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MG MGB Technical - O/D - Transmission Identification and Operation

This may be complicated, so please bear with me while I explain the situation. About four years ago I had the engine rebuilt in 1973 MGB. While it was apart it was suggested by the shop (an excellent shop with a great reputation) that I put in a new/rebuilt transmission with an O/D. I thought this was a great idea since the original transmission needed to be rebuilt anyway. When I got the car back, everything worked great and I had O/D in third and fourth gear. Then about two years ago something went wrong in the transmission (the cluster gears?) and the transmission had to be taken out and “rebuilt”. When I got the car back, I only had O/D in fourth gear. Now, I
travel out of the country on business for extended periods and since the car ran OK, having no O/D in third gear was never a high priority on my repair list. I was always told it was just an adjustment and they could make it one time when my car was in the shop. Well, recently I have
been having trouble with the O/D just jumping out and turning off. It was not in the switch on the steering column, but something at the transmission. Therefore, two days ago I put the car in the shop. This morning I picked it up. They said that the switch on the O/D was not set correctly and needed a different spacer - just as I have read about in the threads on this site - and this why it was jumping in and out of position. Now the kicker - I was told today that when the engine
was rebuilt and they originally installed the O/D transmission, the transmission was from a later model car which did NOT have O/D in third gear. Confused? I certainly am. So here are my questions:

1) How do I determine by looking at it, what year car my transmission is from?
2) Did the later model MG cars with O/D, only work in fourth gear?
3) If the transmission is from a later model year, then how/why did the O/D work in third gear for over two years?
4) If this is a later model transmission and it only had O/D in fourth gear, is it possible to make it so the O/D will work in third gear too?

Comments? Your thoughts? This just does not make sense to me.

Thanks,

Robert
Robert Browning

Hi Robert:

If my memoery serves me correctly, the later(75- 80)MGB 4 synchro gearbox had only O/D in 4th gear. The real difference O/D functioning wise is that the 68 to 74 gearbox, which allows for O/D in 3rd and 4th gear uses a different shift tower. You can make a later O/D gearbox, with 3rd and 4th O/D by using the earlier shift tower. I have heard comment the Leyland made the change to reduce warranty repairs regarding snapped 3rd motion shafts, but I don't have any documentation regarding that subject.

Thanks: Rich Boris 67 B roadster

Rich Boris

Robert,
I'm guessing you are seeking technical expertise to question (use against) this "excellent shop with a great reputation." Suggest directing your written questions to either John Twist (University Motors, MI), Chris Betson (Octarine Services, UK) or John Esposito (Quantum Mechanics, CT). Any professionally rebuilt OD transmission should function properly for years (if not decades). Sounds like they might be doing some CYA. How much did they charge for the rebuild(s)? Just wondering if they did a clean and replace a few O-rings rebuild or a total disassembly and replace all needed parts type rebuild (e.g., who did the rebuild and what was replaced)?
Steve Buchina

Steve,

Actually I am NOT trying or planning on doing anything against the shop. I am just curious, which I guess is even more dangerous. Tonight I drove the car for an extended period and it runs great and the fourth gear O/D shifts in and out like a smooth piece of glass. However, I would just like to know why I had third gear O/D for two years with "this transmission(?)" and then after the second rebuild I didn't. Also I would like to know if I can - is there a simple way someone can explain to me - to identify that this transmission really is a late model or if I am just being jerked around by the shop - which I do not think I am.

As for the rebuild, it was suppose to have been a transmission that completely gone through with all new bearings, seals, etc. I do not remember the cost, but they did not charge me anything for the second rebuild or for the adjustments made this week.

Thanks,

Robert
Robert Browning

Robert,

On my 69 MK11 gearbox, known to be original to the car, there are a number of ID number sets. One of them is the casting date for the housing. Mine appears as '24 10 68'. Twentyfourth of October 1968. This is about a fortnight prior to the casting of the engine block. The casting number on mine is one of 7 number sets. Unfortunately I have not noted where this number appears on the box.

On my overdrive there is the number/notation of '2806 68 D'. This is on LHS of the overdrive case. I think this may be the casting date again.

