and Haynes manuals go into the rebuild procedure, there is also the Ducati workshop manual, all are readily available. However, there is a sequence of shimming to follow, starting with the crankshaft endfloat in the crankcases. Time, patience and accuracy are required to assemble, measure, disassemble, re-assemble, re-measure the components and assemblies. That's why Ducati stopped making the bevels, a rubber band could do the job and was easier to manufacture and set up than a train of gears. Use the 'Search' facility on this website to read previous Q&A threads on shimming the bevels and if all that doesn't answer your specific question, open another thread with a specific 'Subject' heading, so that we can all find it later if we come across the same problem.

It's a very frustrating and time consuming process due to the design, but no shortcuts. From my 2 goes at this, the cases need to be put together and split numerous times to first get the crankshaft endfloat right, and then the same again to adjust shimming of the bottom bevels because the gear in the crankcase can't be removed with the crankshaft in position. Maybe 10+ times depending on how quickly the correct shimming is found.
And similarly on the top bevels, although I reckon that's a bit easier.

[George]

Once crankshaft end float is set are not the bottom bevels shimed by using the special tool (or if not to tight gently knocking out) to remove bottom vertical bevel bearing housing after removing circlip from shaft and adjusting shims therefore leaving crankcase joined.

[Bevel bob]

George is correct . You dont need to open the cases to set up bevels . As you have had one go at this ,and its not worked ,I would suggest you research and be prepared to spend days doing the job, Buy a good selection of shims and a micrometer .You are going to have to question every step you take and do it over and over till its right. Nothing else is good enough . A close study of sectioned drawings of the system to understand how and where the loads are controlled will help. Ask lots of questions , If you dont have questions then you have not thought hard enough. Good luck .

[Jordan]

Gear not properly hardened?