YZ250 Full Rebuild Issue

Acidreamer
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Acidreamer
I did a 100% full rebuild on my yz250 motor. New tranny bearings, new crank bearings, new crank, new seals, gaskets, piston, cylinder, everything.

Basically the engine has a "heavy" feeling when i turn it over with the piston/connecting rod. It takes about 2/3lbs of force to turn it. About half the weight of my hand on it if that makes sense. When it does turn though, it doesnt have a gritty feeling like bad bearings. It doesnt have any change in resistance or the heavy feeling. It has the same feeling all the way around the rotation. Very smooth, but not extremely free as when i took the old crank out which im assuming has been in there since 2001 when it was built by yamaha.

Im very confident that i did everything by the service manual. I've rebuilt my old kx125 motor like this about 4 or 5 years ago and dont remember what it felt like. All i know is i rode it up until a year ago with no issues and sold it for my 450.

Is this just from everything being new and needs broken in? It feels good if i turn everything over with the kickstarter. About the same if not more force required for my kx450.
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xnationalwrench 2/12/2017 1:48pm
2/12/2017 1:48pm
When you assembled the engine did you use assembly lube or anti seize on the bearings (including the rod bearing)? If you didn't it could take time for the bearings to get lube and break in
Acidreamer
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Acidreamer 2/12/2017 3:00pm
2/12/2017 3:00pm
xnationalwrench wrote:
When you assembled the engine did you use assembly lube or anti seize on the bearings (including the rod bearing)? If you didn't it could take...
When you assembled the engine did you use assembly lube or anti seize on the bearings (including the rod bearing)? If you didn't it could take time for the bearings to get lube and break in
I didnt use assembly lube. I put some 2 stroke oil down in the crank and obviously when i put the cylinder on. The oil in the crank helped a bit.
Paw Paw 271
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Paw Paw 271 2/12/2017 3:47pm
2/12/2017 3:47pm
More that likely the crank is not centered in the bearings. It's almost impossible to get them truly centered when doing a new install. I have had luck using a dead blow hammer on the clutch side of the crank and or the stator side to get this done with out damage to the crank or bearings. Tap it a few times from one side and check. If still stiff feeling then do it from the other side. This only works if the crank was tested for run out prior to the install. I have seen very few new cranks come out of the box in real good state of true. I will always suggest that any new crank be trued prior to the install. If the crank is out of true it will remain stiff and can cause excess wear on the parts.
If you elect to not do this it will come into the correct position with a few heat cycles prior to riding it, assuming the crank is in a good state of true.

Paw Paw
Acidreamer
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Acidreamer 2/12/2017 5:51pm
2/12/2017 5:51pm
Paw Paw 271 wrote:
More that likely the crank is not centered in the bearings. It's almost impossible to get them truly centered when doing a new install. I have...
More that likely the crank is not centered in the bearings. It's almost impossible to get them truly centered when doing a new install. I have had luck using a dead blow hammer on the clutch side of the crank and or the stator side to get this done with out damage to the crank or bearings. Tap it a few times from one side and check. If still stiff feeling then do it from the other side. This only works if the crank was tested for run out prior to the install. I have seen very few new cranks come out of the box in real good state of true. I will always suggest that any new crank be trued prior to the install. If the crank is out of true it will remain stiff and can cause excess wear on the parts.
If you elect to not do this it will come into the correct position with a few heat cycles prior to riding it, assuming the crank is in a good state of true.

Paw Paw
Thanks for the tip. I'll give that a shot and see what happens.

The Shop

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Hondas4Life3
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Hondas4Life3 2/12/2017 6:21pm
2/12/2017 6:21pm Edited Date/Time 2/12/2017 6:23pm
The crank isn't all the way pulled into the crank bearing. If you can attach a crank puller to one side of the crank tighten it tighter than you had it before it should pop into place. It may take a few good turns of the wrench to get it. Or you can tap it with a soft hammer to help it along when you have pressure on it.

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