Two Stroke Motocross

Two Stroke Motocross Forum => Technical => Topic started by: mhall41 on May 12, 2012, 04:24:05 PM

Title: Bottom End Rebuild: Necessary Parts
Post by: mhall41 on May 12, 2012, 04:24:05 PM
Hello, I recently acquired a 1991 YZ 125  have decided to go ahead and rebuild the bottom end as I think the crank bearings are shot. What other parts are necessary on a bottom end rebuild?

So far I know these parts are necessary:

-Crank bearings and seals
-A full gasket set

I think oil seals may be as well, but i'm not sure if those are included in a full gasket set? Any help would be greatly appreciated
Title: Bottom End Rebuild: Necessary Parts
Post by: SSGHopkins on May 12, 2012, 08:56:23 PM
If the crank bearing are toasted then the crank might be toasted too. You can find good deals on Ebay for weisco bottom-end kits, I think I picked up one for my yz250 for $118
Title: Bottom End Rebuild: Necessary Parts
Post by: mhall41 on May 12, 2012, 10:21:20 PM
Quote from: SSGHopkins on May 13, 2012, 03:56:23 AM
If the crank bearing are toasted then the crank might be toasted too. You can find good deals on Ebay for weisco bottom-end kits, I think I picked up one for my yz250 for $118

That was my original plan, but they don't seem to offer that kit for a 91. But if anyone knows if the 98-00 bottom-end kits are interchangeable i would definitely go that direction.
Title: Bottom End Rebuild: Necessary Parts
Post by: arnego2 on May 16, 2012, 02:08:56 PM
Just replace the 2 roller bearings their oilseals and the conrod needle bearing plus spacers. I don't think that the crank goes as easy if you have the bearings with little movement or an unsmooth roll.

I change bearings every 150 to 160 hours if I don't get a shot crank bearing first. Every second to third time of crank bearing change (incl. conrod kit) I give it a new crank. So the crankshaft sees about 450 hours. About the same amount for a cylinder. 4 times piston change and 4 to 6 times replating and a new cylinder is most advisable.

Now the small bearings of the water pump and/or PV govenor are  even more sensitive. A good brand somehow won't make it over 80 hours.   

You'll need a flywheel puller and a case splitter, if you have access to a press, the crank bearing should be no problem. If not put your bearings into a freezer, better even on dry ice and heat the cases to about 150 degress celsius. drip some water on the case when it sizzles you are good. Work with gloves  ;D  That way those bearings should drop right in there.

Good luck