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Overdrive conversion

Posted by Lightfoot 
Lightfoot Avatar
Ralph Bell
Martinez, California, USA   usa
1979 MG MGB

Man, I read a ton about installing overdrive transmissions into MGBs. I've been reluctant to look hard at one for fear that I would buy it and never get it installed. My 79B runs great now that I've spent the winter working on it, but in the back of my mind, I still covet that extra gear. HONESTLY, just how difficult is it to install an overdrive and just what is involved. I seem to recall that there were some electrical hook ups that did not previously exist on a non OD car. Can I really do the conversion in a day...The truth only please. Lastly, what do you look for in a used OD transmission? As always, thanks for your insights.
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Simon Austin Avatar
Surrey, BC, Canada   can

If you've already got your engine and old gearbox out, then yeah, you can probably do it in a day. Otherwise, give it a couple.

I installed a late-model o/drive in my '74 some years back. The only difference between wiring is the late models have the switch in the shifter knob and the early (not steel-dash cars) have it on the wiper switch. l hooked up the o/drive wiring from the gearbox into the existing wiring in the car. For your car, unless you can find a shifter with the o/drive knob, you can easily hook up a new switch on your dash. All CB cars had the o/drive wiring on the wiper switch regardless of whether or not the car originally had o/drive installed.

You can use your old drive-shaft with the o/drive as well as the existing cross-member.

It's a good time to renew your clutch components as well unless you know them to be good. All these parts are the same whether it's an o/drive or not.

As for purchasing one, the old adage "buyer beware" prevails. Hopefully you can find one that the PO can verify it's operation. Others will have ideas on what to look for in detail.

HTH
Simon
John Hamilton Avatar
Pensacola, FL, USA   usa
1965 MG MGB "The Great Pumpkin"
1968 MG MGB GT "Buck"
1970 MG MGB "Dad's Car"

I bought a rusty '69 that hadn't been on the road in years, just so I could swap in the OD and the wire wheels. All I did was replace the clutch, drain the fluid, replace the o-rings and filter, refill the gearbox and install it as normal. After years of sitting, it works just fine. Be sure to check all of your switches and wiring before you install it to make sure it'll engage and dis-engage properly. Definitely a worthwhile project, one day if you're familiar with pulling and installing the engine/gearbox assembly, two days if not. Replace your 40 year old gearbox and motor mounts while you have it out.
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chris Avatar
Chris Roop
Pendleton, OR, USA   usa

I stick an o/d into all of my restorations or complete rebuilds just because of the added value at sale time. To put one in is no harder than to put in a 4 synchro except for hooking up a couple of wires.
Is it that much better??? NO. It is only the cool factor that makes o/d worth having. Would I want it in my car??? You bet! To flick that switch, lose a k or two in revs (savings both in gas and wear) is priceless.

Member Services:
Not just for MG parts. Quality new parts at 15% off of list. No sales tax in Oregon.Can kozees at $2 each, black hoodies $25 both plus postage. Check out the Vendor Market page.
Lamb-Man Avatar
Gary Lamb
Tracy, CA SF Bay Area, USA   usa
1970 Triumph Spitfire "To Be Determined"
1975 MG MGB "Twiggy"

Lightfoot Wrote:
Quote: I seem to recall that there were some electrical hook ups that did not previously exist on a non OD car. Can I really do the conversion in a day...The truth only please. Lastly, what do you look for in a used OD transmission? As always, thanks for your insights.

Your 79 should be pre-wired with a plug at the firewall.
I would give yourself 2 days (of course everything I do seems to take 2 days)
As far as what to look for what to look for; it's like looking for a wife, low miles, functioning and of course good teeth. grinning smiley



The answer my friend, is blowin' in the wind!
Wiley1 Avatar
Alan Carpenter
Seattle, USA   usa
1973 MG MGB

John Hamilton is absolutely correct. I pulled my 73's engine , changed the clutch and installed the OD . Started at 8:30 Am was driving the beast in OD at 4:30 PM - same day.
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Ralph Yingling Avatar
Monroe, WA, USA   usa
1973 MG MGB "Molly"

It sounds as if he is unsure about whats required. Sombody be sure to tell him to get the entire OD transmission and not plan on converting his!



Ralph
'73, SUPERCHARGED! tubeless wire wheels, OD, Advaced Dist. recurved dist. w/Pertronix, de- smogged, ANSA
Jim Duke Avatar
James Duke
Memphis, USA   usa
1977 MG MGB

Changing to an O/D is very straight-forward, and should be a one (long) or two (short) day project. The cross member needs to be moved back one 'notch', but the body already has holes for it - you just put it back in the next holes. As for wiring - your harness is already complete except for the O/D 'pig-tail' which you'll need to acquire from British Wiring. I don't know where the switch is on the '79 B, but installation of the gear-top switch is dead simple, after you acquire the switch (Moss?), it is just plug & play with the harness/pigtail.

