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MG MGB Technical - 5 speed gearbox or not

The gearbox on my 64B is on its way out , should i source a repalcement 3 syncro box and fit an overdrive , or should i go with a 5 speed conversion.
I am into driving my cars and not concours , so not too worried about originality. What should i do? .. Thanks Roy
R Mcknight

When my '67 trans first started to fail, I replaced it with another 3 synchro non/od trans. About a year later, I installed a 4 synchro non/od trans and was very impressed with the smoothness of the shifting. About 2 years later, I bit the bullet and purchased a rebuilt 4 synchro/od trans from Moss that had been overhauled in England. That was over 25 years ago and I don't regret it. It totally transforms the way you can drive the car. The 5 speed conversions are very nice, but they are quite pricey. When I converted my car over to the later design transmission, I had access to a '68 GT parts car. I was able to get the backing plate, flywheel and starter from the later model car and this made the conversion much easier. The only thing that I needed was the shorter 30" driveshaft. I even modified the safety on the transmission so that I could use od on all forward gears. I did this by installing a cutout relay, in the reverse light circuit, to kill power to the od if I accidentally left the od switch in the on position. The later transmissions all have the reverse light switch and add installing the cutout relay is a simple procedure. I've been driving my car like this for 25 years and the trans and od still work like brand new. These are tough units and are hard to kill. If you go this route, try to source the black label model, used from '68 to '74, as these are the strongest ones made. RAY
rjm RAY

Ray, good summary. I need to replace the 2nd synchro on my 3 synchro 65 non/od. I'm thinking about the 4 synchro non/od switch as od boxes are hard to find. Sounds like you felt it was worth the effort and stepping away from originality. Of course it does add weigth.

The biggest risk seems to be finding a good gearbox. But, maybe it is much better with these stronger boxes than the old 3 synchro which are often trash.
Robert McCoy

4-synchro O/D boxes seem to be cheap and plentiful in the UK.
Dave O'Neill 2

Hi Roy,

This is a very contentious subject, those for "originality vs those for driving.
When I did my conversion to 5 speed, I took note of Ray's comments about him having all the bits that need to be changed, which you would need to source. I was offered, at an unknown price, a 4 synchro box, but when I found out it was not a direct swap, and required body bashing to physically fit, due to the greater width of the box, I opted for the 5 speed. I used a Datsun Bluebird box and it went in without too much hassles. I am fairly handy and have always done my own servicing and repairs to cars, but had never tackled anything on this scale, so I was fairly nervous about doing it, but found it was just do what the instructions said and use common sense where they were deficient.

My total cost was around $A 1,200. A rough breakdown:-

Gearbox including driveshaft $220
Driveshaft modification and new UJs $220
Clutch plate $ 45
new throwout bearing $ 36
Bronze bushes $ 7
New smaller pre engaged starter motor $250
Custom speedo cable $135
Rear mount $ 26
90 deg speedo takeoff, second hand $ 30
Clutch slave cylinder and hose $ 55
Rubber boots for clutch lever
and gearshift $ 70
Sundries $ 50
Frustration much
Satisfaction in achieving it
and joy of driving priceless

You will need to do or have done some welding and possibly some machining.

If you like I have annotated an article, by a fellow club member, highlighting my experience in doing it, that I can email to you.


Herb Adler

If you have a 3-synch without OD and aren't bothered about originality then a 5-speed will probably be easier and cheaper. A 5-speed as an alternative to an original OD (on 3rd and 4th) unit just doesn't stack up in my book. I wouldn't even replace my 4th only V8 box with a 5-speed, flicking the extra gear out and in is just too convenient.
PaulH Solihull

My view is that having overdrive on 3rd & 4th gives great flexibility and allows you to set the car appropriately for many traffic conditions. I have two 5 speed cars and driving around London one is constantly stirring the box. In my 73B I select a gear and flick the OD in and out as needed, which is easier on the left leg. This is one of the joys of OD equipped Bs.
Richard Thompson

I'll stick with my original four-speed with overdrive. I've driven an MGB that has a five-speed, and it just isn't the same.
Stephen Strange

