250 mk3 1974 crank issues - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 22 (permalink) Old Jun 24th, 2019, 7:29 am Thread Starter
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250 mk3 1974 crank issues

Ducati 250 owner in Edinburgh Scotland.
Just had the crank fitted with a new conrod kit (Lacey Ducati) the crank cases fit together perfectly with just the crank but discovered the cases wont fit cleanly together if I use even the thinnest shim.
Looks like the LH main bearing journal is the problem with the radius to the shim thrust face not being correct, if I fit the original main onto the journal I can see about 1mm clearance between bearing and thrust face on the crank.
If the crank rebuilder (and myself) had known of these issues he could have cleaned up the journal radius and thrust face whilst the crank was split but we were unaware.
I have had advice from Lacey Ducati to say that these journals can be an issue and that chamfering the inside lip of the shim can help, I tried that but I think I would need to remove a lot of material before the shim could sit flush.
A machine shop friend thinks he can fit the assembled crank in a lathe then turn by hand to clean up the journal and thrust face
If this doesn't work, any other suggestions on the best way to sort this would be appreciated.

Regards

.

Last edited by 250-1974; Jun 24th, 2019 at 10:44 am. Reason: mistake
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post #2 of 22 (permalink) Old Jun 24th, 2019, 9:07 am
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looks like your crankshaft is wider after installing the new rodset . how much sideclearance has the rod ?

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post #3 of 22 (permalink) Old Jun 24th, 2019, 9:13 am
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which rod Kit did you do? Lacey has two different kits.

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post #4 of 22 (permalink) Old Jun 24th, 2019, 10:18 am Thread Starter
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Apologies, I made a mistake in the first post, the crank cases go together perfectly with just the crank. it is when a shim goes on I get the problem.
Will check the crank pin clearance tonight but it certainly feels good, very little side movement.
The new crank pin is the Samarin make, Nigel Lacey is happy with the brand and it looks great, come with the thrust washers etc which have all been fitted.
The old main bearing will not sit flush on the thrust face so must be some issue preventing seating.
With the crank installed with no shims I get at least 1mm end float, as soon as any shim goes in, the crank cases will not close, showing a gap about 1mm.
It appears to be an issue with the LH journal not allowing proper seating of bearings or shims.
I marked the journal and spun the old mains on it to see the contact area but I will be rocking it as I turn it so making little contact all the way but if held up to the light there is a gap between bearing and shim face.
Con rod alignment looks like I need to shim lots on LH side
second pic shows case gap with an original .32mm shim installed
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.

Last edited by 250-1974; Jun 24th, 2019 at 10:47 am. Reason: mistake
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post #5 of 22 (permalink) Old Jun 24th, 2019, 11:27 am
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looks like your bigendpin is protruding and preventing the shim to seat .

i was gonne answer your question on motoscrubs but i have problems logging in

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post #6 of 22 (permalink) Old Jun 24th, 2019, 1:00 pm Thread Starter
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Hi Eldert,

I think the motoscrubs forum has died so I joined here today.
That is an optical illusion thanks to my photography skills, the crankpin is flush or just a fraction below....I just checked again.
The shim is stopped before getting to the thrust face.
Just checked the worn original shims, ID is 32mm ! so they have been ovaling around with the crank over time. If positioned in a certain way they can be made to sit on the thrust surface but when in the engine it is impossible to get them to be central around the journal so if one part sits on the journal radius it won't sit flush.
Either I have to machine the journal radius perfect or have the shims machined to fit exactly which may be very difficult.
What would the perfect radius be? I guess machining the journal to the thrust face at 90 deg is not good practice? but looks like that would sort it.

Regards,

Alex

.
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post #7 of 22 (permalink) Old Jun 24th, 2019, 2:07 pm
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Hi Alex

the bearing has a large radius on the inside . the shims are flat . what i do sometimes is get a old bearing and
a shim . put the shim on the crankshaft ( when the crank is on the bench ) put the old bearing over the crankshaft journal
and give it a good whack with a large hammer . that way you have a shim with the correct radius .
you might give that a try

Good luck Eldert
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post #8 of 22 (permalink) Old Jun 24th, 2019, 2:38 pm Thread Starter
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Hi Eldert,

That is a great idea, I can try that if I can't square it up on a lathe tomorrow, one worry would be, sticking the old bearing on the journal radius.
Similar to your idea I thought about a quick tack weld on an old main bearing to lock it up then apply coarse grinding paste and grind the journal to shape that way.
I will give my machine shop pal a go tomorrow to try clean it up by hand turning on his lathe, what do you think about a 90 deg zero radius?
No idea what happened to this crank in the past, bike only done 7000 miles, no idea if the crank pin was original but if so, poor Ducati machining.

Regards,

Alex

.
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post #9 of 22 (permalink) Old Jun 24th, 2019, 2:52 pm
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I just installed the same rod kit on a 350 Mk3. I ll let you know if I have any problems with it when it goes into the cases.


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post #10 of 22 (permalink) Old Jun 24th, 2019, 3:39 pm Thread Starter
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Hopefully your crank journals will be in good shape.
The new Samarin rod kit looks good with a strong rod profile.

.
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