250 MK3 crankcase rebuild

[DBDBrian]

[quote="marsheng"]
I mulled over the problem during lunch and decided to press on.

As they say, measure twice, cut once, a one way street job isn't to be rushed into , nice job Wallace.

[marsheng]

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.
Nigel

I made new brass sleeves for the side cover and both the crank end and cam drive seemed to be perfectly aligned when I put it together. As these were correct, I was surprised that the oil pump was out. Looking carefully at the oil pump castings, it looks like it was the problem. The drive brass bush in not in the center of that castings by a similar amount to what I had to move it.

Getting close to closing up the crank !!

[marsheng]

Oldham couplings cope with axial misalignment.
Could one be used at the pump drive without mangling things too much?

There are other options but now that they are aligned as well as can be, there will not be much sliding movement between the drive and oil pump which I like. It is a permanent fix and very easy to reassemble at a late date.

[LaceyDucati]

Jordan,

The principle of the coupling as it stands should cover "minor" errors up to 0.25mm (3.5mm tang, 4.0mm slot). As Wallace discovered if you are unlucky the errors can add up to more than that, like his 0.4mm. As I've previously mentioned, one of the advantages of the aftermarket pumps, is the 8mm shaft allows for a wider narrower tang (3mm) which from my experience will overcome/accommodate all these errors. Pumps I fitted 20 years ago with the 8mm x 3mm tangs show no signs of wear or distress. It is however not possible to have a 3mm wide tang with the 6mm shaft without risking rounding the tang off.

The errors that lead to the missalignments are:

The idler gear bushing relative to the casing dowels
The pump recess in the timing cover relative to the dowels
The pump body itself
The centralness of the tang and slot relative to the shafts
The slop in the pump body holes relative to the mounting screws.

To many variables and sometimes you get lucky and others you don't.

When you understand how these parts were produced on multi axial machines one after the other fixed to jigs, you can see how errors can creep in. The parts were manufactured by mostly unskilled/semi skilled operatives and if the fixtures weren't cleaned or the operator was in a bad mood and taking it out on the job, "banging em out"..... Add to that it was all influenced by Jig errors/wear, machine setting and wear issues creeping in. I'm sure these issues were widespread in manufacturing in the period and still exist today in the "cheap" end of mass production.

The wildest errors I've found so far are in the alignment of the cam and tacho drive boxes, which makes the small issues of 0.4mm missalignments seem tame. Hence the long suffering coupling "widget". The issue here is two sloping faces with holes/thread in them relative to two bores. If you consider the implications of tolerancing these componets or try to draw it out you will see where the issue lies. Even the gasket thickness is crucial to alignment. I will also add that some available aftermarket drive boxes are next best thing to useless and are likely to end up in a nasty mess in the top bevel housing.

Fortunately most other components are less dependant on so many different alignments/perimeters in such a short distance.

Nigel

[DBDBrian]

Jordan,

The wildest errors I've found so far are in the alignment of the cam and tacho drive boxes, which makes the small issues of 0.4mm missalignments seem tame. Hence the long suffering coupling "widget". The issue here is two sloping faces with holes/thread in them relative to two bores. If you consider the implications of tolerancing these componets or try to draw it out you will see where the issue lies. Even the gasket thickness is crucial to alignment. I will also add that some available aftermarket drive boxes are next best thing to useless and are likely to end up in a nasty mess in the top bevel housing.

Fortunately most other components are less dependant on so many different alignments/perimeters in such a short distance.

Nigel

My rev counter drive was in a bit of a sorry state, as it required a new input shaft, I decided to make it longer and dispense with the coupling.
To insure alignment I dowelled the cover onto the head , and line bored it through the camshaft bearing housing, with the gasket fitted. Then fitted a bronze sleeve to bring it to size.
The drive box also needed a new output flange.
I also replaced the roll pin with a cap head bolt.

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[Duccout]

Great work Brian!

[nalimugmug]

Hi Brian,

Please excuse my ignorance, but what head is that in the last picture?

I ask because I have a head in my garage which appears very similar and I've no idea what it is, I'm leaning towards a mototrans head but of course I may be well off. This head has a 250 cam end cap which may mean nothing.

Thanks

Bob

[LaceyDucati]

Nice work Brian and nice to know I'm not alone in my "I don't believe it" moments

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Nigel

[Jordan]

The principle of the coupling as it stands should cover "minor" errors up to 0.25mm (3.5mm tang, 4.0mm slot).

Thanks for the description, all interesting.
When I first read (the quote above), I thought you were referring to an Oldham coupling, but of course you mean the standard tongue and groove arrangement.
As Wallace mentioned, there could be some sliding in an Oldham - deliberately designed that way to allow shafts to be not coaxial.
What I don't know is if there could be such a thing as "too much" sliding action.
Would say half a millimeter misalignment be too much for an Oldham coupling in this application?
It doesn't seem so much.

[DBDBrian]

Hi Bob, the head is off a 1959 200 SS, fitted with a 27 mm SSI DellOrto, I am no great authority on originality, so I can only assume it is the one which was fitted when it left the factory.


I like that Nigel, I had my first Italian motorcycle in 1961, when I was sixteen, one would think by now I should have grown out of it.
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