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MG MGB Technical - Diff Ratios

Hi there, I am wondering if anyone knows of a way of changeing my diff in an early mgb roadster. It runs the standard 3.9 ratio and I would like something like 3.7. Does any other car run the same Salisbury diff as the B ?. I thought perhaps a TR3 or Morgan perhaps would be a start. My email should read.. rentonmurray@xtra.co.nz
I am having trouble changeing it ??
R F Murray

RF,
The Salisbury axle didn't appear on MGBs until the advent of the BGT in '65. The last TR3 came off the line in '62 and at that time MG and Triumph were competitors. It wasn't until BMC was brought into the BL fold did they became part of the same group. Even towards the end they didn't share a lot of major components - with the 1500cc engine and trans from the Spitfire as used in the midget being a notable exception.

Not sure about what parts Morgans used over the years.

MGB automatics used the 3.7:1 ratio as well as some MGCs.

Tom
Tom Sotomayor

The MGC's ran 3.7, 3.3, and 3.07. They are normally in high demand by not only C owners but also the V8 conversion crowd.
J Heisenfeldt

I find this UK link for crown wheels etc. If you have a Banjo looks like the ratios are 3.9 (standard) or 3.7.

If you go with this let us know how it works out.

http://www.autogear.co.uk/product/?groupid=5&model=&manufacturer=5
Robert McCoy

Here is some information on the different axles used by MG and some of the ratios available. Seems that the 3.7 for the banjo rear is going to be very rare except if you can find some aftermarket company making new ones. Converting to an Over Drive or 5 Speed transmission is probably easier than finding and installing a 3.7 http://www.mgcars.org.uk/mgcc/sf/sf100104.htm
Bill Young

I'm sorry I made the mistake of "Salisbury' the diff I have is the early version that comes out the front of the housing...is that a 'banjo' type. ??
R F Murray

RF,
Yep, the early axle where the gear set comes out the front is the Banjo, the later more robust one is the Salisbury. The BGT, MGB automatics and the MGC all used the Salisbury (late) type.

Tom
Tom Sotomayor

The MGC 3.7 rear axles are less sought-after. I have one that I would gladly exchange for a 3.3 or, best of all, a 3.07. You could then buy mine and ship it to NZ (for a "pittance" 8^)...). But frankly, I don't think the 3.7 is noticeably different from a 3.9. I think what you need is overdrive. An appropriate three-sync OD for your early MGB might not be easy to find, but if you're not an originality freak, a four-sync OD will fit in there with just a bit of "dressing" (don't you love that word!) on the tranny tunnel where the starter bulge is located, using a BFH.

FWIW,
Allen
Allen Bachelder

Thanks to all the comments.I have done the 3 to 4 synchro conversion on my other 66 B roadster with OD and used 'the big hammer' and a little cutting for the gear lever...all very satisfactory, BUT the new project is for my 63 historic B racer....ex UK...its running out of legs down our longish straight....It has a close ratio box ,so rather than getting another 3 synchro OD box AND doing the close ratios for it as well I thought of the diff change...I agree that it is not very significant...another 10 mph at 6500 I figure. At present i don't like taking it past 6500...I have another engine on the bench that I will hope will rev to 7000 ..but that is a few dollars away and time as well. I just had the idea that a car like the TR or Morgan...may have had a similar diff that could be persauded to fit the casing..."I had a dream "that a Jag diff had been fitted to a B. The article on Diffs from Safety Fast was a good read...thanks again. Renton
R F Murray

Overdrive will solve all your problems.
Bill Spohn

Bill,

I've got a 68 BGT that I put an overdrive in, and in my opinion, it is still geared too low for the open highway.

RF,

When I do the math, I only come up with about 5 mph diff for your situation with the 3.7 vs the 3.9. Seems like RPM should be about 95% of what it was with the 3.9, or the other way around, speed should be about 105 when it used to be 100 mph.

Charley
C R Huff

Use this calculator to play with different transmissions, tires, and differential ratios: http://www.geocities.com/z_design_studio/

Rick Stevens

Charley,

Yours is an interesting observation. I have been quite happy with my 73 B/GT with OD. On our trip to the west coast last year, we came back through Montana and North Dakota - both states with 75 mph speed-limits, allowing us to cruise for 1000 miles straight at 80 mph with relative impunity. The car can no doubt do that without OD, but I could never do it! The engine was still relatively quiet - less noisy than turning 3600 rpm without OD at 65 mph. But that's because all the wind noises at 80 mph (even in a GT) drown it out.