In my case these numbers are consistent with a kit having been shipped from England, assembled and sold, as is known to be the case, in October 1969.

Clausager states (pp 84-85) that changes were made to gearbox and OD from the start of rubber bumper production. Gearbox casing changed to side fill (position of number sets could have changed as well) and the OD was modified and the Laycock label changed from black (as on my car) to blue. Further change was made to OD operating only on fourth for North American cars only from February 1977. It always operated on 3rd and 4th in RHD cars.

Hope this may help. Others may have the precise location of the gearbox casting number.

Regards
Roger
Roger T

Robert,

Just gone back to the original notes I made when the gearbox was out of mine. I noted that gearbox casting date as located "flat area front of fork entry port - slave mounting area". That means it would be readable if you had a view of the slave mounting from under the side of the car. As I mentioned above though, this may have changed on side-fill boxes.

Regards
Roger
Roger T

Roger,

This is a great start - thank you.

Now can anyone tell me where on the casting I find these numbers and can I see them while the transmission is in the car?

Also, FYI my transmission does not have a dip stick and is a side fill (passenger side), if that tells you anything?

Robert
Robert Browning

Overdrive Transmissions

by Tom Sotomayor February, 2003

Les Bengtson's Classic MG Pages: Articles:

This article is not intended as a “how to” installation guide, but rather to assist in selecting the correct model overdrive transmission (or OD for short) for any given year MGB. Selecting the proper one from the start will likely be the most cost-effective choice in the long run!

An OD gearbox is one of the most endearing and sought after options supplied on our beloved MGBs. It allows the engine to turn fewer revs per mile, meaning less noise, heat, fuel, wear and tear. Most operate on 3rd and 4th gear. Unfortunately, it was not that common an option as installed by the factory. Time has taken its toll on MGBs; the net result is there are a lot of spare, used components available – including OD transmissions.

There exists a fair bit of confusion as to what parts to use. After all, the MGB had an effective life span of 18 years and had numerous design changes during production. What concerns us here, are the basic grouping of the four different transmissions used (two are non-synchro first gear, two are all synchro transmissions). Identification is fairly easy using the information provided. It will be simplest though if we treat the non-synchro first gear transmissions separately from its later cousin, the all synchro unit:
Non-Synchronized 1st Gear Transmission

1962 through 1964 and 1965 through 1967 (chrome bumper)

1. Laycock Type D OD (note external solenoid)
2. A hole in the bell housing where the starter nose pokes through
3. “Shield” shaped access cover
4. 1020 TPM for OD and 1040 TPM for non-OD
5. OD on 3rd and 4th gears

The gearbox input shaft, flywheel and engine backing plate were changed with the advent of the 5 main bearing engine in 1965. Therefore, the transmission for a 3 main bearing engine (1962-1964) will be different than it’s later counterpart. To swap a 3 main bearing car to the later (5 main) 3-synchro transmission or vice versa, requires changing all of the above mentioned parts. The spigot bush in the crankshaft must be changed as well. The 3 main bearing used a .62 inch ID, and the 5 main used a .85 inch ID.


Fully Synchronized Transmission

1968 through 1974 (chrome bumper)

1. Laycock Type LH OD
2. Rectangular shaped access cover
3. Oval clutch fork boot
4. Dipstick (for checking oil)
5. Black label on the OD solenoid cover stamped “22/61972”
6. 1280 TPM for OD and non-OD
7. Speedometer drive gear (on the mainshaft) is blue
8. Speedometer driven gear (on removable drive housing) is white with 21 teeth
9. OD on 3rd and 4th gears

1974-½ through 1980 (rubber bumper)

1. Laycock Type LH OD
2. Rectangular shaped access cover
3. Square clutch fork boot
4. Side fill plug (NO dipstick)
5. Blue label on the OD solenoid cover stamped “22/62005”
6. 1000 TPM for OD and non-OD
7. Speedometer drive gear (on the mainshaft) is red
8. Speedometer driven gear (on removable drive housing) is red with 20 teeth
9. OD operates in 4th gear only in units made from February 1977 onward

Along the way the factory not only changed the transmissions, they also changed the speedometers. Occasionally the change was visually dramatic (changing the gauge face and the size of the case), other times it was simply the calibration – what’s commonly referred to as “TPM” or turns per mile. This refers to the number of times the speedometer cable will turn for every mile traveled.

