An introduction to overdrive and 5-Speed gearboxes.

At 06:58 PM 4/14/02 -0700, Dave Hammond wrote:
<I'm starting the complete restoration of my 62 MGA MKII and would like to find an overdrive unit to mate to the transmission. ....>

Quite a laundry list of troubles here, and I don't personally recommend any of this, but I do have some information. To begin with, the MGA was never built with an overdrive option, so it is not designed to accept one as a bolt-in conversion. Almost anything you do to try to fit an overdrive into an MGA will require some cutting and welding, and usually some modification of the center tunnel and frame, floorboards, maybe even the seats and the handbrake location.

An overdrive gearbox uses a shorter mainshaft, after which the overdrive unit generally bolts up to the back of the main housing in place of the original tail housing. Overdrive mainshafts for the MGA gearbox may well be non-existent, as the car was never built that way. This problem even applies to the MGB which did have a couple of different overdrives available, but the mainshafts for those overdrive gearboxes are currently not available as spare parts, so you generally have to buy a complete overdrive gearbox just to get the mainshaft.

So this problem gets down to how much cutting and welding and other modification you are willing to do to fit a non-standard overdrive gearbox into the MGA. The biggest problem with the overdrive units is that they are rather wide aft of the main housing, which generally requires widening the tunnel to make them fit. The early MGB 3-syncro unit is a bit smaller (narrower) than the later 4-syncro unit, and would be easier to squeeze in with less radical modification of the tunnel. But the earlier MGB unit is also more rare, and either one requires some cutting and welding to fabricate a new rear mount for the gearbox.

The MGB overdrive gearboxes are currently in high demand and (relatively) short supply, so you might expect to pay something like $400 to $700 USD for one used (written April 2002), and it may require some more money to repair or prep it for use, or a reconditioned one may cost as much as a 5-speed conversion set (including the gearbox).

Now if you don't mind some non-MG parts in your car, and you have about $1400 burning a hole in your pocket, you can buy a 5-speed gearbox setup that is (for the most part) a bolt in conversion for the MGA. You can start here:
http://www.classicconversions.co.uk
and there are other suppliers and other 5-speed units that can be made to fit, generally easier than the any overdrive unit.

Or if you want to get a little more wacky you can do a different 5-speed gearbox and slip in a 160 HP fuel injected V6 engine along with it. I still don't personally recommend this, but for kicks you can check here:
http://www.classicconversionseng.com
These kits are designed to fit the MGB, but a little more money can go a long way towards a little more adaptation to fit the MGA.

Quite a few people would look fondly on a change of final drive gearing to the MGB 3.909:1 gear set, especially if the engine is uprated so it would run past red line in top gear. This is a bolt-in swap, very easy to do, not very expensive, and definitely not visually apparent. The tickler problem there is getting the speedometer and odometer to read right afterward.

A fair number of people would look fondly on a 5-speed gearbox. These are usually English Ford origin, but a Japanese 5-speed may also be acceptable if properly installed. I think the general idea is, out of site out of mind, and the odd gearbox is not particularly noticeable to casual inspection. None of these options will increase the value appreciably, so you would be spending the money strictly for your own pleasure. On the other hand, modifying the frame and tunnel to install an overdrive unit will likely reduce the value of the car.

Oh, one other thing. Cutting and welding to modify an MGA is rather frowned upon in the collector car community. Some of the 5-speed conversions may be (in theory) reversible, but I have never known one to ever be returned to the stock 4-speed configuration once converted. And in spite of the large cash consumption, an overdrive or 5-speed conversion may not make your MGA worth any more for resale, and will most likely reduce the value of a concours car.

Regards,

Barney Gaylord (trying to remain non-judgmental)
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ADDENDUM October 9, 2004:

At 05:50 AM 10/9/04 -0600, Rod Bunn wrote:
>"I have converted back from five speed to four speed with no difficulties. I had to do this, because the twit who converted my 1600 Mk11 did so to make the car easier to drive on classic rallies. Sadly, this is not allowed. So I had to convert it back."

Okay, now I can eat my words. There is now one recorded case of an MGA 5-speed being returned to stock 4-speed configuration. Hey, Rod? Can I have your old 5-speed?

Barney