Question, though, is could a more-or-less stock 1800 pull a taller ratio rear end. And if so, what would it be? A 3.7 would only drop that to about 3420. To get really quiet cruising, you'd have to drop that to about 3000. The next tallest ratio from MG would be the 3.307 MGC. Perhaps from some other car, a 3.5 or so might be about right if coupled with a transmission offering very low first and second gears. Or if a Moss S/C is used - giving a lot of low-end grunt.

FWIW,
Allen
Allen Bachelder

Allen,

Truth is I don't really know how the B would do with the 3.307 rear because I have never tried it. My guess is that it would pull it if you didn't mind sacrificing some acceleration for the low rpm cruising. In my opinion, the B already has a very low first gear evidenced by the fact that it can easily be started in second on flat ground. I expect a high gear rear would take more shifting in steep or curvy terrain, but that's what the transmission is there for.

I notice that if I'm not trying for rapid acceleration, I can be comfortably into 4 OD at 45 or 50 mph (I'm guessing because my speedo quit a few months ago). So, It doesn't make sense to me that I have 5 gears to get me to 45 or 50, and then I have to use that same gear all the way to 80 or whatever.

Maybe because I spent so many miles as an over-the-road owner operator, I like torque and I'm not too comfortable with high rpm. My truck red lined at about 2150 or 2200. With it's gearing, you had to be at every bit of red line in 14th before you could get it to pull in 15th. Often, 15th wasn't useful, but it was there if needed.

Does anybody know if hubs, brakes, axle shafts, etc. from a disc wheel rear can interchange into a rear from wire wheel car? In other words, if I find a high gear wire wheel rear, can I put my disc wheel parts on it? It seems to me that most of the MGCs had wire wheels.

Charley
C R Huff

Charley, The disc and wire rear ends are a different width and therefore their axles and hubs will not interchange. The ring and pinion assembly mounted on the diferential cage should swap out between the two units. RAY
RAY

The wire-wheel rear end is 1-3/4" narrower than the disc wheel rear end - so the axle shafts are not interchangeable. Everything else is interchangeable. You probably can use the whole wire-wheel axle with a combination of spacers and/or minimal-offset disc wheels since the wheels will then be 7/8" closer to the center on each side.

You can also convert the B to 15" wheels, but I don't think that significantly affects rpms. I have a 3.7 in my MGC and I'm definitely (and desperately) searching for a 3.07. I don't like high rpm either. Many MG owners claim that the B block engine will cruise all day at 4400 rpm, but I can't stand to listen to it. Sure I'll rev to 5000 in first or second on occasion but for steady cruising, my normal limit is about 3500, and I wouldn't object to having it run slower than that. Probably wouldn't hurt gas mileage any either. I'd love to figure out a way to get 40 mpg out of my B. Does the Moss FI system help that?

Cheers,
Allen
Allen Bachelder

Ray's post came through while I was writing mine. Since the hubs are integral with the axle shafts, I agree with him about the non-interchangeability of the hubs.

Allen
Allen Bachelder

Hubs are not integral with halfshafts on the Salisbury axle. You can mount disc wheel hubs to the WW shafts, with the narrower track Allen notes.

FRM
FR Millmore

R.F. not sure what the rules may be for your race car but you could easily install the complete Salisbury rear end in the car with no modifications. At least then you would have a wider selection of gears available and would not be modifing anything on the car.
John H

Racers use the Banjo diff to keep unsprung weight down, if you have picked up and moved the 2 types of axle one after the other you will be amazed at how clunky the Salisbury is. I can believe that a 3.7 Banjo with a close ratio box would work well on a fast circuit with a tuned car, if you know the revs you want to pull and the speeds you want to reach it's an easy sum to do. The road going o/d is not useable for racing, although there used to be a special one you could fit, not so smooth shifting but a lot more rugged. I dont know if you can still get them.
Stan Best

Thanks Ray, Allen, and FR.

I guess as long as the ring and pinion swap, that would be good enough. I do have to agree with Allen that it is hardly worth the trouble for the 3.9 to 3.7, unless you could swap out the entire axel.

John,

I don't know about R.F's rules either, but some rules let you swap it as long as it came in some model of your car regardless of the year. He was initially asking about parts from a different car, so it seems likely that his rules allow it.

