250 MK3 crankcase rebuild

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Duccout
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Re: 250 MK3 crankcase rebuild

Postby Duccout » Tue May 26, 2020 11:37 am

Another question: if the cranks were intended to run with zero end-float, would the engines not have been fitted with angular thrust bearings as per all the vee twins?

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Tue May 26, 2020 11:54 am

Duccout wrote:Another question: if the cranks were intended to run with zero end-float, would the engines not have been fitted with angular thrust bearings as per all the vee twins?


Probably correct, however this is a 1960's design, low power motor and I guess regular bearings would do. If I'm not mistaken all the British singles of the time used standard bearings. They did last as there are many bikes with quite high mileage around. The 50% axial loading I think is the reason they work.

Now coming around to assemble the casings and getting 0.00 fit, where do I get shims in 0.01 increments ?

Fortunately, I have a surface grinder so I can make to size.

Question, why is the drive gear SOOO big and the gearbox gears so small. The gears in the box only mesh in half the width of the gear as well.

Cheers Wallace.

Jordan
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Re: 250 MK3 crankcase rebuild

Postby Jordan » Tue May 26, 2020 1:52 pm

Duccout wrote:Another question: if the cranks were intended to run with zero end-float, would the engines not have been fitted with angular thrust bearings as per all the vee twins?


Low resolution sectioned drawings of a widecase single. One view shows angular contact main bearings. I've never seen or heard of such in reality.

At a guess, deep groove bearings are made in much bigger quantities than other types, so could be a lot cheaper.
They do seem to stand up fairly well to side loads in some situations.
I owned two bikes with them in the steering heads as standard - MZ ETZ250 and Norton Commando. They both had tubes between the inner races, unlike spindle bearing setups.
Ducati350-Widecase-engine-sections-sm.jpg

ducwiz
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Re: 250 MK3 crankcase rebuild

Postby ducwiz » Tue May 26, 2020 5:56 pm

Jordan,

a very interesting drawing! Must show the latest version ever made, as it has the 2-cable-feedthrough from the alternator, indicating a Ducati Elettrotechnica version with CDI. Do you also have a high res version available? And from where did it come?

Angular contact bearings are found in a lot of two stroke engines, for scooters, mopeds etc. The only british bike I know with similar bearings is the late Velocette single series, i. e. Thruxton, Clubman and possible others. Here we find tapered roller bearings, preloaded by 1-2/10 of a mm, afaik.

Hans

Duccout
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Re: 250 MK3 crankcase rebuild

Postby Duccout » Tue May 26, 2020 6:55 pm

Don't get me started on Velocette main bearings! The adoption of taper-roller mains was economic and engineering suicide for the company. The crank had to be shimmed for 4 thou pre-load, by placing shims under the outer bearing rings in the crankcase, which required the cases to be heated up and the rings shocked out of their housings by bringing the crankcase half down with much force onto a block of wood, then the shims had to be changed, and the bearing races replaced and the whole operation repeated until it was correct.

Only a lunatic could have believed that this would be an expedient way to build an engine, and why taper-rollers anyway? Did Gold Stars use them? Did Manx Nortons use them? No wonder Velocette went bust.

Actually....I can't think of any other application where there was not a threaded adjustment provided to take the play out of taper-rollers.

DBDBrian
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Re: 250 MK3 crankcase rebuild

Postby DBDBrian » Tue May 26, 2020 8:13 pm

Duccout wrote:
The singles have a crank end-float range of 0.0012 - 0.020 thou, which means that if this amount of float is not factored into the bevel shimming then as the crank is pulled to the primary side during running, the crank bevel will be loading the bearings of the vertical bevel, or am I missing something?

My genuine Ducati Manual list 0.03 - 0.05 MM / 1.181102- 1.968504 Thou. I think we my need to move the 2 up one place :)

Not had too much to do with Velocette's but taper roller mains never made much sense to me either.
I have fitted a needle roller, with a dedicated oil feed in the timing side of my Goldie.
Brian
Made in England

LaceyDucati
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Re: 250 MK3 crankcase rebuild

Postby LaceyDucati » Tue May 26, 2020 8:30 pm

The Swiss army Condor A350 with a Ducati single engine fitted, used angular contact bearings as standard, I have replaced them with standard bearings on the engines I've built without issue. I will check the Condor manual to see what is specified for crank shimming.

Wallace, you are probably correct with your guess at 0.044mm expansion width wise, but you missed my point regarding the bearing expanding in diameter as the fit in the aluminium expands relative to the steel bearings. That in itself will add to the side clearance as well.

Personally I'm happy with zero to 0.01mm when cold as my optimum. I've not had any issues doing so for 30 years of building both my own and many customer engines, that includes road and race engines with success. Many things as original with these engines are often not aligned as well as you may think, therefore running everything tight as a drum may not be the best plan. I could write a book on the inaccuracies, design issues and design errors that i've discovered over the years with these engines.

Brian you are correct with with the original Ducati Factory specified clearance ( I think I quoted it as 1 to 2 thou earlier). Many have of course used that without issue. Personally I'm certain that if the crank is shimmed between zero to 2 thou (0.05mm) there will not be any issues. Regarding preloading I've never done it so I can't offer any opinion either way.

Nigel

DBDBrian
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Re: 250 MK3 crankcase rebuild

Postby DBDBrian » Tue May 26, 2020 9:40 pm

I'm with you on this one Nigel, I am in the process of assembling my case's I think I will aim for zero end float, I really can't see any advantage in preloading.
I have been sorting the intermediate / pump drive today, the drive slot was worn, and far too much end float on the shaft.
I removed the shaft from the gear, cleaned off the spot welds. ground the end to reclaim the slot, rewelded the gear flange in a position, giving the shaft a nice rattle end float.

P1070757.JPG
P1070758.JPG
P1070760.JPG
Brian
Made in England

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Tue May 26, 2020 9:47 pm

LaceyDucati wrote: I could write a book on the inaccuracies, design issues and design errors that i've discovered over the years with these engines.
Nigel


Try doing a Koing 350 4 cylinder. Some of the worst engineering I have seen. For one, the head bolts went through free air. When you tightened them down, they dished. The crank would not turn when reassembled. Had to remove some material from the casing. Etc Etc.
1964 - 250 MK3 ... 1980 - 900 SSD ... 1977 - 500 GTL Parallel twin ... 1980 - 500 Pantah ... Plus a 'few' others.

Jordan
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Re: 250 MK3 crankcase rebuild

Postby Jordan » Tue May 26, 2020 11:20 pm

ducwiz wrote: Do you also have a high res version available? And from where did it come?
Hans


I'm sorry, I can't remember where I found the drawing.
It was on the internet somewhere I think.
I also wish I could have a high resolution version.
I did slightly improve the original copy with Photoshop, as the "original" was even worse.


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