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Hello all, my 250nc build is almost finished, i need to choose a couple of sprockets and am not sure on sizing.
The bike is a 4 speed but the engine should be pretty fruity, mk3 piston, big valve ported head, 32mm mikuni etc, can anyone give me advice as to appropriate sprocket sizes please?
The bike will be road registered and primarily ridden there (enthusiastically), i may go on track when it has some miles under its belt.
Tom

[quote= pptom ...
" i need to choose a couple of sprockets
The bike is a 4 speed
can anyone give me advice as to appropriate sprocket sizes please? "

____ Here follows a link to a related thread that may likely be of help with guiding you towards a fairly good choice of sprocket-ratios... viewtopic.php?f=3&t=1805&p=13290&#p13282
__ The highest stock overall-gearing on any 250-model was found on the 1965 Mark-3 and it sported 40/18t sprockets (in the #428-size). _ While that extra-high sprocket-gearing makes 1st.gear rather quite 'tall', the 5-speed 250-trans can still just manage it fairly well enough.
However while it would make fairly logical-sense that the 5-speed*trans.versions should ought naturally be rather more suitably inclined to better cope with such highly-raised overall-gearing, (* accordingly-assuming thusly up-to a 25% wider gearing-range than a mere 4-speed),, somewhat inexplicably however, the 4-speed trans.version actually rather has a lower 1st.gear than does any of the various 5-speed versions ! _ Yet all 250/350/450-models employ the very-same top-gear (internal)-ratio, so-thus the 4-speed trans.version rather actually enjoys the widest ratio-span between 1st & top gears !
So therefore the 4-speed transmission is rather the better-enabled version to deal-with the extra-tall (and nicest riding) 250-Mk3.gearing, since the 1st.gear-ratio of the 4-spd.trans isn't quite so highly-tall for taking-off from a stop with.
__ If you still think you'd prefer top-gearing that's not quite so high as that of the n-c.250Mk3, and would rather consider some other logically-based gearing-ratio, (such-as setting-up your 4th.gear-ratio to be equal to the 5-spd.40/18-raitio's 4th.gear,, or rather perhaps, setting your 1st.gear-ratio to be equal to the w-c.40/15*-ratio's 1st.gear -[* of the 250-M3D.model], [which would accordingly make your top-gear near 5.8% higher than the stock-DESMO's 5th.gear],, or of-course, even the likes of whatever your-own choosing),, then let me know, and I'll next figure for you what possible sprocket-sizes will result with pretty-much the same gearing-ratio.



" the engine should be pretty fruity, mk3 piston, big valve ported head, 32mm mikuni etc, "

____ What camshaft-model do you have to go with that, and what are you using for a muffler ?


Duke-Cheers,
DCT-Bob

Thanks Bob. Good advice on the 18/40 combo.
Running a green and white grind and a custom reverse megaphone.

[quote= pptom ...
" Good advice on the 18/40 combo. "

____ Just to make it certainly clear, I really wasn't actually advising to particularly choose that n-c.Mark-3 sprocket-ratio. _ (However I would tend to suggest 45/18t GT-gearing or 45/17t Monza-gearing, and rather recommend 43/17t sprockets.)



" Running a green and white grind "

____ While I wouldn't discourage the employment of the 350-Mk3/G&W.cam-model (which merely is as a HOT Mach-I cam),, I'd be somewhat fairly concerned about taking full-advantage of a 250-F1/G&W.cam within an old 4-speed motor, (as I've never before heard of that having ever been tried).
You may be the first to possibly learn of any unforeseen consequences by running an old/ordinary 4-speed motor with such a top-end that's so-much inclined for higher racing-revs. ...
__ When Ducati-themselves ran with such an extra wild cam in a 250,, it was rather done either as a w-c.DESMO, or at-least the valves were rather controlled with extra-strong 'F1' type valve-springs, (since the 250-engine could continue-on productively-revving well beyond 10-grand with the F1-cam).
__ If your 250 project engine indeed has the 250-F1 cam.model, (which has pretty-much the very-same valve-timing as the std.DESMO-cam),, then you may be more-so inclined to rather choose to actually-employ the same sprocket-ratio as do all the stock DESMO-models, (which is a 2.67:1 ratio). _ (Of-course not to imply that such a gear-ratio is any more safe than higher [or lower] gearing,, but rather, only that the factory apparently expected increased engine-RPM [combined with the lower 40/15-gearing of the w-c.M3D] to compensate for a resulting top-speed near-around that of their other high-performance models [such-as the n-c Mach-I/Mark-3 & Mark-III] which rather employ a 18t.drive-sprocket.)


Duke-Cheers,
DCT-Bob

The head has also had the lacey valve spring conversion, nigel also rebuilt my crank with one of his arrow conn rods, so i'm hoping the engine is up to the task!
Thanks again

[quote= pptom ...
" so i'm hoping the engine is up to the task! "

____ Probably-so then.
__ And since the old-motor's transmission-components could at-least adequately handle the stronger low-end torque of the milder engine-models, those motor-parts also ought-to cope well enough as well.


____ So now what choice(s) so-far, have you become inclined to accept for your sprocket-gearing ?
__ Do you happen-to already have a selection of sprockets to choose from ?


Hopeful-Cheers,
-Bob

Thanks again Bob - I've no stock of sprockets so will be buying them and the chain new.
At the minute I'm inclined to go 17 / 43, this seems like a sensible starting place

[quote= pptom ...
" I've no stock of sprockets so will be buying them and the chain new. "

____ I might be able to help you with that.



" I'm inclined to go 17 / 43, this seems like a sensible starting place "

____ Indeed so ! ...
I'm sure you'll find that sprocket-ratio provides a nice enough riding experience,, as it's overall-1st.gear-ratio would be 17.39:1 (with a 4-speeder), and that's still lower than a 5-speed Monza's (which is 16.74:1), thusly 3.8% lower,, and yet also, the resulting top-gear ratio would be 4.7% higher than the 5-spd.Monza'a 5th.gear,
(the resulted top overall-ratio being 6.12:1 [vs the Monza's 6.41:1]) !
So that 43/17-gearing's resulting ratio-span providentially happens to nearly-evenly over-encompass that of the Monza's (overall gear-ratio extremes).
(Note that the 5-spd.Monza & 250-DESMO overall-trans.ratios are pretty-much the same, [with the DESMO's being less than 1% lower].)
__ For track-racing,, you could always simply swap the 17t.drive-sprocket for a 16 or 15 or 14t, depending on the particular race-course.


Duke-Cheers,
-Bob

Brillian info thanks Bob, i'll probably buy the sprockets over here in the UK as i fear that postage over the pond may be a killer.
Tom