Charley
C R Huff

I've never had much luck at transfering tables to this BBS, but if you copy & paste my posting and then use your spacer bar, you should be able to arrange the following info into useable tables:

Road Speed* in MPH/KPH w/ 3.909:1 (11/43) crownwheel & pinion gearsets** (BMC Part # BTB 586) for the Hardy-Spicer B-Series Banjo-type rear axle, and crownwheel & pinion gearsets** (BMC Part # BTB 653) for the Salisbury Tube-type rear axle, @:

2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM
1st
12.0 MPH/19.3 KPH 18.0 MPH/29.0 KPH 24.0 MPH/38.6 KPH 30.0 MPH/48.3 KPH 36.0 MPH/57.9 KPH

2nd
17.4 MPH/28.0 KPH 26.0 MPH/41.8 KPH 34.8 MPH/56.0 KPH 43.5 MPH/70.0 KPH 52.2 MPH/84.0 KPH

3rd
25.8 MPH/41.5 KPH 38.8 MPH/62.4 KPH 51.8 MPH/83.4 KPH 64.7 MPH/104.1 KPH 77.7 MPH/125.0 KPH

3rd Overdrive (LH)
31.6 MPH/50.8 KPH 47.4 MPH/76.3 KPH 63.2 MPH/101.7 KPH 79.0 MPH/127.1 KPH 94.7 MPH/152.4 KPH

4th
35.8 MPH/57.6 KPH 53.7 MPH/86.4 KPH 71.6 MPH/115.2 KPH 89.5 MPH/144.0 KPH 107.4 MPH/172.8 KPH

4th Overdrive (LH)
43.6 MPH/70.2 KPH 65.5 MPH/105.4 KPH 87.3 MPH/140.5 KPH 109.1 MPH/175.6 KPH 130.9 MPH/210.7 KPH

* Presumes road wheels that have an Original Equipment rolling radius of 23.5
** Note: These gearsets require either differential cage BMC Part # BTB 328 for Hardy-Spicer banjo-type rear axle, found in mass production MGB roadsters 1962-1968, or differential cage BMC Part # BTB 866 for Salisbury tube-type rear axle, found in mass production 1969-1980 MGB Roadsters, 1965-1980 MGB GTs, and 1969 MGCs with Overdrive.
Note: All of the Hardy Spicer B-series banjo rear axles use the same carrier, bearings, and shims. There is no need to change a bearing unless it is worn. All of the crownwheels can be simply bolted onto the standard differential cage (BMC Part # BTB 328) und the mass Production MKI MGBs with no machining required, with minor alignment adjustments made by means of shims.


Road Speed* in MPH/KPH w/ 3.7:1 (10/37)** crownwheel & pinion gearset, (BMC Part # BTB 1244), Salisbury Tube-type rear axle, @:

2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM
1st
12.7 MPH/20.4 KPH 19.0 MPH/30.6 KPH 25.4 MPH/40.9 KPH 31.7 MPH/51.0 KPH 38.1 MPH/61.3 KPH

2nd
18.4 MPH/29.6 KPH 27.5 MPH/44.3 KPH 36.7 MPH/59.1 KPH 45.9 MPH/73.9 KPH 55.1 MPH/88.7 KPH

3rd
27.4 MPH/44.1 KPH 41.0 MPH/66.0 KPH 54.7 MPH/88.3 KPH 68.4 MPH/110.1 KPH 82.1 MPH/132.1 KPH

3rd Overdrive (LH)
33.4 MPH/53.7 KPH 50.1 MPH/80.6 KPH 66.7 MPH/107.3 KPH 83.4 MPH/134.2 KPH 100.1 MPH/161.1 KPH

4th
37.8 MPH/60.8 KPH 56.7 MPH/91.2 KPH 75.6 MPH/121.7 KPH 94.5 MPH/152.1 KPH 113.4 MPH/182.5 KPH

4th Overdrive (LH)
46.1 MPH/74.2 KPH 69.2 MPH/11.4 KPH 92.2 MPH/148.4 KPH 115.2 MPH/185.4 KPH 138.3 MPH/222.6 KPH

* Presumes road wheels that have an Original Equipment rolling radius of 23.5
** Note: This gearset requires differential cage BMC Part # BTB 866 for Salisbury tube-type rear axle, found in mass production 1969-1980 MGB Roadsters, 1965-1980 MGB GTs, and 1969 MGCs with Overdrive.
Steve S.

Yes.....all things considered ,an OD box would be good,but they are rocking horse .... I believe. I am interested in weight saving [very] so the Salisbury diff sounds wrong. I will search for a 3 synchro OD box and put the heavy duty OD springs in it. I'm not too sure about some of the regs,but I thought initially another diff would look the same and not be noticed.The tyres I'm using by the way are Bridgestone, Potenza RE 540 size 185/60 14" they are fantastic. When hot,the grip is amazing....sliding YET gripping..amazing. Thanks for all the info. Renton
R F Murray

Mr Murray - the stock OD unit will stand up very well to racing use - if you use it carefully.

