250 MK3 crankcase rebuild

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Jordan
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Re: 250 MK3 crankcase rebuild

Postby Jordan » Tue May 26, 2020 11:45 pm

Duccout wrote: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.


BSA and Norton went bust too, without help from taper roller bearing mains. :)

To be fair, Velocette were successful innovators in their day, with for example two strokes that had oil pumps, the same in principle to Yamaha's (who somehow got away with claiming to invent the system). Velo also invented the gearbox as we know it today, with a two way ratchet sequential shift, a modern rear swing arm, etc. All before the 1940s.

My friend Warwick is making a Velo KSS head in aluminium to replace his iron one. He told me something I can hardly believe - the oil feed to the head is by flooding the whole bevel tube with oil. Nobody gets everything right all the time.

LaceyDucati
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Re: 250 MK3 crankcase rebuild

Postby LaceyDucati » Wed May 27, 2020 9:39 am

Hi,
Wallace,
I'm sure there are far worse engines than the Ducati Bevel Single engine, most of the problems with the Ducati engine are down to the translation of the conceptional design to the production engines with financial restrictions influencing decisions. Ultimately the bevel single is a great engine to work on for engineers ans enthusiasts alike.

Regarding Condors/angular contact bearings
Just to muddy the water further I have checked the Condor (which uses angular contact bearings) manual and it states a crankshaft axial clearance of 0.05 + 0.10 mm whatever that means... Then again they also state a main bearing journal run out of 0.04mm (0.0016") which I would deem to be less than ideal. My normal maximum acceptance is 0.02mm which is in line with bevel twin cranks which are physically far greater in size. One would naturally expect a single crank run out to be ideally less. Maybe The Author of the Condor manual had had a brush with reality at could be expected with the engines they had been supplied. Regarding crank run out, I normal run the crank on the main bearing journals and clock the noses of the crank as that gives a larger relative reading. However I don't clock the oil feed nose as that is always worn, instead I clock the bevel gear diameter by sliding a 6202 bearing on it. Clocking that way I usually like to see a maximum run out of 0.025 mm (0.001"), ideally perfect.....we can always hope :-)

Brian,
Be careful regarding the oil pump drive slot in the gear relative to the pump drive tang, as there is invariably a misalignment of the two components. Dry build the pump and gear components before assembling the cases, so you can check the assembly spins free by either spinning the points shaft if it is installed or flicking the idler gear around through the main bearing. The misalignments in this area are responsible for the wear on the components (slot, tang and pump bushing) in the worst case scenarios the drive tang will get ripped off. If the assembly is tight it will slowly eat itself back to the worn state you found it within the first 50 miles. As the slot is 4mm and the tang 3.5mm it is obvious the alignment errors can add up to and be greater than 0.25mm in some cases. I think I've touched on this subject before regarding replacement pumps.

Regards Nigel

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Wed May 27, 2020 11:03 am

DBDBrian wrote: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


Some figures from my bike. Crank end float with original spacer was 0.27 mm with no gasket. If the gasket was 0.3 mm then the original float would have been around 0.6 mm.

Could this explain the damaged teeth ? Everting else in the motor is good for its age. This was the main damage (other than the valves dents in the head)
DamagedGear.JPG


Cheers Wallace.

DBDBrian
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Re: 250 MK3 crankcase rebuild

Postby DBDBrian » Wed May 27, 2020 3:44 pm

Thanks for your advice Nigel, I took onboard your previous post / comment awhile back, re pump shaft alignment.
The drive shaft had excessive end float, resulting in the pump tang only just entering the slot, causing wear, so grinding the end off the shaft, reclaimed it, with plenty of depth left in the slot.
I had a choice between three pumps, fortunately one being in very good condition all round. trial run with everything fitted in the crankcase, checking through the main bearing bore, everything lined up perfectly, with about 10 thou max, end float. Deep Joy.


marsheng wrote:
DBDBrian wrote: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


Could this explain the damaged teeth ? Everyting else in the motor is good for its age. This was the main damage (other than the valves dents in the head)


Cheers Wallace.

Wallace, my comment on Duccout's figures, was a bit tongue in cheek, as I presume he meant 0.002 Thou, not 0.020 as posted.

(other than the valves dents in the head) ?
Brian
Made in England

Duccout
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Re: 250 MK3 crankcase rebuild

Postby Duccout » Wed May 27, 2020 4:14 pm

Well I suppose that if end-float was set at 0.020 there wouldn't be any worries about side loadings on the bearings!

DBDBrian
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Re: 250 MK3 crankcase rebuild

Postby DBDBrian » Wed May 27, 2020 4:21 pm

:)
Brian
Made in England

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Mon Jun 01, 2020 12:00 am

LaceyDucati wrote:Be careful regarding the oil pump drive slot in the gear relative to the pump drive tang, as there is invariably a misalignment of the two components.
Regards Nigel


I did a dry test and sure enough, the drive gear for the pump would not rotate freely once assembled. On closer inspection, this is what I found.

OilPump.JPG


I don't like this option but I see on other way. I'm going to turn down the oil pumps outside housing by a mm or two and make an offset spacer to realign it to where it should be.

Cheers Wallace

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Mon Jun 01, 2020 2:44 am

Being a holiday today, I took full advantage.
I made a post 6mm for the pump side and 4 mm for the drive side. I used this to check alignment.
oilAlignment.JPG


I mulled over the problem during lunch and decided to press on. I removed 2 mm from the pump top from a diameter of 50 to 48 mm for 5.5 mm.
I tried the bits in hand tight. (PS I had to drill the 4 holes to 7 mm from 6.5 to get some movement. ) After a few trials and some measurements, I found I had to move the center over 0.4 mm. I made a spacer as shown.
OffsetSpacer.JPG

Assembled it all and perfect rotation. All new sleeves made for the shafts.

PS I have had all the parts vapour blasted. It is a pleasure working on the motor now.

LaceyDucati
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Re: 250 MK3 crankcase rebuild

Postby LaceyDucati » Mon Jun 01, 2020 8:25 am

A well thought out solution Wallace.

While you're checking things in the timing chest I'd check the crank end bushing while you are on a roll. Line boring through mains and an oversize O/D bushing sorts any misalignment issues.

Nigel

Jordan
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Re: 250 MK3 crankcase rebuild

Postby Jordan » Mon Jun 01, 2020 9:04 am

Oldham couplings cope with axial misalignment.
Could one be used at the pump drive without mangling things too much?
Last edited by Jordan on Mon Jun 01, 2020 10:00 am, edited 1 time in total.


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