This from my desmo.org.uk site I think it would be better to have this info in more than one place…
Largly written and researched by Alan Yardley and members of the Ducati owners club- and frankly anything I have written is in the main things I have picked up from DOC members!!
Also, With big thanks to Ditch and Luca from the Pantaheads E mail list and all who contribute to the DOC(GB) E mail list.
I am gradauly cutting bits out of this posting and pasting them into seperate posings for each subject.
Fuel Tank
Later Pantahs with removable fuel cap allow ingress of substantial water which rusts the bottom of the tank. Solution is to treat tank with chemical liner and regularly drain.
Ignition
Ignition pick up wires can cause problems and are best replaced with Teflon coated or oil/heat resistant wire.
Gearbox
Second and fifth gears can blow but others seem to last forever. Primary gears have good life. Pantahs after frame number 660901 had a fifth gear ratio altered from 1:0.9 to 1:0.931.
Instruments
Replacement speedos can be found from Honda 550 K3 and 750 F2 but are a little optimistic in their readings.
Carbs
Transmission snatch below 3500RPM can be cured by filing out the cutaway on carb slides by an eighth of an inch if air screw adjustments don’t cure it. 40mm Dell’Orto pump jet carbs can be fitted if replacement manifolds are made up. K & N filters and ditching the airbox is a popular modification.
General
My Contribution Ignition Volt drop from battery to ignition units was found to be high (2.3 Volts) when start button pressed. This caused unreliable starting as ignition would only fire on release of start button. Cure was found by adding relay to switch directly from the fuse box to the ignition units. Result was a volt drop of only 0.2 volts and reliable starting even with a half dead battery. Due to regulator system, battery discharges even when ignition is off. Without replacing regulator the best solution is to remove earth battery lead when leaving bike standing for more than a few days (cheap) or use a battery charger suitable for permanent connection (£45). Electrical connectors are rubbish and have all been replaced by quality crimps fitted with a ratchet crimping tool. Fuse box corrodes badly. I found out that my bike is on it’s third fuse box. If it looks like happening again I will replace with something of higher quality. Koni dial-a-rides found to be under sprung and over damped (I’m 15 stone in battle dress. Front fork preloader tubes cut to 135mm. 200ml of 20 grade JP1 oil used.
Piston, bore and stroke choises
Now, to resume the complete matter, since I have already considered all mix-matches of parts (and as yet done none) and a friend has fitted 82 mm pistons to an Alazzurra 350 (BxS=62x51):
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All Pantahs were created with only three crankshaft strokes: 51mm, 58 mm and 61.5mm
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All Pantahs were created with identical lenght cylinders, so they can all fit the same belts.
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Pantah pistons came in 62 and 70.5mm bores for the 51mm crank; 74 and 80 mm bores for the 58 mm crank and 82 and 88 mm bores for the 61.5 mm crank.
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All Pantahs have the same conrods (900cc NOT included), so big ends and small ends are identical in all engines: on the conrod, on the shaft and on the piston.
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any Pantah cylinders will fit in any Pantah crankcases, with the only relevant exception of early SL500 cases which require their specific cylinders
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the standard combinations of shafts and pistons will give capacities of 350, 400, 500, 580, 650 and 750 cc
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the above features and apparent interchangeability will tickle the fantasy of home tuners who are already imagining an 88x51 super-short stroke Pantah 622cc
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unfortunately, being the cylinders all the same length, the pistons of shorter stroke engines must be longer than those of longer stroke in order to reach the head. Exactly half-stroke-difference longer
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therefore, a piston matched with a crankshaft of shorter stroke will give a greater squish height; a piston matched with a crankshaft of longer stroke will hit the head. (e.g. if an 82 mm bore piston coming from a 61.5mm stroke motor is fitted to a 58mm stroke motor, the squish band will be 1.75 mm
higher) -
all Pantahs were created with three types of heads: one with small ports and a small dome, which was fitted with 30.5 and 33.5 valves and coupled with the 62 and 70.5 bore pistons, one with medium ports and a medium sized dome, fitted with 33.5 and 37.5 valves and coupled with all other pistons and one with big ports and big dome, fitted with 36 and 41 mm valves, coupled only with the later version of 88 mm pistons (F1B)
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this means that if you fit a different piston-cylinder pairs to a certain Pantah engine without modifications, not only squish height will be affected but also squish band width, compression and valve-to-piston clearances.
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if on the other hand you shorten the cylinder to adjust the squish heigth but you retain the standard head, you might end up with the compression ratio of a diesel and the belts will be too long and will stay loose.
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it is possible to fix compression values by machining the heads and/or the piston domes and belt tension by elongating the belt adjuster hole, but at this point the camshaft will have rotated a few degrees. when machining the piston domes you will probably also have to machine the valve grooves.
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you can still machine a new groove in the pulley to re-position the camshaft with the correct angle, but at this point you will have: shortened the cylinder; machined the head and piston dome; modified belt adjuster, machined the belt pulleys and perhaps re-balanced the crankshaft and fitted bigger valves and seats. Your mix-matched motor will at this point have costed you more than a bigger, standard motor (Alazzurra and Elefant 650 are cheap and plentiful).
the early 500s (those with the “banana” fairing) had smaller holes in the 'box to accomodate cylinders, and the cylinders had accordingly a smaller outer diametre in the part that fits in the 'box. The holes were enlarged to accept the 600’s bigger bore cylinders and the modification was standardized on all Pantahs, including the 500, 400 and 350. This mod went more or less along with the new stronger “3 dogs” gearbox so if you have the old motor you will also have the weaker gearbox According to the Pantah’s part list (available at ducatimeccanica.com) the “old” cylinders and gearboxes are on motors nos. 600001 to 601653 and 602238 to 602528 while the new gearbox was fitted to motors from no. 603246 onwards My Pantah 500 has motor no. 600508 so it has both of the old things. ciao Luca
750cc in a Pantah. Just what combination of parts will I need to make a
650 Aly into a 750 motor?
I just checked in the manual, the Aly has an 82mm bore and a 61.5 mm
stroke.
Does this mean that I can bore out the liner and drop in a set of 88 mm
pistons? (Discounting the Nikasil liners or course) Or do I have to
find a set of barrels from an F1?
both options are possible. F1 (or Elefant-Indiana-early modern SS) cylinders will fit without mods.
Will the heads still fit? Would the crank have to be rebalanced. Can I
Changing 659 into 750….
do this on the cheap?
F1A had the same head as the Alazzurra so F1A pistons will match the standard head. Not F1B pistons. The Alazzurra was already under-valved as a 650 so imagine as a 750!
