MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Overdrive choice

I have several LH overdrive transmissions. One is the early dipstick style - this one you can activate the overdrive in both 3rd & 4th gear. Of course it has the messy inside the car fill and the wrong speedo gear, the later ones are correct for the car and have only 4th gear engagement. Do you think the 3rd gear engagement is worth it or should I just install one of the later transmissions?

Thanks Fritz
Fritz Warner

Fritz
My experience is that the 3rd OD is so close to straight 4th that it's no advantage having it, The other disadvantages of ot being the wrong box for your car wwould (for me) push it out of consideration.

I am looking for pne of these early boxes for putting into an early car, sp if you have no further use for it, please drop me a mail off board

Dominic
dominic clancy

Fritz,

My OD operates on 3rd and 4th. I find 3rd OD really valuable here when driving in 50k speed zones (which increasingly apply to suburban areas), particularly when in traffic which can have you varying speed around this limit.

Open road driving sees me engage OD from 4th gear.

Regards
Roger
Roger T

As far as I know the gears the OD works in is governed by the inhibitor switches on the gearbox not the OD unit itself.
Ron
R. Algie

Whilst OD 3rd is quite close to 4th on the earlier D-type OD it is much closer to mid-way between 3rd and 4th on the LH - 4.43 as opposed to 5.4 and 3.909. As such it makes a very useful 'extra' gear for long inclines and twisty bits where otherwise you would be continually going up and down, or running constantly at a bit higher or lower revs than is ideal. Beats me why anyone chooses to swap the electric OD system available at the flick of a switch for a 5-speed box, have all that extra clutch malarkey and lose a gear into the 'bargain' ...

Ron is correct about the inhibitor switch controlling which gears OD is available in. Late-model US spec cars had it restricted to 4th gear only as the same switch was used to control the vacuum advance, which had to be limited to 4th gear only to prevent 'surging' caused by all the emissions baggage.

50 mph cruising? I'd be in OD 4th, let alone straight 4th!
Paul Hunt 2

Paul,
That 50 in Roger's post referred to kilometres not miles!

Roger,
I neglected to thank you for measuring that MK II badge location - very slack of me.

Cheers,
David
David Overington

Third gear OD is very handy in town. YMMV.

warmly,
dave
Dave Braun

David,

Quite OK, we all get busy on our cars. Trust it helped.

Paul, 45-50 mph is about where I switch my OD in on 4th gear. Generally I don't bother with 3rd OD when simply accelerating up from, say, a light take off or on joining a main road. Seems to work for my car and style of driving. I would agree with you an the reference to 3rd OD on inclines. I should also have mentioned mine is the LH behind the 18GG engine from 1969.

Regards
Roger
Roger T

50kph/31mph - fair do's, but I'd still be in 4th and not OD 3rd if trickling along at that as a constant speed. Personally I say that OD is used just like any other gear - 'when the conditions and/or traffic dictate'. Therefore if I'm accelerating up from a standstill to an ultimate speed of 70mph, say, I would engage OD 4th at a higher speed than if I were just accelerating up to 40. Likewise I may hold 1st until 40 or 50mph if heading for 70, whereas if heading for 30 will have gone through all the gears by the time I reach that speed.
Paul Hunt 2

I find 3rd useful not only for climbing hills, but also on the highway. When I come up on slower traffic, I can shift between 3rd OD and 4th OD without touching the switch. With a 3rd-only OD gearbox, you would have to switch in and out of OD instead, or just put up with winding it up and down in 4th only.
Steve Simmons

Compared to an overdrive, a 5 speed trans is more efficient and more reliable. The ring or sun gear system for an overdrive has more gear tooth engagements and more bearings making for substantially greater friction than the typical 5 gear tranny. The overdrive ring gear & clutch system is much more complex, less reliable and more expensive to fix.
A Sierra or Datsun 5 speed trans is about 50 lbs lighter than an overdrive.
I must admit it is cool to be able to flip the switch and change gears. I've got a good operating late model o'drive sitting in my garage that I took out for a Datsun 280z trans. I wouldn't go back.

Having 4 close ratio gears, with a second gear that is halfway between 1st and 3rd gears more than makes up for the flip switch. 5th is .745/1 compared to about .8/1 for the overdrive.

Barry
Barry Parkinson

I use 3rd OD quite often and find it very useful. It is worth saying I have a dash mounted switch which I can reach without taking my hand off the steering wheel. Times when I use 3rd OD are
:- hard acceleration onto motorway (can easily get well above legal limit in 3rd OD)
:- town traffic (flick of switch is more relaxing than changing gear all the time)
:- twisty roads (never fast enough for 4th OD through the bends, but need more than 3rd gear. Not having to take hand off wheel is a real help here!)

Personally I wouldn't change to a 5 speed box even if you offered to give me one and fit it for me. The OD switch is just part of the character of the B to me! Having said that, if I had a gearstick mounted switch I would seriously consider mounting it on the dash instead. Plenty of people seem to like the 5 speed option, so I guess it is a personal thing.