It's your car and your decision. I have had my 3 synchro o/d rebuilt twice during the time I've had the car. It needs thought to use effectivley with a stump puller first, 2nd is clearly geared so once rolling you can get by without double de clutching into 1st. So that causes the long gap 2nd to 3rd. Then the o/d on 3/4 with vacumn interlock takes a while to get to grips with, but its fun and can be used as a semi automatic, it also means the car is very comfortable on Motorways. For me the period charm and character of the transmission is one reason I have kept the car so long. The type 9 conversion works well, its slick shifting and especially with the longer 1st gear makes a big dirrfence to acceleration through the gears, but if I wanted a Mazda MX5 I would buy one.
ps The MGB change is a lot slicker than the cable opersted one on the mgtf.
Stan Best

I have a good 3 syncro with overdrive in my '67 BGT and a Ford Sierra 5 speed in my '58 MGA.
Having O/D on both top and third is nice, but I very rarely use 3rd O/D.
IF I had to start again from scratch, I'd go with a five speed in the MGB rather than the 4 speed with O/D.
Top O/D and 5th are nearly the same gear ratio, so no arguement either way, there. However, the Ford box gives me syncro on 1st gear; the second gear ratio is closer to third and is a more useful gear.
Difference in cost? Hard to compare.
The 3 syncro box and O/D was about $1700 CDN in 1992, from M&G International in Birkenhead.
The 5 speed conversion was about $2400 CDN from Five Speed Conversions.
Both costs include all carriage costs, duty, taxes and the usurious brokerage fees, all of which roughly double the cost of the bits.
Fraser Cooper

Installing a non OD 4 synchro trans in a pre '68 car doesn't require any modifications to be done to the tunnel. With the later LH OD equipped trans, the OD's bulge requires some mild enlarging of the tunnel around the OD itself. The gear lever sits back about 2", but the tunnel adapter is easily modified. There are even some fiberglass tunnel covers on the market today that are designed for this swap. The shift lever can be installed 180 degrees out to locate it closer to where the original went. You can get away with using the original starter and flywheel by drilling a hole in the bell housing for the starter's Bendix drive to pass through. The only other thing that you will need is a 30" driveshaft. I have a friend with a perfectly restored Iris Blue '64 that has a '71 4 synchro/od trans installed behind the original 3 bearing engine. It uses the original starter and flywheel, as well. It totally transformed the car. RAY
rjm RAY

I must disagree with Herb and say that installing the 4synchro gearbox with or without overdrive, does not require any body bashing on a 1964 car. I did it 25 years ago and it added so much to the car's driveability. Plus the late gearbox is much stronger and you are highly unlikely to need to recondition it again once it has been installed.

Ray's advice is (as usual) excellent but I also did not need to enlarge the transmission tunnel for the over drive. Using the overdrive is great and increases the driving pleasure and uniqueness of the MGB.

The closest contact point is the bulge on the bellhousing for the starter motor and I filed away a small amount of the aluminium to avoid having to dent the body. I can take a photo of the amount of metal removed if someone would like me to, because that gearbox is now not in the car. The car is now a V8 with a factory gearbox.

I will concede that I did install one simple dent with a ball pein hammer where the angle drive for the speedo cable comes close to the tunnel.

Besides the valuable information in this thread there is also heaps in the archives.
Ian Buckley

I have a functioning 4 speed with overdrive and find it fine, sort of. I did have Herb's write up of the 5 speed conversion, and as a tinkerer thought it might be a project for later, but having looked at the account Herb, I think your ability, and skills are greater, than mine, so will not tackle this for a while. There is a question I must ask while we are on this subject. I can select over-drive in 3rd and 4th, fine but when de-selecting ie, swithing off I sometimes find it does not come out immediately? As I slow down suddenly the revs rise and I think to myself, Ah, overdrive's off? I cannot think this is normal. Consequently, I am not using it too much cause I'm scared! What do you think? Thanks Mike
J.M. Doust

It sounds like your relief valve isn't releasing all the way. The valve can be serviced in the car. Look for the various videos on You tube, by John Twist, on this subject. It is a job that just about anybody can handle with ordinary tools. RAY
rjm RAY

Thanks Ray, Mike
J.M. Doust

I did a Datsun 280ZX conversion in my '63 - it did require a little bashing to make a bulge on the left side of the tunnel - but so does fitting a MGB 4-synchro;

given that 4-synchro synchros are nearly impossible to find, particularly the 3-4 rings, the writing is on the wall regarding those vintage boxes

- the only downside is that some vintage race clubs such as CSRG do not allow 5-speeds

( essentially - CSRG does not allow anything that is not bone stock in appearance)

(I know several guys running 5-speeds in VARA)

Larry Moeller

The sheer number of 4 synchro transmissions available, due to their strength, means that parts, from these transmissions, can be used to rebuild quite a few complete, reliable units. RAY
rjm RAY

"but when de-selecting ie, swithing off I sometimes find it does not come out immediately?"

