Beck TD, Part 48 - Understanding My Transmission

I've undertaken a big project, one that has intimidated me forever: rebuilding a transmission. When I installed the new-to-me Volvo B18 engine in Beck TD, I installed a used trans at the same time. My existing transmission had what I considered unacceptable wear at the front bearing. As it turns out, the replacement trans also has a problem. I decided it was time to get over my fear...


I've spent an inordinate amount of time lately just learning how transmissions work, and in particular, how the M40 Volvo transmission is arranged. I found two YouTube videos that were invaluable in that process, from an British company called Amazon Cars Ltd. "Amazon" refers to a particular model of Volvo. This two part set details disassembling and rebuilding the M40 trans:

teardown: https://youtu.be/7b3XCJqw-Tk

rebuild: https://youtu.be/d6JbNWbFP6c

These videos are incredibly detailed, and I have watched both multiple times. This post won't duplicate all that, but rather record what I've learned in the process, so I can review it in the future and quickly refresh my memory on how the build goes. I'll start with the obligatory "All Apart" picture, showing just how far I've disassembled the transmission.

This is not just vanity - this photo has some important info for me when I reassemble - so much so, let's have a few close-ups. All the gears and components are arranged around the shiny shaft with the gear in the middle. That's the "main shaft" and the gear is permanently made as part of the shaft. Each component around it is in order, and the "down" face is to be rotated so that it goes toward the center of the shaft. First, the input side:

And the output side:

The selector shafts interact with the gearchange lever, and select the gears. These are arranged in order, with the components in the correct orientation:

Incidentally, the grub screws that attach those components are very tight, and the video details an expensive screwdriver with a built-in hex so that a box-end wrench can be used to tighten and loosen. Since it's a British video, he called it a "ring spanner." Anyway, I made my own out of hex stock, precisely sized in thickness and width to match the screws:

UPDATE: The driver above did work, but since I didn't have the means to harden and temper it, the tip twisted under the torque. I wound up grinding an old screwdriver to appropriate thickness and width to tighten the grub screws. END UPDATE

There was another special tool needed. There is a huge snap ring in the center of the main shaft, and it must be removed to fully rebuild the transmission. The videos above call it a "fearsome old brute." In other research, I found commentators calling it the "J****" clip (name omitted to avoid offending via blasphemy), because of the difficulty of dealing with it. Jake R. on the Volvo-Engined MG Group tells me it's called that because that's what you yell when it flies off the pliers across the shop or hits you in the eye!

Anyway, it turns out the tool of choice is the Snap-on SRP4 pliers. My first look somehow found a site that wanted $500 for those pliers, but later I did figure out you can get them for about 70 bucks in the US. But Jake found them for me on eBay, used, for $25 plus shipping. Sold! As you can see, they are very large pliers:

The key is that the pliers have an indentation in the tip that matches the angled tip of that snap ring (left). It fit perfectly (right), but took on a slight twist, probably due to my ham-handedness. Jake assures me that it will probably be fine, because the ring snaps into a groove that will straighten it.

I started this exploration with almost no real understanding of how a transmission does its work internally. I had seen those big gears, of course, and assumed they move around somehow. However, that's not it at all. The large gears on the main shaft and on the secondary layshaft are in constant mesh, but the main shaft gears can mainly "float" without driving anything. The exception is the two straight-cut gears at the center. They provide reverse gear, and require a secondary gear to reverse the rotation. That gear is mounted on the side of the transmission case.

When someone says a trans has "bad synchros" he is usually talking about these brass thingies:

Those are more properly known as "synchronizer cones," and they are just part of the synchronizer assembly. The entire synchronizer encompasses this assembly. I really should have called the top thing the "synchronizer ring" in this annotated photo, and that's how I'll refer to it in the text.

I was surprised to learn that, when  you shift gears, all that moves is the synchronizer ring, and it only moves about half an inch in each direction. This photo shows how the synchro gear fits inside the larger synchro ring:

Those small teeth in the outer ring are what mesh to connect the engine to each gear. All the torque of the engine passes through those small teeth! However, it helps that there are a lot of them, so even transmissions like the famous M-22 "Rock Crusher" transmission from the early Corvettes have these smallish teeth.

In the photo above, you're looking at the gear on the end of the input shaft, and the mating synchro ring for the 3rd-4th gear synchronizer. That's what matches when the shift is made - the synchronizer ring slides over the teeth on the input shaft when selecting 4th gear, making a "straight through" connection to the output. The brass synchronizer cone helps the gears align and spin at identical speeds, so that the gears don't grind.

When the synchronizer ring slides, it uncovers gear teeth in the ring, and as the gear speeds synchronize (helped by the brass cone) it slips into place on the matching teeth of the gear being selected. In this photo, I simulated that on the left by shimming the ring to uncover teeth, and then on the right slipped the input shaft into place, showing the mesh:

But how does the synchronizer move? It is slid along by "shift forks" that are engaged by the shift lever. It took me quite a while to be able to visualize this. Here are a couple of drawings I made to try to help. The key is that there's a pivot point on the shift lever, and therefore the action in the transmission is the opposite direction from the way you're moving the lever.

The transmission end of that lever engages a slotted mechanism in three shift rods. The middle rod is 1st and 2nd gears, the leftmost handles 3rd and 4th, and the rightmost handles reverse. I made this visual representation because I kept getting it wrong when trying to test! Expand to read the legends, and pay careful attention to the orientation of the notched receivers at the right. The front of the transmission is to the left.

Just a few more photos to remind myself of things I can get wrong - pieces that have a specific orientation, which can be installed the wrong way around. During reassembly, I realized I had some of these wrong. See red corrections.

The 3rd-4th synchronizer ring has a groove that faces the center of the transmission:

The matching 3rd-4th synchro gear has an extended boss on one side that faces the center also the front of the transmission.

The dogs on the 1st-2nd synchronizer are slightly assymetrical. The longer side goes to the rear front of the transmission.

The bearing housing at the rear also holds a spiral gear for the speedometer drive. It has a machined shoulder at the top, and I believe it goes to the rear of the transmission. If that is wrong, somebody tell me! Nobody told me, but I was wrong! It goes toward the front of the transmission.

This has been a very interesting learning experience, but the proof is in the pudding. I've ordered parts, and when they arrive I'll renew the wear items in this transmission, reassemble it, and see if it works. Wish me luck!

Continue on to Part 49...