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MG MGA - 3.9:1 Conversion

I know questions on this topic have been asked many times before, but I have not got my head fully round all the items I need to change if I convert to this differential.

I am running an 1800 engine with Sierra box. The back end is a standard 1500 axle, modified to take WW. This involved new half shafts (KO184/10), hubs (ATB7204 and ATB7205), and brake drums (ATB4165). Do any of these items transfer across? Do I use whatever comes with the alternative differential? Do I need additional items? I am happy with the speedometer recalibration details.

Steve
Steve Gyles

Depending upon the differential, you may have to deal with different spline counts on the half-shafts. I believe that most 3.9's come from early b's and there fore have more splines that a standard 1500 diff. You would either have to use the 3.9 gears on your diff, or switch axles to that of a ealry B.

j
JohnB

I have a 1960 MGA Roadster with 1600 engine, original gear box and wire wheels. I did a 3.9 conversion last year with the help of a friend who had done it several times himself. We took an early MGB rear end and removed the pumpkin. We then removed the pumkin from the MGA rear end. Both pumkins were disassembled and the side gears from the MGA pumkin were put into the MGB 3.9 pumpkin thereby matching the half shaft splines of the MGA half shafts. The MGB pumpkin was rassembled with the MGA side gears and installed into the existing MGA axle. This was a relatively simple job and we were all done in one evening. The car has been driven several thousand miles since with one trip being to Mackinaw for the GT. Revs have dropped about 500 at highway speeds and the whole driving experience has become more refined. The lower gear performance loss was hardly noticeable. Just as an FYI I have had some fellow enthusiasts caution me and others about doing a 3.9 conversion with a 5 speed gear box. The power loss at the top end is significant apparently. FWIW !!

Mike
1960 MGA
1973 MGB
Michael Hosier

If you've got plenty of power to spare, then

3.7 Banjo's are available from here.

http://www.autogear.co.uk/

I've purchased one the other month.

Should be great for cruzing across australia.

All the spider gears can be moved across quite easily, if you still have course spline shafts.

Cheers <MARK>
Mark Hester

Can't see that he'd need the 3.7 AND his 5 speed.

One or the other should do it.

Miachael is right on with his advice on axles - you couls also just try and find an early MGB wire wheel axle complete, or just an MGB steel wheel axle and bolt on Triumph wire wheel adaptors

http://www.rhodo.citymax.com/albums/album_image/2528536/833578.htm
Bill Spohn

Mark,

Can you give us an impression of how the 3.7 changed the 'behaviour' of the car? We've emailed about this a while back but I still haven't bought a replacement diff for the MGB yet. (Spent too much time playing with the 'new' GT V8 :-D)

Alex
Alexander M

Bill if those Triumph wheel adapters were used on a stock MGA rear end would the spacing work? or would the offset be too far off? My son wants wires on the 58 coupe. Where do the adapters come from? is that how Triumph does wire wheels?
R J Brown

RJ,
The TR adapters I've seen are from a tr3. This is how they are set up from the factory. I believe that they do space the rear out a bit more as they are going onto the disc rear end.

j
JohnB

Steve,

As stated by others its a simple job. Everything you have already stays the same apart form a couple of changes in the diff.

If the splines at the diff end of the half shafts are different on your 3.9 diff, all you do is undo the two large bearing caps (2 bolts on each so 4 bolts in total)and remove the complete crown wheel assembly which just leaves the pinion in the diff casing. Then remove the crown wheel from the planet geras by undoing the circle of short stubby bolts.

Do the same with the other diff and transfer the crown wheel of your 3.9 onto the planet gear assembly of your old 4.3 diff and reassemble in the 3.9 case.

You should at this stage check the back lash in the gears ans adjust it as stated in the workshop manual, although I have never bothered.

Parts you will need are the hub to half shaft gaskets (X2) a new diff to axle casing gasket and I think the crown wheel bolts have tab washers. You could also replace the pinion oil seal at the front of the diff if you wish. Some fresh oil to fill it with and away you go.

We look forward to the results appearing on your website.

