250SX differential swap !!!DONE!!!

[Name Brand]

The pinion bearing failed and caused my differential gears to get chewed up pretty badly; I need to replace them. I'd like to find a complete differential and just swap over the complete unit; I'm sure this would be far cheaper than buying new gears and having the local Honda shop rebuild the unit. I just need to find one now.

Are there any other machines that shared the same differential unit as a 1985 250SX? 250ES or a Fourtrax 250? I see that even the new Honda Ranchers have a very similiar looking differential, would it fit? I'll be going to the salvage place on Monday.

I do have a '86 Fourtrax 250 (at my mother house 200 miles away...) with a complete rear swingarm and suspension; if I can't change out just the differential, would the complete swingarm fit my SX? I'm already going to run the 9 inch rims off of it, everything else is setup the same. The engine is even exactly the same as my 250sx. If the frame is the same width at the swingarm mounts and the driveshaft connects I don't see why it wouldn't work. Anybody got a Fourtrax nearby they can measure for me?

[Dirtcrasher]

I don't believe the swinger will fit. But the ES and TRX diff is the same, there is some talk that the ES diff has a lower gearing hence the 25" tires. I'd like to know if the Rancher would fit....

[Name Brand]

I have some 25-12-9 Maxxis Sur-traks on order for this machine; the tires on it now measure 23 inches tall. I don't think a slight change in the gear ratio would hurt anything. I'd lose some top speed, but I've also never found enough flat land to completely top out this machine. This 250SX is my 'go anywhere' machine; if I want to go fast I have a 250R I need to put together.

Any idea if the TRX is geared the same as the ES; I know the TRX had 25 inch tires too. Or better yet, does anybody the actual gear ratio for these machines? I just counted the teeth on the 250SX gears; the gear ratio is 3.4:1.


- As soon as I read this again I realized that whatever speed I may lose in the gears I will be making up for in the taller tires. I guess the gearing isn't a big issue; what can be bolted up is though.

[Name Brand]

I had a freind bring up what's left of my 1986 TRX250; the Fourtrax. The differential is a match for the 1985 250SX; all the bolts line up. I don't know if the 250SX axleshaft will fit the differential though, I didn't take the TRX swinger apart that far to check the splines.

Instead I'm looking at using the complete TRX swingarm. It bolts directly to the SX frame using the TRX swingarm bearings and seals. The TRX driveshaft slips right onto the output shaft of the engine and lines up perfectly. The TRX swingarm is about 2 3/4 inches longer than the SX swingarm; I imagine this will help clear the 25-12-9 tires I have coming and help the SX climb even steeper hills. I will likely run the SX rear shock, but I will need to fabricate a lower mount for it on the TRX swingarm. If I use the TRX lower shock mount the swingarm is pulled up too high, the shock is sitting at a large angle and the battery would have to be moved/removed. By placing my own lower shock mount I can ensure the machine will sit at the proper height in the back. I can still adjust the height with the SX shock too; the TRX shock does not have a hieght adjustment. The TRX swingarm has an enclosed brake drum; this means smaller seals and less chance of water getting in. I will likely use the foot brake cable from a TRX and hope my SX handbrake cable will reach far enough.

Doing this swap also gets me some much needed swingarm and brake parts for my other 250SX!

I also looked at some new Ranchers/Foremans; the differential is totally different.

So far so good on this swap; I'll post more as I get it done.

[Dirtcrasher]

Keep us posted, I'm curious to see if the swinger and driveshaft will work.

[Name Brand]

I just finished putting my SX back together using the TRX swingarm. I made a bracket to locate the bottom of the shock a few inches forward of the axle tube, welded on some mounts for it, covered everything in grease, sealent or paint and everything else bolted up just as it should. The rear tires sit a little farther back (about 2 inches); I have 22-12-9s on it now, I should be able to clear the 25-12-9s I have ordered without a problem.

I just rode it down the street a few times and the rear brake is working very well using the stock SX cables. I didn't get far enough into the diff to count teeth, but the gear ratio of the TRX diff is about 3.9:1.

I'm going to go on a shakedown run right now to the Knik River area; I'm pretty sure everything will go well and nothing will break. :Bounce

[pavementking68]

you say the swingarm is 2 inches longer?any pics,that would be cool to have an extended arm on an sx
:pics:

[Name Brand]

The stock wheelbase measured about 48.5"s, with the TRX swingarm my wheelbase is about 51"s. The difference in the swingarm lengths is 2.75"s. The 22 inch tires line right up to the back of my rack, or to the back of the factory grab handle. 25 inch tires will stick past the back of the machine, but I don't think it will be a problem at all.

I made my bracket with three mounting holes for the bottom of the rear shock. I'll play around with it till I find the right heigth and ride. I have pictures, but you all will have to wait till I fill my cheap camera.

My ride on Saturday night went well, no problems except one of my rear tires had lost alot of air by the next morning. Just have to find the hole now.