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stiff crankshaft replacement

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denvoyager
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PostPosted: 16:20 - 18 Apr 2019    Post subject: stiff crankshaft replacement Reply with quote

Hi,everyone,
I've just replaced a crankshaft on my Yamaha YP 250 Majesty 2001 4str.
It has gone back in ok, and I have fitted the two crank case halves back together. The oil/water pump shaft is rotating correctly, and is not jammed.
The problem is that the whole assembly seems a little stiff, I can only turn it with a spanner on the spline end - it goes 360, but it's stiff - any ideas,please , anyone, or is this normal, and will it turn on the starter, and 'bed' itself in do you think?
Cheers, Den, Swansea
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MCN
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PostPosted: 16:38 - 18 Apr 2019    Post subject: Reply with quote

Did you put lots of engine oil on the parts as you clashed it together?
Piston rings new? In the correct way?
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Robby
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PostPosted: 16:55 - 18 Apr 2019    Post subject: Reply with quote

Just the crank in the cases, or is the top end on too?

If its just the crank spinning in the cases, it should turn with light finger pressure. No resistance from anything, just rotating mass to get moving.

If the top end is on, you have the friction from the piston rings in the barrel, which will need a spanner on the crank end to turn it.
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Grubscrew
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PostPosted: 16:56 - 18 Apr 2019    Post subject: Reply with quote

Some big end or main bearing shells can only go one way round, and make sure you’ve not swapped shell or shell bearing retainer for any of the others.
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Riejufixing
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PostPosted: 17:01 - 18 Apr 2019    Post subject: Re: stiff crankshaft replacement Reply with quote

denvoyager wrote:
The problem is that the whole assembly seems a little stiff, I can only turn it with a spanner on the spline end - it goes 360, but it's stiff - any ideas,please , anyone, or is this normal, and will it turn on the starter, and 'bed' itself in do you think?


I don't think it's normal.

Are you sure it's the correct crank?

Did you replace the main bearings or buy the crankshaft ready-built?

Are the bearings correct, and going "home" properly?

Have you still got the old crankshaft?


Last edited by Riejufixing on 17:43 - 18 Apr 2019; edited 1 time in total
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RhynoCZ
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PostPosted: 17:14 - 18 Apr 2019    Post subject: Reply with quote

What did you use to seal the crankcase? How much torque have you put on the bolts?
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Riejufixing
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PostPosted: 17:44 - 18 Apr 2019    Post subject: Reply with quote

RhynoCZ wrote:
What did you use to seal the crankcase? How much torque have you put on the bolts?

I was wondering along the same lines (crankcase gasket), but it's hard to see it would make such a difference on such an engine.
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MCN
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PostPosted: 18:26 - 18 Apr 2019    Post subject: Reply with quote

Grubscrew wrote:
Some big end or main bearing shells can only go one way round, and make sure you’ve not swapped shell or shell bearing retainer for any of the others.


I think its a roller bearing not shells.
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Pete.
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PostPosted: 18:43 - 18 Apr 2019    Post subject: Reply with quote

A common issue when fitting roller bearing cranks in vertically-split cases. The bearings are tight on the journals so pulling the halves together puts an axial pre-load through the bearing. If you try to run the engine the bearings will fail very quickly. If you check the manufacturer's assembly procedure you might find that the case or the bearing should be heated to make sure that you get no pre-load (as an example, Aprilia say heat the case to 80 degrees for the RS125 engine IIRC).

You need to put a shock through the bearing to release the pre-load. You have a couple of options, you could give the crank a crack on each end with a hammer to bump the bearing deeper into the seat, we're talking fractions of a mm here. Protect the end threads with a piece of soft metal like ally. Put the nut/bolt on the end if you can (but DON'T hit the nut). You want a sharp crack with no real weight and no follow-through. Bit hard to explain but easy enough to do. Or you could do the same with a tube spacer to give the case a crack over the bearing outer ring for the same effect. You should also put a snug fitting wedge in the gap between the crank webs opposite the big end to avoid putting a bending moment on the crank pin.

Neither of these things are what you'd call great practice and they do carry a bit of risk and best left to someone with a good bit of mechanical empathy but if you do nothing you're going to knock the new bearings out in no time flat. I've put together cases that needed nothing but a small tap to free up the rotation and the odd one that needed a bit more.
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stinkwheel
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PostPosted: 19:52 - 18 Apr 2019    Post subject: Reply with quote

OP IS turning the oil and water pumps too. This may have a bearing.

How did you fit the cases? Just drop them on and wind them up? Hot cases onto cold crank? Did you tighten them together using rhe crankcase bolts or pull the case onto the shaft with a spacer tube and the crank nut?
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Pete.
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PostPosted: 22:48 - 18 Apr 2019    Post subject: Reply with quote

stinkwheel wrote:
...or pull the case onto the shaft with a spacer tube and the crank nut?


Another good way of releasing any axial load through the bearings (much better in fact). I never thought of doing that Thumbs Up
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Riejufixing
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PostPosted: 23:26 - 18 Apr 2019    Post subject: Reply with quote

The bearings on this crank seem to be pre-fitted on the crankshaft.
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Pete.
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PostPosted: 00:03 - 19 Apr 2019    Post subject: Reply with quote

Riejufixing wrote:
The bearings on this crank seem to be pre-fitted on the crankshaft.