Iain
70 BGT
www.mymgb.co.uk
I D Cameron

But Iain, who steers when you want to change from 4OD to 3rd? Personally, the dash switch irritated me when pressing on and I prefer the geartop mounting - all the gears in the same place.
Steve Postins

The Mark ones did not have syncro on first.
However;
I was under the impression that the third gear overdrive was disabled for the V8 because of strength issues and, well, left that way. Paul is probably right as usual, however it could be restored easily enough.
The 3rd gear O/D inhibition was accomplished (I thought) by grinding a small part of the corner off the rod/base that the gearshift lever locates on. This results in that switch not being depressed as it is with the 4th gear. If this is infact the case then the 3th gear overdrive can be easily enabled by restoring that ground out bit. Replacement with the earlier part or blobbing on a bit of weld, or even epoxy gluing on a wrap around bit of metal. It might even be done with the box in situ because the switch is to all intents and purposes external to the gear box.
Any thoughts anyone?
If it were my car I would much rather have the extra 'overdrive' gear. I used to have an overdrive mark one, and the extra 3rd overdrive did add to the fun factor significantly.
Yes, modern gear boxes are much much better, I have a Toyota supra behind my V8, stronger lighter better ratios; but definitly not quite as much fun as excessivly elaborate electric overdrives!.
Peter

The O/D I installed on my '73 GT is in fact the wrong one - a later model without the dipstick. However, it definitely engages in 3rd as well as 4th. The only difference is that I had to swap out the 1280 tpm speedo for a 1000 tpm one in order to get correct readings. BTW, the 1280 vs 1000 tpm speedos have nothing to do with the presence of o/d. Both o/d and non-o/d MGBs went from the 1280 to the 1000 about the same time as the switch to rubber bumpers.

Here in the mountains of Appalachia, I often find myself climbing in the 35-50 mph speed range where all my shifting is simply between 3rd and 3rd O/D. Since I have the stalk switch, this means my hands never leave the wheel. On a 3.9 MGB, it I'm estimating that 3rd O/D yields about 13 mph/1000 rpm. It's rather like driving my old 4.55 ZB Magnette in normal fourth gear. Climbing a long straight hill near where I live, I can achieve speeds in 3rd O/D that I cannot get in straight 3rd or 4th.

While accepting Barry's reservations about the o/d box, I'm inclined to agree with Iain: it's part of the MGB experience. But when it came time to add overdrive to my 1800 Magnette (different thread, I know) I chose the Rivergate/Datsun 5-speed because it could be installed without irreversible alterations to the tunnel. Therefore, I kept my Magnette MORE original by using the Datsun box. By the way, that makes for a VERY nice combination.

FWIW,
Allen
Allen Bachelder

Hi Folks:

You can make either 4th only o/d or 3rd & 4th o/d by changing the shift tower. The 75-80 shift tower locks out 3rd. Seem to remenber a MGB list thread regarding the reason was snapped 3rd motion shafts (warranty claims). Use the 68-80 tower if you want 3rd & 4th o/d.

Good luck: Rich Boris 67 B roadster
Rich Boris

But Rich, my transmission is from a '76 and it shifts into o/d in third. Your comment makes sense, but could it be that it refers only to 77-80? Or was it the tcsa switch on these models that also locked out o/d in third?

Allen
Allen

my view is often that a 3rd OD ratio is very helpful, like when exiting roundabouts with around 3500rpm in 3rd, and then engage OD and very often you find yourself out grunting cars with ease.

however for hill climbing im not such a big fan, 3rd OD requires greater power to push than straight 4th due to the gearing combination.

a .82:1 and 3rd ratio requires nearly as much power as straight 4th. and then when you have accelerated the 3rd OD ratio you engage 4th and the car feels sluggish once more.

i tend to stick to the straight gears when driving in town.

my car is a 1967 roadster 3syn.
d buck

I've checked up, I had the wrong end of the tower in mind. From Roger Williams book.
The part number is 22B 386 and goes on the gear selector shaft. It is a gearbox out job.
Peter

77 models lost OD 3rd because the same switch was used for the TCSA which operated in 4th only.

3rd OD for hills and twisty bits is more about rev range and hence torque than power.
Paul Hunt 2

Has anyone who has professed to know why some O/Ds have O/D on 3rd & 4th and some only on 4th, actually taken the box apart to find out why?????? I have an O/D on my 73, and it engages on 4th only. If I wanted to have O/D on 3rd as well,I would tend to think that it would involve more than just the switch. When I am in 4th gear, the "button" on the end of the electrical switch inside the trans is depressed allowing power to the O/D solenoid and causing the O/D to activate. Whatever is causing the electrical switch to close (be depressed)in 4th gear, is not causing the switch to close when I am in 3rd. I think that there is a small part on the 3-4 selecter shaft that is not used on 4th O/D only boxes. Just guessing....
Alyn

It is just a case of the end of the selector shaft being the appropriate shape to operate the lockout switch in both 3rd and 4th through to the end of the 76 model year, or 4th only for 77 and later. See Peter's posts above.
Paul Hunt 2

This thread was discussed between 20/05/2006 and 29/05/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.