85 Big Red differential

[troyky29]

I was wondering if the rear differential out of a 85 250 Big Red will work on a 86 Big Red. I picked up a 86 with a bad differential. I had a differential out of an 85 plus a 85 parts bike that is complete. I know the rearend is different on a 86 but the differentials look the same. I just want to make sure before I pull it all apart.

[Flyingw]

Opps, my original reply was backwards. The seaing lips on he 86 axle tubes are taller than the 85 so if you try to install an 85 diff in to the 86 swinger and axle tubes then some grinding will be required to get the axle tubes to sit all the way in to the diff. Confused??? I'm doing yard work now but later I'll post a couple of pics to show where the problem is.

[troyky29]

Think I got it. So the differential from the 85 won't seat against the 86 axle holder, correct? Could I just swap out the entire rearend and swingarm from the 85? Don't really want to grind on anything and possibly ruin any parts.

[Flyingw]

You could swap rear ends but really the grinding isnt too severe. The raised lip on the axle tubes that protrudes in to the diff housing is what needs to be modified. They just need to be about 1/8 shorter and it doesnt have to be perfect.

[troyky29]

Just wondering which would be easier. I've taken off a couple differentials but never the complete rearend. Just my luck, out of all the big reds I've owned and the parts I have, they all have been 85's. I assumed this one was a 85 too.

[Flyingw]

It depends. Pulling the entire rear end is easy assuming the swinger pivot bolt isnt corroded and the bearings are still serviceable. Swinger pivot, upper shock bolt, two brake cables, two vent hoses and the whole rear end will roll out from under the frame.

Whats wrong with the 86 diff?

[troyky29]

Suppose to be froze up. Previous owner said he was riding it and the rear just locked up. I jacked it up and the rear wheels would not spin at all. Went ahead and pulled the differential off. Some water and what looked like fluid/water came out. Axle then spun freely. Which would eliminate brakes being locked. After trying to turn the differential seems as if something broke free. Differential is turning now. The axle, axle holders and differential all had alot grime everywhere. I'm wondering now with a little cleanup and fresh fluid if the differential might be ok to use. Any thoughts?

[Flyingw]

The main failure of the ES/SX diffs is the pinion bearings fail. Thats the shaft the propeller shaft slides in to. When you rotate the diff, how does it feel? how does the propeller end of the pinion feel? loose sloppy etc.... If the diff seems tight, remove the fill cap and up end it to get everything out then get yourself a couple of bottles of rubbing alcohol and fill it up. Cap it, rotate it, swish it around and drain. Repeat this 2 or 3 times. The alcohol will carry out any water molecules without damaging the seals.

[troyky29]

The propeller end has just a little play when moving it from side to side and up and down. There is a little more restistance in the original one when turning it compared to the spare I was going to put on.

[Flyingw]

Ya I say the diff is bad. The pinion should not have any side to side play or up and down play. The big bearing on the propeller shaft end is pre-loaded and with everything working and assembled, there can be up to .018 backlash between the pinion gear and the ring gear. Open that thing up and see what the inside looks like. Unfortunatly, the tools needed to remove the pinion are about 300.00 so most guys with bad pinions are screwed. The ring gear bearins and seals dont require any special tools.