Regards...John Bray
J Bray

John Bray,

"remove the complete crown wheel assembly which just leaves the pinion in the diff casing"

Aren't the crown and pinion a matched set? If I understand your comment correctly, you state that the pinion for the 3.9 and 4.3 are the same?

j
JohnB

Thanks everyone. Most useful. Just need to source one now!

I must say, as a non engineer, but always keen to learn, that I have never really understood backlash, depite reading the manual. It tells you how to set the diff up to minimise it but does not tell you what it is. Can someone explain in simple terms?

Steve
Steve Gyles

The procedure as given by J Bray is WRONG!
The differential carrier (the casting the crown wheel is bolted to) and its bearings are parts which affect the setup of the gears in the case, DO NOT change any of these!
It sounds as if you have all 1500 parts. Normally the axle housing must be changed to switch from steel to wire wheels, as the axles are narrower for WW cars; however, the half shaft numbers you give are not BMC numbers, and they may be special conversion items. If they are not special, and the steel wheel housing was kept, then the half shafts will not be fully engaged in the side gears.
To adapt the MGB final drive to the 1500 half shafts, you need to switch the axle side gears from the 1500 diff into the MGB diff.
Take off the 4 big nuts and remove the diff carrier with crownwheel. Be sure that the bearing caps are marked so you get them back in the original position. The crosspin on which the two planet gears are located is retained by a small pin driven into the carrier, through the crosspin. The hole for this small pin is peened over to retain the pin, on the side opposite the crownwheel. Drill out the peened over metal from the hole, and drive the locating pin out from the crownwheel side. You need about a 1/8" punch, as the hole is smaller on the crownwheel side to prevent the pin passing all the way through. Now you can drive out the planet gear cross pin, and remove the planet gears and washers. Now remove the side gears with the splines, and their washers. Fit the side gears from your old diff and reassemble. Don't forget to peen the hole to retain the small pin! None of this affects the gear settings in any way.
"Backlash" is the clearance between gear teeth, and is only one of the critical settings in bevel or hypoid gears. If you change ANY parts in the final drive except the axle/planet gears, you will upset all the critical settings.
FRM
FR Millmore

Thanks FRM. You are correct with my 1500 parts. I am using the 1500 casing. The ww sit slightly wider.

Steve
Steve Gyles

Just for clarification FR Millmore is correct J Bray is incorrect. Go to this Moss website: http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29225
Part # 10 in the diagram is all you are changing. If you look below it came 3 ways 10, 25, and 26 spline. These are the only parts you need to change. Disturb as little as possible and mark and return all other parts to their original position. If you change the whole assembly contained within part #6 as J Bray suggested it will change the set up of the diff. You may get lucky and have no symptoms or it could cause noise and or premature wear.
RJB
R J Brown

See photo tour of the process here:
http://www.chicagolandmgclub.com/photos/a_diff
Just swap the sun gears in the differential to match the splines on the halfshafts.

WW axle housing and halfshafts are 7/8 inch shorter on each side to accomodate the length of WW splined hubs while maintaining the same track width. The stock WW halfshafts cannot be used in the DW housing, as there would be insufficient engagement of the splines.

If you have special longer halfshafts with WW hubs which could mate properly with the differential splines, the track width would be 1-3/4 inches wider. Bolt on WW hubs used with the DW axle have similar effect. The wider track width may cause rubbing of the tires on the rear fenders, even using 165-15 tires.
Barney Gaylord

Thanks Barney. Easy to follow. Just need the bits now.

Steve
Steve Gyles

I stand corrected for which I apologise, however in my defence I would like to point out that it is easier to do the conversion rather than describe it.

My wrong method lasted for 10 years of racing MGA's (using 4.55, 4.3 4.1 and 3.9 diffs) without any problem, and the same method was also used when I converted my road car to a 3.9

Once agaim my apologies...JB
J Bray

JB

No need to apologise on my part. I am grateful for all the inputs and issues raised. It all combines together to increase knowledge.

Steve
Steve Gyles

Sourced a 3.9 diff. Should have it by the weekend.

Steve
Steve Gyles

This thread was discussed between 12/04/2006 and 18/04/2006

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