The O/D really makes the B a road car. You can accelerate smoothly past 75 mph, and cruise with comfort at interstate speeds.

Best investment for a B in my thought.

Jim D
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Michelle Pierce
Elyria, OH, USA   usa
1978 MG MGB "Mabie"
1995 GMC Suburban "The "Burb""
1999 Porsche 911
2007 Saturn Aura

I spent 2 1/2 hours last night pressure washing an O/D tranny that I bought, I think I needed a wet suit. Mine is a 78 and I am putting in an O/D from a 74, I have talked to a couple different mechanics and they said I could do it myself but to call if I have questions. Since most 77 and later cars have the shifter made to handle the wiring of the switch, there are only a few things that I need in order to make the change. Personally I can't wait to try it out. I agree with the member who said to buy the complete O/D unless you are parts and tech savy the conversion of an existing one would be over my head smiling smiley. The next couple days will be to finish the cleaning of years of grime, check out the inside for any missing teeth then set her aside and work on pulling the engine. So perhaps it depends on your level of comfort, a lot of these guys can pull the engine and tranny in no time but I think it is years of experience that guides them. But you have to start somewhere.
esd3104 Avatar
Simon Dix
O'Fallon MO, USA   usa
1972 MG MGB "Scarlett"
1977 MG MGB "Tigger"

Lots of good advice already! I've done this a couple of times now and I have to say for me it is the best "upgrade" that's is available!

1. Best to find a complete OD & tranny - you can convert but you need the main shaft from an OD tranny in addition to the OD itself. Find a complete one.

2. Watch out for the type. Your 79B *should* have a 1000 TPM (turn per mile) speedo. As such you want a "blue label" OD from a 75 or later car or you'll have to deal with recalibration of you speedo. There are other resaons like top fill vs side fill but the 1000 vs 1280 TPM is a biggy.

3. I've gone the "slap it in" and "overhaul" routes and both have worked. Depends on the level of trust/knowledge about the condition if you go the slap it in route. The overall route was somewhat forced on me as the 2nd time I did this I had brought the tranny and OD back from UK, but as I checked it in as luggage on the trip back, I broke it down and left the bell housing behind.

4. Search here in the forums for pictures of the "transmission crosss member modifications". This REALLY makes a HUGE diffference to the ease of putting the X member back. PLEASE take my word for it on this one!

5. The Switch is much debated but you can really put it anywhere and if you choose somehwere non standard you can make your own wiring harness pigtail/extension. On my 77 I put the switch in the top of the ash tray and we wired into the switch on the pedal box housing that controlled the vac advance on 4th gear on the later cars. For my 72 it turned out the switch was on the wiper stalk (but unmarked) so I could simply use the standard OD harness extension and connect that right into the main harness.

6. If you've never pulled and engine/tranny before then this is not a 1 day job. Even if you have I'd suggest it's longer than one day to get everything back together and address any wiring setup you may need to do. All about experience as with many things. Find a friend to help either way!

7 Useful resources? Here, of course and http://www.octarine-services.co.uk/ and http://www.quantumechanics.com/

Hope this helps. It truly is a great upgrade - I love the OD!

Simon





Simon STL

77B On Tail of the Dragon @ MG 2006 & The 72 at home and essentially complete

underdog Avatar
Jim Underwood
Pittsburgh, USA   usa
1972 MG MGB
1980 Triumph TR8 "Fabulous Trashwagon"
1999 Chevrolet Corvette "Darth Vader"
1999 Chevrolet S10 "Spare Change"
2003 Jaguar S-Type "Eleanor"

Ralph Yingling Wrote:
Quote: It sounds as if he is unsure about whats required. Sombody be sure to tell him to get the entire OD transmission and not plan on converting his!

Since you brought that up.. I saw a conversion setup on ebay from the UK. Had the unit, mainshaft...everything but the trans itself. I suppose they figured better chance of selling it to USA with just the pieces than shipping a whole trans assembly. You catch that one Eric? I know you were looking into having some imported.
reggo3636 Avatar
reggie dernbach
cuyahoga falls, ohio, USA   usa
1970 Triumph GT6+ (MkII) "Jeremy Fisher"
1974 MG MGB GT "Marjory Stewart Baxter"
1979 Triumph Spitfire
2003 Audi A4 "Kenneth"

I put one in over the winter and it wasnt too bad at all. Do it. You'll love it
Lightfoot Avatar
Ralph Bell
Martinez, California, USA   usa
1979 MG MGB

Thank you all for the sage advice. I'm on the lookout for a transmission now. I saw one for sale in southern CA, but did not pursue it due to the doubts I had. I'll look again and hopefully find one in good shape. Wish me luck.
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