"It sounds like your relief valve isn't releasing all the way."

Sounds more like the solenoid to me, not coming back far enough to release the majority of the pump pressure. My V8 is doing something similar - when switching out on the overrun after a highish speed run it is like it is switching between OD and not OD (or could be drive and no drive it is difficult to tell) until the speed drops. It only started a couple of years ago, but has become more noticeable, and changing the oil and cleaning the filters and valves made no difference. Ex-Laycock opinion is that it is the solenoid plunger sticking in the solenoid and failing to release enough pressure.

I don't use OD 3rd that often, but in twisty bits or on inclines it proves its worth.
PaulH Solihull

I find it difficult to believe that the solenoid's plunger is sticking. The clearance, between the two parts, makes it an unlikely culprit. Have you dropped the solenoid base plate and examined the plunger and solenoid? When my OD was misbehaving, last Summer, I put it through all of the usual tests. It passed them all, but when I double checked the power draw of the solenoid, it read 0 AMPs on my gauge. It turned out to be a faulty solenoid. It was, after all, 39 years old. After replacing the solenoid and cleaning the rough spots on the plunger, the OD has worked perfectly ever since. I would think that when the power is shut off to the OD solenoid, the 400+ psi of oil pressure would easily push the plunger aside and allow the the sliding member to return to direct drive. RAY
rjm RAY

If you drive the car flat out from first and travel through the gears i have found a dead spot in third gear .It seams MG got the gearing slightly wrong.The Ford Sierra 5 speed box (standard gearing)seems to cure this problem and i read some where that you can get to 60mph 2 seconds quicker.
DCP Parker

Hope you dont mind me commenting on this subject, I almost qualify as I have an MGA with that is fitted with a tuned 3 bearing MGB engine and a ford sierra 5 speed box.
For the MGA fitting the O/D box is a difficult proposition and most owners are reluctant to modify the transmission tunnel and chassis so much to make it fit.
So the sierra box is the easier option here in the UK.

The ratios are a little better spread than in a standard gearbox but 2nd gear is still a little low in my opinion and I find I dont tend to use it once I am on the move. There is just so much torque with the MGB engine.
(Stan in the UK is probably correct in that 2nd gear was originally kept so low to compensate for the non synchro 1st gear being so difficult to select whilst the car is on the move)

For anyone who has a non O/D 4 speed box, fitting a 5 speed doesnt really change how the car drives as the 1st 2nd and 3rd gears in the box are pretty close to the original and 4th is still 1 to 1 like in your 4 speed. Ist gear is still too low to use once you have gotten moving (except on steep hilly terrain maybe)
So the driving experience is not spoiled and you can still drive it as a 4 speed if you wish.

The advantages are that the over-driven 5th gear makes those long freeway runs quieter and if you are in traffic or climbing a steep hill you can select 1st gear so much easier.

So I wouldnt suggest it is better than overdrive, but its certainly a really great alternative to a 4 speed.


Colyn Firth

!00% agree with Colyn thst for an MGA the Sierra type 9 might have been designed for it at Abingdon. I have driven a very original MGA 1498cc coupe 4 speed and a Twin Cam and a mildly tuned 1588 both fitted with the 5 speed. A bit more power and 5 useable forward gears transform the car (lighter and less profile drag than the B). With its looks and handling , if not interior space , already irreproachable you can see why they remain so popular today. The 1588 car had the longer 1st gear which is well worth the cost uplift, it was just amazing in situations like joining an A road. When you add in better fuel economy and the fact it's invisible when the car is parked personally for modern traffic I wouldnt consider an MGA without this upgrade. For an every day driver MGB I think the 5 speed has it on reliabilty grounds, for our car, it's staying original in this area, the gap from 2nd to 3rd being masked by the 1950 cc overbore.
Stan Best

This thread was discussed between 03/02/2011 and 16/03/2011

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