Can anyone tell me if I can change the ratio of a diff from 3.9 to 4.8 by just changing the crown wheel and pinion. I have broken an axle in my good 4.8 and stuffed the carrier hat thing that the crownwheel bolts to. It would be an easy fix if I can make one out of a 3.9 diff but I'm not sure if the hats are the same.
Thanks in advance olboy

The crown wheel will mount up to the carrier if it is from the same "H series" diff.
So yes you can mount another crown wheel and pinion up but keep in mind that the assembled centre will most probably need re shimming of the bearings to ensure the crown wheel and pinion are mated properly. If not set up properly you will most likely destroy another crown wheel and pinion.

I'm guessing that you have an H165 differential.
My parts book says that the crownwheel bolts to the 'case - diff' This houses the side & 'pinion' gears [not pinion as in ring & pinion, very confusing]

The part number [38421 - W8500] for the 'case - diff', sometimes called the hemisphere, is the same for the 3.9 & 3.7 to 1 ratios, but someone would need to look up the number for the van diff to confirm if it is the same for the ratio that you have, & therefore interchangeable.

If you are going to mess with the diff, it would be preferable to give the job to someone who has the right tools & the experience to set it up properly as tooth mesh can be critical & if you get it wrong, you could be most unhappy later on.

P.S., the part number is from my series one & two 910/ Bluebird book.

Thanks mate I have found a series 2 bluebird works fine. The diff is in a motorkana car so I'm not worried about setting the diff up that right. It never goes that fast to be a problem. its in a lightweight 1200 with an A15 and an auto box so power is never going to break the diff. I keep breaking axles though. Any thoughts on why. its a welded diff in a long axle vanett stanza size. thanks Olboy

The welded diff is causing the axles to load up beyond their design limits when cornering, particularly on hard surfaces. I believe that an LSD would be the answer to your axle breakage problems.

also the greatly increased amount of torque the axels are under with that superlow final drive ratio wont help in ur halfshafts life-span.

Yeh sounds like that but it is fine on tar, it only happens on dirt. I think it might be something to do with the suspension. It has fairly soft suspension at the back and it has a single link on the top about 1/3 the way across to stop axle wind up. Surley it shouldn't break axles this often ( 2 to 3 per year )h165s are much bigger than standard, the car is lighter (perspec windows etc ) it has't got much power and its an auto which should be easy on the axles. Got me beat Olboy

I'm sorry to have to tell you this "but you ARE breaking axles in the dirt" because of the welded diff. It differs in perfprmance on the tar because of the uneven loads applied on rough slippery or hard dirt surfaces. Dirt use is always going to be harder on the diff centre especially if its a little rough or uneven on a hard packed surface.
Only solution is to go to a bigger diff/axles or allow some slip in the diff via a well designed spring loaded locker, or even better as Dodgeman has suggested, an LSD.
The problem will be added to a little because of your soft suspension, tightening the rear up will most likely give you more oversteer and possibly less traction in the dirt. Its always a compromise though!
If you throw another crown wheel and pinion in it without getting your diff set up properly I am afraid that you are only asking for trouble. Broken axle and maybe broken crown wheel or pinion

My 2 cents worth ...
A couple of thoughts (no solutions though!)
Is it the same side breaking? ie a slightly bent housing one side causing a stress point on axle ?
Or - being a motorkhana car - the tight radii of the course and rapid forward / reverse movements whilst negotiating turns may be stressing and/or shock loading the axle (even with an auto).
Sounds like a 29 spline H190 is on the way
Cheers

Thats what I thought but I was looking for an easier answer. Problem is It needs to be a big ratio. Anyone know how big an L300 mitsy diff is , I think they have some big ratio's and might be fairly strong. H190s don't come in 4.8s do they.
olboy