Proper procedure requires that you always pause slightly by backing off the throttle when shifting up - none of this 'leave it on the floor and hit the switch' nonsense.

It also requires that you only switch out of OD when off throttle, preferably after slowing down into a corner and while you have it in neutral when downshifting. I find it quite easy to flick the switch to disengage OD while downshifting to 3rd (or 2nd, depending on what the corner requires).

Treated with respect like that, a stock OD unit will bear up very well to competition use. Without the upgrade to V8 style springs etc., it will not take well to full throttle shifts and you will burn it out.

You will also trash it if you use synthetic oil in it.

Frankly the 3.7 diff doesn't offer enough difference to be bothered with and dropping it further to a 3.3 or lower is all very well, but a normal 4 cylinder will not hold 4th gear on steper hills that a car with a 3.9 would have no trouble with.
Bill Spohn

Bill Spohn-
What are these V8 springs that you mentioned, and how do they work?
C. Werner

Hello again....I had a look at an MGB project in Samoa I think it was...a Jewellery Co had done a wonderfull supercharged B with a Ford Sierra [TX9?] conversion in it.My NZ racing rules allow a 5 speed box to be replaced by another 5 speed box so...as MGB 3 synchro OD boxes are so rare I might find it easier to put in the Ford conversion. Does anyone knoe if it fits right in without body chopping...gear lever position etc... any info will be very welcome. Thanks Renton Murray
R F Murray

Renton,

If you haven't already, take a look at the thread titled, "Anybody put in a Quantum Gearbox?".

It was last posted to a couple days ago so it is still current. You will find some info there about one of the 5-speed boxes.

Charley
C R Huff

I've been contemplating a V8 conversion so I've been checking into diff ratios. There is a TR7 with the 4 cyl automatic here that I'm told would be either 3.45 or 3.09 and I'm thinking that it would fit. The Triumph expert in our club tells me that his TR8 uses a 3.09. For a V8 that would be ideal but it might be too tall for the 1800. The TR7 is late enough that it should be BL and would fit both MG and Triumph. A lot of the V8 conversions are using the diff from the MGC w/automatic because it is either 3.07 or 3.09.
Good luck with this project.
Ken
1974 MGB roadster
Kenneth Thompson

Renton,

I am the one that has the American Samoa MGB. The Ford 5 speed requires no modifications at all. Bolts right in perfectly. Very complete kit.

The gear lever is already modified to fit the MGB exactly where the stock lever is located.

Ray
Ray 1977mgb

The Ford type 9 5 speed conversions are offered here by Higear. They do a close ratio re-con box and a raised 1st gear only box.
http://www.hi-gearengineering.co.uk/
I know several MGBs and MGAs and a TD fitted with this upgrade and the results are great, it does remove the character of the o/d in an MGB so for our fun only car I would not consider it, for a daily driver it's a no brainer and it does sound a good choice for racing use on circuits where you run out of revs on a 3.9:1 axle . I believe the type 9 can take everything a modified 1798 B series engine can produce.
Stan Best

Agree,

I have mine on a Supercharged MGB and the T9 works great with it. Once you have it, you will like it.

Ray
Ray 1977mgb

Hello Ray...I have been looking at the T9 [Sierra]box and apart from the lower 1st gear ratio it looks ideal.No changes to bodywork and clutch jobs in the future can be done with engine in place...Do you find the low 1st gear too low or did you get 1st gear uprated ...Thanks for help. Renton
R F Murray

An MGA at our natter had the raised 1st gear fitted, it works perfectley allowing the car to accelerate briskly and safely out of minor roads onto fast A roads. This car has a mildly tuned 1588cc engine which allows it to pull the overdrive ratio 5th as well.
Stan Best

Toyota gear boxs might be an option,
They are small strong and light. I'm using a supra W58 gear box with my V8. Thats about 20kg less than the old MGB box and it fits into my '75 transmission tunnel with plenty of space, to spare, so the earlier tunnels should not be a problem. Very smoothbox as well, nice.
A cheaper option might be CROWN, CORONA, CELICA AND CRESSIDA BOXES. Still lighter, but not quite as much.

http://www.dellowauto.com.au/

There's some one new Zealand also doing what dellow does, but I believe he charges allot more. You could get the gear box locally and the bell housing from Dellow.
Peter

This thread was discussed between 09/05/2008 and 24/05/2008

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