Final drive on Bugeye

[ncbugeye]

When the final drive is changed from the standard 4.22 to 3.9 is it necessary to replace the entire differential or can one just replace the 9/38 pair of gears with the 10/39 pair of gears?

 

[Gray_Cat]

Chris:
You can change out the gear set only - that means you will basically be rebuilding the differential to do so. Seems a little scary but is very doable!
Roy

 

[ncbugeye]

OK, so you just told me the answer to the question I wanted the answer for, but did not ask:

Is it usual/sensible to replace the entire differential? Answer: Yes.

 

[scoutll]

Much easier. I have done it both ways on different cars. It is much easier to change out a whole carrier.

 

[Gray_Cat]

Absolutely easier to change the entire pumkin (differential) than changing out the gears only - if you locate one ready to go. If you do change out the entire pumkin be certain to inspect and check the gears and bearings prior to doing so.
Roy

 

[v8mgbal]

also remember to change the seal while its out.

 

[ncbugeye]

So I have managed to lay my hands on a 3.7. I have never replaced one of these before. Is there a howto guide anywhere? Or can anyone tell me what needs to be done.

 

[vagt6]

Chris, these "how hard is it" questions regarding LBC projects are entirely related to our mechanical skill, tools, and know-how. For example, if you are an experienced amateur wrencher with dial gauges and all the tools needed for the project, even then it's a tricky job to change out the ring and pinion set.

The ring and pinion set must be very carefully installed to very close tolerences. If it's just a little off, the gears are toast in short order. Not a job for the uninitiated, and most certainly not a job for a slightly skilled LBC enthusiast.

There are plenty of folks who can do this job, but I would not trust it to anyone who has not (successfully) installed a ring and pinion set before. A repair shop so inclined will charge you their hourly rate, and I'd assume that it's at least a three hour job, all told to do it properly.

It's still cheaper than installing a foreign, non-original (i.e., Datsun/Ford) gearbox or overdrive and in my opinion, the best way to go.

Let us know . . .

 

[Gray_Cat]

Chris:
If you are replacing the entire pumkin (differential) then the process is fairly straightforward.
Get the car up in the air and remove both rear wheels.
Disconnect the drive shaft from the differential.
disconnect the brake line junction from the differential.
unbolt the differential from the axle housing.
you will need to remove the rear axles before you can pull the pumkin away from the axle housing - do that by removing the retaining screws you will see on the outside of the axle hubs. You can just pull the axles partially out to clear the differential and keeps things a little tidier. Of course you may want to replace the hub gasket if it (they)tear so would need to pull them out completely. No special tool is needed to remove the axles. Be very careful to not pull out lube with the axles and avoid getting it on the brake shoes!
Reassembly is just the opposite and as someone else suggested, you may want to replace the oil seal in the front of the pumkin. You would need to remove the flange to do that but isn't difficult. Also, be certain to replace the gasket between the pumkin and axle housing (you can make one out of good paper stock if you don't have a new one)
Take a look at the Moss or VB websites of you don't have a manual to get a "visual" before proceeding. It isn't a very difficult task.
Now if you are replacing the gears only, that takes this to a whole nother level!
ROy

 

[ncbugeye]

No, I am going to replace the entire diff. I just asked the initial question the wrong way.

I am going to pick it up from Carl Cason (legendary Sprite racer) this afternoon.

Will I need to do anything with the speedometer to compensate for this change?

 

[jlaird]

Easy to replace punken, Pull axels un bolt drive shaft, unbolt punken and you have done it.

 

[ncbugeye]

Just to clear things up, this is the unit I have, and in the second pic it shows 11/41 which is 3.7272:

 

[Gray_Cat]

Chris:
Very good looking unit - if you change your mind about the install you can put it on the Greyhound to me!

And, yes, the speedo will need to be recalibrated to account for the new gear set. That I can't advise you on though other than find and go to a local shop that does that work there.
Roy

 

[Gray_Cat]

Not to beat this subject to death and maybe I'm stating the obvious but this would be an excellent time to change the differential lube while you're at this job.

 

[Gundy]

Nice score on the 3.7 diff.
I really like this gear in mine.
Using a GPS I've found with my tires and the 3.7 the speedo is
about 10 mph lower at 55 mph. That is the speedo shows 45 when the GPS shows 55. Your results will vary depending on tire size.

 

[ncbugeye]

I will change the differential oil, thanks for the suggestion. I presume it will have to be completely drained to do this job anyway, so no point in putting in what could easily be, at least in part, 50-year-old oil.

What grade of oil?

 

[Sarastro]

I think it's standard 80 or 90-weight gear oil, but check a manual. In fact, all this stuff is in any decent shop manual, so it would be a good idea to get one. The Haynes manual is pretty good, and you can find it on eBay for $10 or so.

3.7 differentials are scarce as hen's teeth, and much sought after. You're really lucky to find one.

 

[regularman]

To correct the speedo. Take the car for a drive at 60 using a gps and see what the actual speed is on the spedo. Stop the car, remove the speedo cover and move the needle up to what the speedo was reading at 60 on the gps and hold the stem with a pair of needle nose pliers. Remove the needle and put it back on reading 60mph. Then let go of the stem. The needle might not settle down to 0 any more but will now be accurate at 60mph. You can verify this with a recheck with the GPS.