Maintenance & Technical > KX250 / KX125

Crankshaft side play

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Cantab:
Does any body know what the service spec is for crankshaft side play?
I am talking about how much the crankshaft can move between the main bearings with the main bearings fully seated in the crankcase housing.

I assume there should be some clearance, otherwise there would be a problem with thermal expansion and contraction between the steel crank and the alloy case and possible side pressure on the main bearings.

Any advice would be much appreciated, using a micrometer it looks like there is about 1.2mm at the moment.

stewart:
it should be a press fit  and not movable

Wizzy214:

--- Quote from: stewart on October 30, 2008, 11:17:48 AM ---it should be a press fit  and not movable

--- End quote ---

if theres side play, you might have bearing failure.

BDI:

--- Quote from: Cantab on October 30, 2008, 10:32:39 AM ---Does any body know what the service spec is for crankshaft side play?
I am talking about how much the crankshaft can move between the main bearings with the main bearings fully seated in the crankcase housing.

I assume there should be some clearance, otherwise there would be a problem with thermal expansion and contraction between the steel crank and the alloy case and possible side pressure on the main bearings.

Any advice would be much appreciated, using a micrometer it looks like there is about 1.2mm at the moment.




--- End quote ---
If there is 1.2 mm of play something bad is wrong. I do know that when the kx500 was raced on a pro level and the engines were rebuilt on a regular basis, the crank would become worn and start floating in the main bearings.  Is your bike by any chance an old race bike...

Cantab:
Sorry I should clarify.  Actually it is a brand new Hot rod crank, new NSK bearings and the crank halfs have been put on a bluing glass to take off any high spots. 

So the bearings are tight into the case halfs, and the bearings are tight onto the new crank rod.  The crank is definately not going to move sideways when assembled, unless forced.

The Clymer manual says that if the crank is hard to turn when the cases are just reassembled, then the crank is most likely slightly out of centre.  It says to tap the end of the crank until the gap between the case and the crank webs are even and the crank rotates freely.  To me this suggests that there is probably some side play in the crank between the bearings.

The reason I ask the question is the old factory crank (probably rebuild many times) is slightly wider than the new HOT ROD crank, by about 0.8mm.  Of course the Hot Rod cranks are supposed to be built to OEM standards, I am hoping I haven't purchased one that hasn't been assembled properly.  Also as mentioned in my first post, with rough measurements there seems to be about 1.2mm difference between crank and bearings.

I am trying to find out if there is some kind of service limit to any crank sideplay as I re-assemble it.  It probably won't matter as the crank isn't going to move, but I am kind of fussy.

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