Project Mini Cooper S | Magazine Series Part 5: Rebuilding Our Mini Cooper’s Transmission

Update by Tim Suddard to the Austin Mini Cooper S project car
Sep 16, 2020

[Editor's Note: This article originally ran in the January 2014 issue of Classic Motorsports; for more updates on our 1967 Mini Cooper S, visit here.]

This may not sound ideal today, but at the time it was seen as a way to save space.

More than 60 years later, the design actually still works quite well. The Mini’s ubiquitous A-series engine backed by the four-speed transaxle has proved to make a reliable drivetrain.

Like the rest of our 1967 Mini Cooper S, our transmission was disassembled when we bought the car. With the help of the shop manual, we figured out how to piece it back together. Surprisingly, we determined that we weren’t missing any pieces.

The transmission still needed to be rebuilt, and Mini Mania fielded our many questions. Finally, company owner Don Racine just asked us to ship them the transmission. They rebuild several a week and could easily handle the job.

We came up with a compromise, though: We’d ship the transmission to them and then fly out to assist with the rebuild. That way, the job would be properly handled as well as fully captured for our story.

Wands, Remotes and Rods

While the Mini’s basic profile didn’t change during the model’s 40-odd-year run, three distinctive shifters were used–and each one came paired with its own transmission.

First up was the magic wand style of transmission and shifter that dates back to 1959, as it was standard on the early cars. This one is easily identifiable by its long shifter jutting from below the dash. It’s also the least sporty shifter fitted to the Mini.

The remote shifter appeared late in 1961, originally offered in the 997cc Cooper, while the magic wand could still be found stirring the bread-and-butter models. The remote-shifter transmission uses a structurally mounted aluminum housing to enclose the shifter and act as the mounting point for the rear of the shifter mechanism. While technically no more precise, many enthusiasts prefer the remote shifter over the magic wand.

Like the magic wand shifter, the remote shifter was also connected to a gearbox that featured a non-synchro first gear. All-synchro gearboxes were phased in starting in September 1968. A year later, they became standard across the board.

In 1973, the rod shift gearbox debuted. Some say that shifting feel again improved, but like with many things, there is some debate.

Standard Operating Procedure

While our Mini came to us in boxes, fortunately the right transmission was included: casting number 22G333. This Cooper S box has closer ratios than the standard units and is physically stronger. It’s also a bit difficult to find these days, as demand outstrips supply.

Rebuilding a Mini transmission is like rebuilding any other transmission. First you disassemble it, and then you inspect it for problem areas. Take lots of pictures along the way so you’ll have a guide for putting everything back together.

If what you see scares you, you can box it all up and send it to Mini Mania. They build transmissions nearly every day, and most jobs cost $800 to $900. Changing the final drive will add another $300 or so to the bill.

We did the rebuild at Mini Mania, a longtime supporter of the marque. Company owner Don Racine inspected each gear with a magnifying glass before determining which ones he could reuse.

Our Mini Cooper S came to us disassembled, and its remote shifter transmission was no exception. Before we rebuilt it, we cleaned the pieces and ensured that everything was present. Surprise, surprise, we had all of the major parts.

Four different transmission castings were used on these early Cooper S models. Our transmission sports casting number 22G333, which is correct for our car.

Fortunately, we didn’t need to source a replacement transmission. If you’re searching the swap meets for a Mini transmission, know the three basic types used on the car. You can identify them by their matching shifter mechanisms. The early magic wand transmission can be seen on the left, while our remote-style gearbox is the one in the center. The later, rod type is on the right.

Never rebuild a Mini transmission without installing new baulk rings–also known as synchro rings–because their mating cone surfaces are typically chipped or worn. A damaged synchro ring won’t properly slow down the gear; as a result, one gear will crash into the next. Rule of thumb: You should be able to measure at least 0.030 inch between the ring and the face of the gearing engagement teeth. Mini Mania sells new Rover OE rings for around $50 each. Aftermarket ones go for around $20 each.

When inspecting a gear, take a very close look at the teeth–especially the small ones since they take a lot of abuse when shifting gears. These small engagement teeth are very critical, and if they’re worn, the transmission will pop out of gear. New gears will have very pointed ends, while the leading and trailing edges will each have a distinct profile. As the gears are abused over the years, the pointed ends wear away and the ramp angles change.

While you have your transmission apart, install a competition oil pickup. The stock pickup only grabs oil from the right side of the pan, meaning it can starve during right-hand turns. A competition-style pickup takes oil from the center of the pan, eliminating the chance of starvation. Our car came with a new Leyland piece, while Mini Mania sells reproductions for around $80.

In addition to the standard rebuild, we made one big change to our gearbox. While steeper ratios were optional, the standard final drive ratio on a 1967 Cooper S like ours was 3.44:1. Since we have slightly boosted power, we figured that our engine could handle a slightly more relaxed final drive. The goal here is to make the car a bit less nervous on the highway.

The folks at Mini Mania highly recommended the 3.10:1 final drive. They keep these gears on hand and install them in lots of gearboxes. This ratio would provide much better cruising at highway speeds by dropping the engine speed by between 600 and 800 rpm.

This modification is relatively simple to make, too, as just some slight machining is required on the final drive gear retainer. In addition to the new final drive, you’ll also need the correct speedometer gear. We found ours at Heritage Garage.

Replacing the laygear bearings is quick and easy, but be sure to inspect the old ones first. If you spot any degradation, the inside of the laygear may well be in trouble. Since inspecting the inside of the laygear is a bit challenging, inspecting the laygear bearings is an easy first step.

The Mini’s differential features two brass thrust washers, one on each side of the pinion gears. Over the years, these thrust washers take a lot of abuse, and very rarely are they reused. Inspect the thrust washer housings, too. If the thrust washers are excessively worn, the housings may also need to be replaced.

The shifting knuckle found at the bottom of the remote shift housing is also a typical wear point that must be inspected.

All early remote gearboxes suffer from very sloppy shifting, and there’s one typical cause. When a driver continues to hold and even lean on the gearshift lever while changing gears, that side load is continually applied to the brass shift forks. The more this happens, the wider the resulting gap between the shifting hub and the fork becomes. Here’s what some gross wear looks like, and the end result is some very imprecise shifting.

Mini Mania keeps a huge selection of new and used gearbox parts on hand. Mini Mania’s master engine and transmission builder, Tony Gambirasi, showed us just some of their selection.

Remote Control

While we had everything apart, we also rebuilt the remote shifter. Fortunately, this job is also rather straightforward. We disassembled the assembly, cleaned and lubricated everything, and then reassembled it with new rubber plugs sourced from Mini Mania.

Don Racine gave us a good tip for this part of the process: Check the end of the remote shifter fork that attaches to the shift knuckle. This is a common wear area, and tired equipment keeps the gearbox from shifting smoothly. Our remote shifter fork was worn, so we replaced it.

To encourage some slightly more relaxed cruising, we decided to replace our original 3.44:1-final-drive gear set with a 3.10:1 final drive. The 3.10:1 final drive’s pinion gear is larger than our original cog.

In fact, the larger pinion gear will no longer fit into the retainer.

To provide the necessary clearance, Tony slightly ground the retainer. Now the pinion gear won’t bind when installed.

There’s a lot of moving parts inside a Mini transmission, but the final, assembled unit is surprisingly compact.

Shifting Gears

Our Mini’s drivetrain is pretty much done, and the body is nice and solid. Several areas still need plenty of work, though, including the subframes, suspension and brakes. We will start covering these systems in our next chapter.