With the exception of the non-synchro first gear transmissions, there is no functional difference between a speedometer for an OD equipped car and a non-OD one. All USA spec MGB's from 1968 through 1974 used 1280 TPM transmissions and speedometers. From 1974-1/2 through 1980 (all “rubber bumper” cars) it was changed to 1000 TPM. A lot of confusion exists since for the non-synchro first transmissions there is a numerical difference from the OD to non-OD speedometer output. It is 1020 TPM for OD and 1040 TPM for non-OD. Overall, you’re looking at a self induced error of about 2% if you don't swap speedometers. That really wouldn't be too tough to live with.

The serial numbers are different between OD and non-OD speedometers for all synchro cars of any given year, but the TPM remain the same. A non-OD speedometer can be used for an OD application; the end result is the same. The serial number followed by the calibration number can be easily found on the gauge face at roughly the 4:00 position. Also quite a few speedometer 'heads' (where the cable attaches) have the TPM etched into it on the threaded barrel. It's usually fully visible once the works are removed from the case. Fortunately, you don't normally have to get this detailed, or swap speedometers unless you've changed the series of transmission you're using.
Additional items to consider:

1. From 62-67 the OD driveshaft was different from the non-OD driveshaft and differed depending on the type of axle used. You’ll want to make sure to get this from whoever is supplying the transmission. The all synchro gearbox used the same driveshaft regardless if an OD was fitted or not. See chart below for applications:
Driveshaft Length Std non synchro 1st OD non synchro 1st Std all synchro OD all synchro
30 inches banjo axle
31.125 inches tube axle banjo axle tube axle tube axle
32 inches tube axle

2. Overdrive speedometer cables are longer than the standard unit.

3. OD switch

* 1962-1967 located on the left side of the dash with a chromed bezel
* 1968-1976 supplied as standard on the wiper stalk
* 1977-1980 located in the cap of the gearshift knob

Let’s look at some possible “mismatch” scenarios and see what can be done to fix them:

1. 1967 or earlier MGB with a side fill transmission
The speedometer is calibrated for 1040 TPM and the gearbox has an output of 1000 TPM. The speedometer will read 4% low. No action required.
2. 1967 or earlier MGB with a dipstick transmission
The speedometer is calibrated for 1040 TPM and the gearbox has an output of 1280 TPM. The speedometer will read 23% fast.
* Get a “converter” box from a speedometer shop
* Send your speedometer out for recalibration
3. 1968 to 1974 MGB with a side fill gearbox
The speedometer is calibrated for 1280 TPM and the gearbox has an output of 1000 TPM. The speedometer will read 22% slow.
* Get a “converter” box from a speedometer shop
* Send your speedometer out for recalibration
* Get a used speedometer from a 1975-1976 MGB or midget (1000 TPM)
4. 1974-½ to 1980 MGB with a dipstick gearbox
The speedometer is calibrated for 1000 TPM and the gearbox has an output of 1280 TPM. The speedometer will read 28% fast.
* Get a “converter” box from a speedometer shop
* Send your speedometer out for recalibration

Mixing early gearboxes with later cars has been intentionally avoided in this analysis for several reasons. They are not as plentiful or robust and they can be quite costly. Best to swap it, or sell it to someone with an early car and get the proper unit for yours.

Hopefully this has answered some of your questions and will enable you to select the proper OD gearbox. Happy motoring!

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright © 2003 by Tom Sotomayor
Steve Buchina

Steve,

This is fantastic and really helpful.
The following seems to fit my car from what I can see:

1974-½ through 1980 (rubber bumper)

4. Side fill plug (NO dipstick)
5. Blue label on the OD solenoid cover stamped ““22/62005”” (??)
6. 1000 TPM for OD and non-OD

I am going to go check on the other items in a few minutes and I will let you know what I find.

