250 MK3 crankcase rebuild

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DBDBrian
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Re: 250 MK3 crankcase rebuild

Postby DBDBrian » Mon May 25, 2020 9:19 am

(Can a slight negative clearance bend a crank? Or have I miss understood Nigel’s comments.)
No I don't think you have Graeme.

Taking Nigel's comment re loading the main bearings to the extreme, of say .010", would bend the crank, so yes, in theory, any amount of preload would have an influence on the crankshaft alignment.
Brian
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LaceyDucati
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Re: 250 MK3 crankcase rebuild

Postby LaceyDucati » Mon May 25, 2020 9:38 am

Hi Graeme

Yes preloading the crank will "bend" the crank, generally not permanently, more a loading. In theory if you estimate the preload correctly when the engine is at full temperature then in theory the crank will have no side play. My concern has always been that these type bearings are not designed for sideways loading/running and are designed to run with the balls at the bottom of the track for there designed load rating. Bearings of this type have side play to some extend and that varies from manufacturer to manufacturer and in recent years this seems to have increased. If I set the clearance to zero, the bearings are already pushed sideways and my theory being the expansion will allow the bearings to unload as such. I guess this will be a few thou and in reality that won't cause an issue in use. Well so far so good, no failed bearings, bevels etc, but like so much engineering it's never perfect and an element of guess work has to be applied. The trick is to to being consistent, observant and do what proves to work. The problem with estimating the expansion amount is there is not only the cases expanding side to side the bearings lose tension in the cases and the side play increases due to that as well.

I hear of quite excessive preloading of bevel twins, but then the same people seem to be mentioning cases splitting too...... my estimation for a twin expansion was around 3.5 thou and I often here 8 thou mentioned. At 8 thou preload the crank on a twin will be running like the proverbial skipping rope ( I know I've done that test) and I can't see how it will will ever be running true as such. It will be forced true in the cases, but it will be trying to fight it's way out as far as I see it.

I think there is a little give and take and there are other issues around the shims fit concerning the large radius where the journal meets the thrust face. If the shims don't sit perfectly against the trust face then the crank will soon shake loose in use.

Just my thoughts and experience, other thoughts and experiences are available :-)

Regards Nigel

LaceyDucati
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Re: 250 MK3 crankcase rebuild

Postby LaceyDucati » Mon May 25, 2020 9:48 pm

Hi Wallace

The gear dogs don't look that bad to me, could be better but in the big picture usable.

Regarding the crank wear 29.99mm is about as large as the journal can be and still allow the crank to move in the bearing. Certainly 29.99 to 29.985mm should be considered to be as new. The wear on the journal will be on one side and be in the vertical direction with the pin at the top. In my opinion 29.96mm is still about serviceable, again not perfect but will run fine for some time to come. Once the journal is below 29.95mm I would then definitely consider a repair like hard chroming or metal spraying as you suggest. However Journal repairs are nowhere near as straight forward as you would think. The issue lies with the fact the cranks were ground as an assembly. If you place a crank half between centres you will generally find the nose of the crank runs true to the end centre, however the main bearing journal will run out to the inner centre. Unfortunately some have fallen foul of this issue and have assumed if they get the journal ground to the centres all will be well.... Only to find the crank won't true up. There is always a unworn area on the journal and this can be used to "re centre" the half. Once the half is running true, repairs are fairly straight forward.

I've seen evidence of the "phantom centre potter", knurler, etc etc, I can see where you are coming from, but ultimately not nice. The issue with loctite is if it sets off centre the crank is likely to then be running out of true. Having built many race engines, that get stripped more regularly (at least ever few years) you tend to find out what happens to many of those "That may work idea's". My experience is that loctite used on the crank journals will usually mysteriously vanish in operation.

As for the casing bushes tolerance, most of those are in the manual, however bushings like the points shaft bushes will need to be whatever size necessary to allow the shaft to run free, much like the crank end bush. Suspend all you beliefs that all the covers and components line up with each other.... I'll stop there.

Regards Nigel

graeme
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Re: 250 MK3 crankcase rebuild

Postby graeme » Mon May 25, 2020 9:55 pm

Hello Nigel,
Thank you for your time explaining, makes sense.

Regards
Graeme

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Tue May 26, 2020 1:44 am

Hi Nigel and all others. Thanks for the replies. As no one has corrected my temp expansion figures, then hopefully they could be right. If they are, then a 100 deg rise creates a 0.04 mm expansion. This is very small. So my opinion is to get the float to be as near to 0 as possible. This means that the crank will not move left to right or visa versa under load and all related timings, gear clearances will remain fixed. The crank should be free to rotate.

Manual? I've looked but only found a parts one. Anywhere I can download one ? In the meantime I have replaced the bronze bushes. Easy to do now rather than later.

May a new post : Cams
The left cam is the one that came with the bike. The head was bit mashed up so I got 2 new second hand ones from the USA. Both had the same cam profiles which looks more like and bucket direct acting cam. Anyone shed some light one this ? All heads had hair springs.
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marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Tue May 26, 2020 2:21 am

Bearings:

1/2 way down the page

https://www.smbbearings.com/technical/bearing-load-rating.html

Deep grove ball bearings such as 6200 or 6300 series may take axial loads of up to 50% of the static radial load rating.

This means that you can preload the main bearings without any issues.

Cheers Wallace

Jordan
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Re: 250 MK3 crankcase rebuild

Postby Jordan » Tue May 26, 2020 7:25 am

marsheng wrote:Manual? I've looked but only found a parts one.


https://www.ducatimeccanica.com/#literature

Duccout
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Re: 250 MK3 crankcase rebuild

Postby Duccout » Tue May 26, 2020 8:56 am

One query I have with the crankshaft shimming is with the effect on the bottom bevel if there is any end float on the crank. A crank with zero end-float allows the bottom bevels to be shimmed accurately, ie bevel vee twins, but 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?

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Tue May 26, 2020 9:04 am

Who needs solitaire ??? Read on.

I stripped this motor 5 or so years ago just to see what it was like inside and rebuildable, and, not paying too much attention to the assembly.

Last week, I ordered all the bearings as suggested by this forum. Now to assemble. There is now way a 6204 is going to fit on the drive shaft side near the kick start lever. Looks like some sort of roller cage goes onto first gear. I don't remember seeing that. Back to google - nothing , all show a 6204 bearing.

Started scratching through all the packets of bits. Nothing. In the past I had seen a container of rollers and wondered where that came from. I do a lot of other bikes and thought it was from one of them. A few hours later, the penny dropped. They are from 1st gear. started putting them into the gear, bugger 1 short. Scratched around in the other bits and eventually found the solitary roller. Put in in and 3 dropped out. More grease and tried again, and again, and again. Needless to say, at one stage 3 dropped on the floor.

Now to get it inside the cage in the casing. What an effort, any misalignment, a roller falls out and you start all over again. Even tried and elastic band, but, with a bit grease on it, it never stayed put!! That must have been a Brits idea on getting the Italians back for making good bikes.

I'm checking the free play and somehow when splitting the cases I need to ensure this gear does not jump out.

So far selector drum, 0.8 mm layshaft 0.2, drive shaft 0.9.

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Tue May 26, 2020 9:07 am

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 point exactly. 0 clearance is where I'm going. The bearings can take a 50% axial load, so having no clearance should be fine.

Cheers Wallace


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