3.90 ring/pinion

[TulsaFred]

I just got out bid on a 3.9 differential on Ebay.
Anyone know a good source for this?

Thanks
Fred

 

[regularman]

They are probably the most common pumpkin. I only have a set of gears, but just hang on and one will show up. The 3.7 is the harder one to find.

 

[Guest]

just hang on and one will show up. The 3.7 is the harder one to find.

X2. They're out there.

 

[TulsaFred]

Ill keep watch
Seems like mostly the 4.22s available which is what I already have

Fred

 

[Guest]

Can you make due with just a gear set?

 

[TulsaFred]

Thanks but i want the assembly so i can swap easily
Fred

 

[AN5Sprite]

Fred,

I've got a spare, PM sent...

 

[Guest]

Fred,

I've got a spare, PM sent...

Hehehe, I need a spare one too for my 3.55.

 

[mightymidget]

The 3.90s are the old 370s on ebay

 

[64rocksprite]

Some 3.9's for sale over at MGExp right now.
https://www.mgexperience.net/phorum/read.php?42,1877316

 

[regularman]

Fred,

I've got a spare, PM sent...

Hehehe, I need a spare one too for my 3.55.

You mean you need a spare pumpkin to swap the 39:11 gear set into? I thought you had already done that.

 

[Guest]

I had one here but the carrier was bent. I don't want to disassemble my 3.90 that's in the car. Once I get the 5 speed in I might want to swap back.

 

[regularman]

I had one here but the carrier was bent. I don't want to disassemble my 3.90 that's in the car. Once I get the 5 speed in I might want to swap back.

You might want to, but the gearing will be perfect for the interstate in 5th and 4th will be good for most places. 3rd will be about where your 3.9 is now in 4th. It will be geared about like other modern cars.
I don't know how many gear sets you have changed out, but I would be glad to give you my nickels worth of advice on the 3.9 to 3.55 swap I did, if you want it. Send me a PM when you get ready to do it and I'll call you and we can talk.

 

[Guest]

Gonna have a guy w/ 35 years experience set them up. I don't want to take any chances with the last new set known to exsist. With new bearings, the gears, another chunk and labor I'll have over 1K in it.

 

[regularman]

Gonna have a guy w/ 35 years experience set them up. I don't want to take any chances with the last new set known to exsist. With new bearings, the gears, another chunk and labor I'll have over 1K in it.

Just make sure he is familiar with the Mowog ones. The offset marked on the end of the pinion is from a jig set in place of the bearings. Its not quite the same as fords and chevys. In the end though, the prussian blue painted on the gears will tell the tale. If he does good work and guarantees it, then its probably worth the money. I didn't replace any of my bearings. I checked them for smoothness and play and they were within specs. So far, so good.
I got too other midgets besides my one with the 3.55 in it. One is a junker and the other is a 77 that I am putting a BE nose on.
The one I am working on and the junker are both rubber bumpers.
They might both had 3.9 or one or both might have 3.7 in them.
If one is 3.7 then I might swap with the 3.55 and give the 1500 the 3.55 and the 3.7 and 5 speed in the 1275.
I go to get rid of that rolling junker before I make a race car out of it :wink:

 

[Guest]

Zimmer Gear out of Baton Rouge.

That should raise you about 200 RPM in 4th at the same speed. if it were me, I'd put the 3.90 behind the 1275, the 3.55 behind the 1500 and the 3.70 on ebay.

I'd love to know how the 3.55 works out with the 1500.

The more I think about it, the more I see I need to just put the 3.55 on my 3.90 carrier. If I go back to 3.90, it'll be to a RX7 LSD diff.

 

[nomad]

I believe the 78 on had the 3.7.

Kurt.

 

[Spridget64SC]

3.727 (11/41) diffs started showing up in the MG1500's in September 1977. Essentially 78 models (8/77-4/78) until end of production in 12/79. Although 3.7's might show up in earlier vehicles due to a PO's replacement. You never know what you might find in a Spridget.

3.55's (11/39) were never a production item. Just a Special Tuning offering to help keep the rpm's down on the racing 1500's.

HTH,
Mike Miller

 

[TulsaFred]

Can someone compare and contrast the diffs available and what is ideal for a given situation.
My B/E will get a 1275 and 5 spd conversion. Seemed to me like the bigger engine would do fine at low speed with a taller rear end gear, and further reduce the highway rpm beyond what the 5 spd would do.

Fred

 

[2091351]

Fred, given your in Tulsa and know that just East of your location are some super roads for a BE, I would try to acquire two (2) 3.9's and use them up! :thumbsup:

Your 1275+ will be able to rip and tear with 3rd & 4th in the hills and the OD 5th will give you great Freeway (Tollway in OK) RPMS.

YMMV

Steve