However, I would still like to get an answer about #9 -
9. O/D operates in 4th gear only in units made from February 1977 onward

So now my questions continue:

Does anyone know where the casting numbers are on the side(s) of the transmission and
O/D and can I see/get to them with the transmission in the car?

The most confusing part of this to me and what I would most like to identify is that I drove this car with O/D in third gear for two years and then all of a sudden I am told this transmission did not have O/D in third gear. I would just like to know what year and design this transmission really is (and if this is a different transmission from the one I bought when the four years ago)?

Thanks,

Robert
Robert Browning

Help!

Can of you make any sense of this?

I just crawled under my car and these are the numbers I could find:

The plate on the bottom of the O/D unit is the blue label and has the numbers: 22-62005 Beside that number is: 055374

On the O/D casting toward the drive shaft are the numbers: G14C3 A.D.C.1

Between the main transmission casting and the O/D units (this may be part of the O/D unit), the casting has the numbers (on the passengers side) 7283/2 LM22

The only numbers I could find on the transmission casting (again on the passenger side): 22B 590 (this could be 228 590, as it is hard to see) The next line reads 2B M2 (or 28 M2).

On the side cover of the transmission (drivers side) the casting reads: MOWOG 22B 309.

Does any this make any sense? What year is my transmission & O/D? Should it have O/D in third gear?

Thanks,

Robert
Robert Browning

One assumes that the O/D oil has been topped off.

Dan Robinson

Yes, the oil level is fine. I did check it, but thanks for asking.

Robert
Robert Browning

Robert;; This may have nothing to do with your car however this happened to me. I had a 1969 Olds Cutlass Supreme purchased new. Car was three years old and I had a leak on engine rear main seal.
Took it to AAMCO for a quick fix. Picked it up next day and it did not shift the same.Checked underneath and I did not have my transmission. It was now painted blue. Fortunately the year before I had marked all major car parts with a new electric pencil. This was not my trans. I called the local sherrif and we paid a visit to AAMCO.
There was my transmission on the shelf.Apparently this was a common thing for that shop as they were charged with fraud . I won my case against them.
Could something like this have happened with your car?
Sandy
Sandy Sanders

Sandy - what you are saying is what I have been thinking - but I hope I am wrong. This is why I want/need someone to help me identify what year my transmission really is. I know I had third gear O/D before the second repair. Now the shop is telling me that this is a late model transmission and never had third gear O/D? How could this be? If this is a late model transmission then I know something is fishy here!! I hope someone can help me identify the year of my transmission.

Robert
Robert Browning

Robert,
It is my understanding that whether the overdrive works in 4th gear, 3rd gear or all four gears has nothing to do with the overdrive. All MG's with OD have a lockout switch which simply stops the flow of electricity to the overdrive switch in reverse, 1st, 2nd and 3rd after 1974. Could it be that the lockout switch or the wiring to it was changed when you had the OD overhauled for the second time?
The most important thing to note is that the OD must NOT be engaged when you engage reverse gear, otherwise you will damage the clutch in the OD and have very expensive repair. On this BB at the moment is another post with a referencet to http://www.mgb-stuff.org.uk/electricstext1.htm Overdrive schematics is what you want. Hope this helps, Greg
Greg Taplin

Greg,

You are absolutely correct about never wanting to go into reverse while in O/D.

From my understanding the transmission for an O/D is different then from a regular transmission. Also, there is a switch which engages from neutral to the third-fourth side of your shifter - this is what I have been told.

So again, my question has to do with the model of the transmission and what year it is and if it was designed to have third gear O/D, and I do not think this is an electrical issue

Thanks,

Robert
Robert Browning

Hello,

I also posted my questions on the "MG Experience" website and someone wrote in and said:

"Blue label transmissions only operated with O/D in fourth gear. Black label transmissions operated with third and fourth gear O/D."

Can anyone verify this?

Thanks,

Robert
Robert Browning

No!

The blue label OD units with the side fill boxes came in in 1974 and OD worked in 3rd & 4th.

