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Gearing up for a gearbox rebuild

I was fearful of what we might find when we opened the gearbox on the MGB GT, especially since it had a Laycock de Normanville type-D overdrive


During this time spent indoors during shelter in place, I have been able to continue work on the MGB GT restoration. The latest project has been tackling the gearbox.

For a bit of history: The 3-syncro overdrive-equipped gearbox in this car is the one in it when the car was delivered. The gearbox was rebuilt about 10,000 miles ago by John Twist, founder of University Motors and the expert on such cars, and should have been fine. 

However, I noticed while driving the car before I decided to disassemble it that first and reverse were very difficult to engage. I know how to shift a non-syncro gearbox so I knew that something was not right. 

Mac from The Paddock and I did a light cleanup on the gearbox before we started the job of disassembly. 

A point of information is that while over the years I have rebuilt many engines, everything from Alfa Twin Cams, to Porsche 356 and 911s, to name a few, I have never before attempted to disassemble, let alone to rebuild a manual gearbox.  They have always scared me a bit as I really do not know how they work. I always think of them as some magic device. That goes doubly for the Laycock de Normanville type-D overdrive in my MGB GT’s gearbox. If the gearbox is magic, the overdrive seemed to me to be like high sorcery, something that only a graduate of Hogwarts could master.

It is possible that you know as little about these overdrive units as I did,  so I thought I would give you a little history on the development of these technological marvels and describe how they work.

Gearbox out of the car and ready for disassembly
Gearbox in pieces

The Laycock de Normanville overdrive was invented by an Englishman named Edgar de Normanville and was manufactured by automotive supplier Laycock Products. 

Normanville is an interesting character, an automotive journalist as well as an inventor. He wrote for Motor Magazine, The Daily Express and Chronicle newspapers, and also wrote a number of automotive books. His initial gearbox invention was an epicyclic 4-speed gearbox for Humbler in the 1930s. In the 1940s he adapted this idea of epicyclic gears for an overdrive unit.  The overdrive is basically an electromagnetic, solenoid-activated planetary gear set that sits between the normal manual transmission and the driveshaft.

At the flip of a switch, the overdrive unit gives a reduction in gear ratio, basically adding a gear. When activated, the overdrive engages and lowers the engine speed relative to the driveshaft, making for better fuel economy.

Mac pleased with looks of planetary gears

With my trepidation about disassembling the gearbox and overdrive, I am grateful that my friend Mac was there to help and tell me the job is not as hard as I thought it was going to be. We disassembled the gearbox and overdrive unit and examined all the parts. We knew we needed a reverse gear and a new layshaft but needed to find out what else had been damaged during the previous owner’s lack of finesse at shifting. 

After about an hour we had transformed the gearbox into a table full of parts. The reverse gear and layshaft can be difficult parts to find as they are basically out of production. Happily, I was able to find a rare new-old-stock layshaft and reverse gear at The parts were still in their original boxes and I bought them immediately.  I then called John Esposito at Quantum Mechanics for advice on what to do next. 

There is a good reason I made this call. Quantum Mechanics is probably the finest gearbox-rebuilding company on the East Coast, and specializes in British gearboxes, and thus the staff includes serious experts with the Laycock de Normanville overdrives. After discussing what I was up to with John, including finding out that there are two different 3-syncro gearbox layshafts, a 2-hole and a 3-hole version, Mac and I loaded up all the parts and drove up to see John to get an assessment and buy the other needed parts.

Quantum Mechanics does hundreds of gearbox rebuilds every year
John Esposito starts his examination

John’s shop is filled floor to ceiling with gearboxes and gearbox parts. If you need it, he is likely to have it. He and the team at Quantum Mechanics rebuild a few hundred gearboxes a year and they are always busy.

John examined each part. One thing I was concerned with was a very rough casting near the bearing side. It looked like a flaw in the casting to me, but John explained it was just casting flash and showed me three other gearbox cases that were all but identical.

He went through each part, making notes. I was happy to hear that I had purchased the correct layshaft. That was a relief. John took extra time examining every part of the overdrive unit and pronounced it good, including the overdrive clutch, which he said looked brand new. The owner I bought the car from had not destroyed it and it still looked as good as when John Twist rebuilt it. The planetary gears, bearings and even the overdrive clutch were all in great shape. This was a relief as replacing those overdrive parts alone can cost as much as the rebuild cost for an entire gearbox.

Per John, we ended up buying a new first gear, all the bearings for the gearbox, seals, gaskets, a second-gear steel syncro, and a reverse gear shaft. The total bill was around $500. 

This week we will be taking all of these parts and reassembling the gearbox. I am still worried, but Mac promises, ”The scene is safe,” so I am confident we can do it.

Andy Reid
Andy Reid
Andy Reid's first car, purchased at age 15, was a 1968 Fiat 124 coupe. His second, obtained by spending his college savings fund, was a 1966 Ferrari 330 GT 2+2. Since then, he has owned more than 150 cars—none of them normal or reasonable—as well as numerous classic motorcycles and scooters. A veteran of film, television, advertising and helping to launch a few Internet-based companies, Reid was a columnist for Classic Motorsports magazine for 12 years and has written for several other publications. He is considered an expert in European sports and luxury cars and is a respected concours judge. He lives in Canton, Connecticut.



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