250/350 Desmo Head

[graeme]

Hello Gents,
Yes I remember Bob well.
Sadly he and Kevin (matchen) are no longer with us.
Take care all and enjoy every day with your single.
Take every chance to enjoy fiddling with, and riding them.
Graeme

[Jordan]

Harvey reported over one millimeter of clearance variation of a closing cam - a whopping amount!
Dew-Cat-Tea Bob's comments about it not mattering are probably correct, and I rely on the combination of momentum and gas pressure on my desmo head, which I run with no springs. However I don't have so much clearance. I'd have noticed!

Years ago when I did a night course on workshop practice (mostly lathe work), I brought in to show my desmo head to the teacher.
He was incredulous regarding the action of the desmo - "How can they make it so accurate?"
Of course we now know that ultimate accuracy was not intended and probably rarely if ever achieved.

[blethermaskite]

I have set up and built from parts several desmo bevel single heads over the years and in terms of cam clearance accuracy have found that in general most cams are just ok but seldom perfect, I think the worst cam I encountered needed 12thou closer clearance to stop binding, and interestingly a fairly recent purchase of a new old stock genuine Ducati desmo cam for my 250 straight out of the wrapping needed 5thou closser clearance to rotate freely,........so even a new and unworn cam (the first one I have ever purchased) wasn't perfect. Of course wear and tear on used cams, rockers and pins all add to errors in the perfect desmo geometry. In practice I have stopped trying to get the "set up perfect" I simply shim opening clearance to factory spec and then shim closing clearance to allow bind free rotation, in most cases this means a closer clearance of plus or minus 2/3 thou. these setting have proved absolutely fine in thousands of miles in normal running with no increase in noise level or adverse effect on performance, in contrast several bevel desmo heads I have opened up which had been set up with zero closing clearance had suffered serious damage to both the cams and rockers.
So for me its "fast and loose" every time
Cheers,
George

[blethermaskite]

Sorry in above meant to say..........closer clearance of 2/3thou. (not plus or minus)
Cheers,
George

[LaceyDucati]

Hi All
I will just add my two pennies worth, of my discoveries re Desmo single heads.

The worst head I found had massive variations and with "Factory" clearances the head locked solid and would not turn even with force. I guess the errors were in the region of 1mm. I will add it was NOS....I wonder why

Having inspected many heads I have come to the conclusion the spindle holes are often neither parallel to each other or the cam. As a test to see for yourself, put 4 x pieces of 8mm silver steel in the four holes. Then hold the head up and let me know what you think! Also having measured various twin and single heads the spindle positions vary by at least 0.25mm in any direction (+- 0.25mm) and are not as a set of holes centred around the cam centre. My conclusion is some repeated errors where due to machine/tooling errors and others just variation in accuracy. Add this to cam grinding errors and rocker variation and you start to see the reasons for potential errors.

All this said I have managed to re-engineer/hand fit many heads to work fine. I have also built various successful Race singles with Desmo heads, however my professional opinion is this was in spite of being Desmo rather than any advantage! The same cam profiles can be run with valve springs and without the closing rocker the valve guide can be positioned higher, creating a higher better flowing port. I do not buy into the valve spring losses, as for racing at full tilt the inertia in the valve will largely cancel out spring load. The inertia in the valve still exists in an Desmo and it has to change direction the same, so the loads are still there. This can be observed by the hard life the collets (keepers) and valve grooves get in a desmo race engine. I hasten to add all my race desmo's had either no springs or the twin snail springs. My experience with this was they both performed the same with regards to wear issues or performance.

My conclusion after 30 years of working on these bikes and 21 years in business is: I would always choose a spring valve head over a Desmo for racing or performance. Also I now refuse to work on Bevel Desmo heads for business as they are mostly a can of worms, best left for the "enthusiast" with plenty of time and patience. It is a case of "the more closely you look, the more you wish hadn't!" I won't even go into potential closing spindle sealing issues that occur.......

Sorry to be negative re "the Desmo dream" but I've spent too long trying to make silk purses and loosing money in doing so, LOL.

Best Regards Nigel Lacey

[DBDBrian]

Thanks Nigel, your thoughts and experience on the subject, are certainly something to ponder.

Brian

[Jordan]

Soichiro Honda knew about desmo, and decided multiple valves made more sense.
Desmos are so romantic though!

[blethermaskite]

Yes Nigel, we won't go into the closer spindle sealing arrangement..............aaarrrrrggh!!!
Cheers,
George

[themoudie]

Thank you for the pearls Nigel.

Good health, Bill

[Ducadini]

I'm not saying that the consistancy and quality control at Bologna were top of the line, but I don't think the cam is Always to blame.
Decades ago, we regularly had rockers (openers and closers) grounded and rechromed (singles, twins and belts).
Often the rockers were from a working head with some very minor binding, just giving the engine a good going over during the winter.
Having the rockers done and refitting the MARKED parts, the cams wouldn't turn and sometimes we had to go to nearly 0.5 mm play at the closer.
Discussing the problem with the rechromer, we found out that Ducati changed the circle of the rocker surface at a certain point.
Don't remember which one was how much, and can't check that right now.
But the changed radius has an impact on the point where the rocker starts to move and the elevation (that has to coincide with the cam).
I don't think there are many closers around that weren't rechromed at a certain moment.
Fitting a Imola in a 350 head gave us the same problem and, not wanting to modify the cam, we abandoned this project.
Still, I'm not taking the Imola's out of my SS and fitting Sport heads just because setting tolerances is a bit difficult.

ciao
ducadini