Which is normal. You press the bearing inner races on the crank then fit the crank assembly into the cases.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Riejufixing
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PostPosted: 00:26 - 19 Apr 2019    Post subject: Reply with quote

Pete. wrote:
Riejufixing wrote:
The bearings on this crank seem to be pre-fitted on the crankshaft.


Which is normal. You press the bearing inner races on the crank then fit the crank assembly into the cases.


Yes, you do.

Can you explain "Another good way of releasing any axial load through the bearings (much better in fact). I never thought of doing that Thumbs Up"? It's unclear to me what you meant.
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steve the grease
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PostPosted: 00:28 - 19 Apr 2019    Post subject: Reply with quote

Tight = friction= hot = wear =fubar.

Something is not right , something is putting axial loads on the bearings.
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Pete.
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PostPosted: 07:58 - 19 Apr 2019    Post subject: Reply with quote

Riejufixing wrote:


Can you explain "Another good way of releasing any axial load through the bearings (much better in fact). I never thought of doing that Thumbs Up"? It's unclear to me what you meant.


Hard to explain in a way that it's clearly visualised.

You take a piece of tube that is roughly the same as the bearing OD and put it over the end of the crank that's poking through the case. Use large washers or a plate with a hole in it on the end of the tube so that the threaded end of the crank pokes through the washer then put the nut on the crank end and use it to PULL the bearing deeper into the seat. Repeat either side until the axial load is removed and the crank turns freely.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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Meatybeaty
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PostPosted: 08:42 - 19 Apr 2019    Post subject: Reply with quote

Little video here showing intent
https://www.youtube.com/watch?v=p0LK30KVWJg
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denvoyager
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PostPosted: 09:38 - 19 Apr 2019    Post subject: stiff replacement crank Reply with quote

Hi, and thanks everyone for the really helpful advice. The bolts are torqued to 10 newtons, as per the manual.
When I back them off slightly , the rotation is a LOT freer.
I think the (complete with bearings already on) unit is the correct one, and that it is almost certainly that the whole needs a bit more movement to bed the bearings in.
I 'froze' the unit , and it went in ok, seemed to mate/butt to the end of the bearing space.
With the second split casing, I pre-heated it, after putting sealant on the first casing, then tapped it down. Again it seemed to go in easily, especially after a 'wiggle' to align the slot of the oil/water pump axle.
I'm going to try backing off again, to see how free it really is. That'll confirm the 'spacing' issue. I'll keep you posted,
Cheers, and thanks again, Den, Swansea
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stinkwheel
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PostPosted: 10:03 - 19 Apr 2019    Post subject: Reply with quote

Does it have a paper gasket in the centre and did you fit one? Missing one out can also cause issues. On my latest engine build, I've got a thick and a thin one to chose from with absolutely no indication which I need to use!

Here's me winding a frozen crank into a casing using spacers (two bits of scrap alloy, an old inner race and the alternator rotor) and the crank nut.

https://lh3.googleusercontent.com/Mp4bajjzcX-_Awf_g5aAAuMfX5C0wiJLgVJQTiivZloVeKfhYZofgkyeYuTM8aQv_W3KnFTlRnLUdm7dWplo2v2iuSPq9oxVzZwKkMZeitxYIBWD0dLQfspKjnUZGqVRTM-xHrLg_w=w493-h370-no

I've ordered a piece of appropriate diameter steel tube to cut into various length spacers (a bit like bicycle steering stem spacers) for the next time I do this (and a second bit to wind the bearings onto the crank because I have to fit mine seperately). I know it will make the job totally straightforward rather than a bit of a rush/worry.

The steel cost less than a tenner, delivered off ebay. A steel stockholders would probably give you an offcut for free.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.
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denvoyager
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PostPosted: 12:21 - 19 Apr 2019    Post subject: Wiggling and slacking off! Reply with quote

Yep, I've slacked off a mill, and its moving, although not finger touch, but moving by hand. Will try Stinkwheels tip above next. Thanks, everyone. Den
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Riejufixing
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PostPosted: 13:08 - 19 Apr 2019    Post subject: Reply with quote

Pete. wrote:
You take a piece of tube that is roughly the same as the bearing OD and put it over the end of the crank that's poking through the case. Use large washers or a plate with a hole in it on the end of the tube so that the threaded end of the crank pokes through the washer then put the nut on the crank end and use it to PULL the bearing deeper into the seat. Repeat either side until the axial load is removed and the crank turns freely.


That's what I thought. Personally, I'm not at all keen on applying significant force to the outer race through the inner race and bearings. Using the inner, e.g. pushing the bearing on, is OK, some are bloody tight. The OP's engine has I think steel inserts which are a tight interference fit already pressed into the crankcases, so the bearings should not be a tight fit in them. The OP's latest is vaguely encouraging.
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Pete.
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PostPosted: 13:24 - 19 Apr 2019    Post subject: Reply with quote

What's all that white powdery crap all over the assembly? I hope it's nothing more than frost.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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