250 MK3 crankcase rebuild

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marsheng
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250 MK3 crankcase rebuild

Postby marsheng » Sat May 23, 2020 5:53 am

Finally getting round to reassembling my 250 MK3

I split the crank and the wear on the conrod and bigend pin was minimal. The rollers were a bit undersize. 3.46 mm I got new rollers and and an aly machined cage. New rollers were 3.50 +- 0.005 mm. All assembled well however there is minimal clearance between the bigend and thrust washers but it is free !!! As the crank is pressed up against the bigend pin, there is no room for adjustment.

Next, I was wondering if I could do away with the main crankcase gasket. I think with Yamaha bond, it does away with gaskets. I have spot welded the casings here and there to repair mating damage and then ground them flat. They are a good clean fit now.

Need to know about the gasket before I setup the crankcase end float. Some say 0.01 mm float, others say minimal crush fit ie -0.1 to -.02 mm. That is normal for the angular contact bearings but not straight ball bearings. I'm in favour for zero to -.01 fit.

Comments will be appreciated.

Thanks Wallace
Last edited by marsheng on Tue May 26, 2020 9:10 am, edited 1 time in total.
1964 - 250 MK3 ... 1980 - 900 SSD ... 1977 - 500 GTL Parallel twin ... 1980 - 500 Pantah ... Plus a 'few' others.

Jordan
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Joined: Fri Dec 24, 2010 11:29 am

Re: 250 MK3 crankcase rebuild

Postby Jordan » Sat May 23, 2020 7:04 am

I've heard of someone who assembled with no centre gasket, reportedly successfully.
Apart from crankshaft end play (I try for zero, why not?) gearbox input, output and selector cam shafts need resetting, obviously.
Also maybe the cylinder mouth should be checked in case it gets too tight.

What do you use to measure 0.005mm?

Bevel bob
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Re: 250 MK3 crankcase rebuild

Postby Bevel bob » Sat May 23, 2020 7:40 am

All sounds good. Mainshafts need to run true to avoid primary gear runout, Mine was like a skipping rope!!, If you have the 5 speed box don't shim the selector drum too tight as the tracks are not perfectly machined. If cases closed up ,head bolts can get tight to fit.

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Sat May 23, 2020 12:15 pm

Jordan wrote:What do you use to measure 0.005mm?
A very small micrometer. No seriously I do have several Mutityo instruments that read 0.001mm. At that precision you have the check the temperature.

The gears on the selector forks have a healthy spacing between dogs.

These gears are so small. If I compare a GT250 Suzuki, they are massive.

I have a 5 speed.

Cheers Wallace
1964 - 250 MK3 ... 1980 - 900 SSD ... 1977 - 500 GTL Parallel twin ... 1980 - 500 Pantah ... Plus a 'few' others.

Duccout
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Re: 250 MK3 crankcase rebuild

Postby Duccout » Sat May 23, 2020 2:01 pm

I don't know about the centre gasket - perhaps Nigel or Eldert would comment. Anthony Ainslie, for those who remember him, told me once that he never used any gaskets in his engines.

I think that there is an article somewhere on this site by Frank Scurria about shimming cranks.

LaceyDucati
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Re: 250 MK3 crankcase rebuild

Postby LaceyDucati » Sat May 23, 2020 7:26 pm

Hi,

I generally don't use centre gaskets and just shim accordingly. It's how I built all my race engines and I just prefer not using them, I like the repeatable accuracy of the build without.

I would say 0.01mm was about perfect to me, I use an imperial tenths clock gauge and just like to see a few tenths movement. Factory quoted free play is I think 0.025 to 0.050mm. I actually work to zero to 0.025mm, been doing that for 30 years without issue.

If there is zero clearance or any clearance the crank will always "bob" up to the top from BTDC. As soon is there is any any pre-load then the crank will stop in any position without returning to the top. I personally don't ever preload single cranks, if you even pre-load it 0.01mm it will squeeze the crank. If you do this and then clock the crank for run out you will find it running out.

Regarding big end rollers ideally they should be "grade 2" rollers +/- 0.002mm variation.

Regards Nigel

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Sun May 24, 2020 7:11 am

A few more questions on the bottom end.

Hopefully and easy one, what is the recommended clearance on the bronze bushes in the cases ?

One crankshaft journals is a bit oval. The good one is 29.98 nearly all the way round, the other is goes from 29.98 to 29.96. I was thinking about getting it metal sprayed but then I'm sure someone on this forum as been there done that. Another option (not sure if I should even mention this) is center punch the worn side and then install with loctite. If the end float is correct, this may work.

Lastly, not have ever ridden this bike, came in a box of bits, I would think the wear on these dogs is ok and changing would be fine. See attachment.

Gear.JPG
1964 - 250 MK3 ... 1980 - 900 SSD ... 1977 - 500 GTL Parallel twin ... 1980 - 500 Pantah ... Plus a 'few' others.

Jordan
Posts: 983
Joined: Fri Dec 24, 2010 11:29 am

Re: 250 MK3 crankcase rebuild

Postby Jordan » Sun May 24, 2020 11:00 pm

marsheng wrote: I would think the wear on these dogs is ok and changing would be fine. See attachment.

Gear.JPG


Your gear sample looks in excellent condition.
The gears should change OK unless there is more than 1/3 of dog depth rounded off.
That is, as long as there is not excessive end play at the gears or overall shaft to case - for all 3 shafts.
Every gear pair can be individually shimmed, a time consuming job to do the whole 'box, and you need the shims.
You are right about the gears being small compared to other bikes, and not overly tough either.
The second gear with the dogs is the most troublesome.

marsheng
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Re: 250 MK3 crankcase rebuild

Postby marsheng » Mon May 25, 2020 5:37 am

The crank clearance has got me thinking.

With the C3 main bearings, I can get a 0.15 mm feeler gauge between a straight edge and the inner race on the bearing. That is a total movement of 0.3 mm from one side to the other of the inner race with respect to the outer.

The expansion of aly is around 13.0 E-6 m/m/c Steel is around 7.0 E-6 m/m/c. Say the crank and casings are 74.00 mm wide at the bearing surfaces. So for a 100 deg rise, the casings now become 74.096 mm and the crankshaft 74.052. Only a .044 difference.

Now if the bearings have no preload, then both inner races will be pressed 0.15 mm toward to the outside of the casings. Ie they will be running on the edge of the race much like an angular contact bearings. With the 100 deg tamp rise, it will only move the inners 0.02 mm each and this will not even center the inner race in the bearing.

The problem that may come in is, in the drive side, with the thrust on the bearing, the inner race may try and center itself. If it does move at all, that will increase the preload on the opposite bearing. That may not be so great.

I'll sleep on this and see if anything else comes up.

Comments Welcomed.

Cheers Wallace
New Zealand.

graeme
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Location: Tasmania Australia

Re: 250 MK3 crankcase rebuild

Postby graeme » Mon May 25, 2020 7:34 am

At the risk of being shot down and being labeled as a dickhead,,,
I’ve always shimmed the crank to slightly negative clearance to allow for heat expansion of the cases.
Shimmed so the crank has a VERY SLIGHT resistance to roll, like stiring paint.
But Nigel’s explanation of run out has me second guessing this idea.
I’ve never had an issue with main bearings.
The helical drive gear is going to push the crank one way on acceleration and the other way on deceleration. So shimming it with no play in theory makes sense?

Can a slight negative clearance bend a crank? Or have I miss understood Nigel’s comments.

Graeme


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