The restriction of OD to 4th only happened in Feb 1977.

My guess is that at the last rebuild you got an exchange tranny.

The OD operation is controlled by the selector arm in the top of the box - not the extension tower.

The only way to restore the OD operation in 3rd is to change the arm for an earlier one.

The casting numbers won't help as the castings didn't change - what will help is that on the casting - usually on the flat part between the mounting rubbers and/or in front of the side access plate there will be a date stamp in UK format - ie 4/12/67 is the 4th December 1967
Chris at Octarine Services

Chris - Thank you very much for this information. I was also under the impression that third gear O/D was done away with in Feb 1977 and not 1974.

Now let me ask, did "all" transmissions have a date stamp? I have just examined the entire transmission with a light and a magnifying mirror for the past hour and I cannot find a casting date stamp anywhere on my transmission. I looked everywhere everyone has said plus everywhere else I could reach and cannot find anything. The flat area between the two rubber bumpers has nothing and neither does the machined flat area in front of the side covering. So, did they all a date stamp?

Thank you.

Robert
Robert Browning

All of the late gearboxes I have in stock have the date stamped on the 1.5 x 1 inch flat area between the mounting rubbers - but you won't see it easily with the box in the car as the mounting bracketery is in the way.
Chris at Octarine Services

Chris,

If I remove the rear cross member and the the two rubber support bumpers, should I be able to see it then - with the transmission stil in th car?

Thanks,

Robert
Robert Browning

Is it possible to have a transmission with no production stamped on it?

I have looked in all the places suggested, plus everywhere else I can to with a light and magnifying mirror, but I cannot find a date stamp.

Robert
Robert Browning

Of course it is possible that no date was stamped on the tranny you have in the car!

Chris at Octarine Services

Robert,

it sounds like something in one of (I think) Pete Burgess's books on improving the MGB (sorry, but the book name escapes me). In that, there was some funky circuitry that effectively turned the MGB 4+OD into an easy to use 6-speed box, and the driver didn't have to worry about engaging or disengaging OD, as the "box of tricks" did that for you.

Sounds cool, but I went for a Sierra 5 Speed.

Grant :-)
G Hudson

North American cars from July 76 are complicated. Some had Transmission Controlled Spark Advance (to overcome problems caused by all the emissions mods) which limited vacuum advance to 4th gear only. Originally this was done by an extra micro-switch on the selector housing which operated in 2nd and 4th, which when wired in series with the OD lockout switch which still operated in 3rd and 4th, gave a 4th only output while still allowing OD to operate in 3rd and 4th. This is the circuit that all the diagrams I have seen show. But a few months later in Feb 77 the circuit was changed so that there was only one switch, which operated in 4th gear only. This left the function of TCSA unchanged but limited OD to 4th gear only, on all cars by this time.

The reason OD was restricted to 3rd and 4th is that the OD isn't strong enough to take the torque reversals in the lower gears. This is why the V8 was restricted to 4th gear only for all but the first few examples. This makes it unwise to use OD in 1st and 2nd. To get OD in lower gears means that the lockout switch arrangements have to be changed, which depending on how you do it can leave OD available in reverse, with potentially damaging results.

I have seen a circuit that claims drop OD out automatically when you change from 3rd to 4th, but that also has to have some additional mechanical work at the gearbox and isn't just electronics. The box of tricks did it for you so you didn't need a manual OD control at all? That would seem to combine the worst aspects of an auto and a manual OD. I can never see why people go for a 5-speed and lose the option of OD 3rd, which is a very useful gear for long inclines and twisty bits, but each to their own.

To go back to Roberts original question, if his gearbox was 'rebuilt', went in with OD 3rd and 4th but came out with OD in 4th only, then either the lockout switch is only just operating in 4th and not in 3rd due to adjustment issues (although wear usually causes the opposite problem); or some parts were switched with a later '4th only' gearbox, in which case the date label will tell you nothing. OTOH maybe the whole box was switched, in which case the date label (if it has one) will show a post-Feb 77 date.
Paul Hunt 2

This thread was discussed between 01/06/2007 and 07/06/2007

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