M40/M41 Transmission: Rebuilding
by Phil Singher

Restoring an M40 or M41 Volvo transmission to perfect working order is not nearly as intimidating a task as it first appears. You will need a few tools beyond what's in a typical basic kit and a certain amount of organization is required, but the only thing most of us won't be able to do at home is pressing in new main bearings. The total cost of tools, parts and pressing is far less than most shops will charge for a rebuild of uncertain quality.

We are not covering rebuilding an OverDrive unit in this article; that's a separate subject for another time. Once the OD unit is detached, the M41 rebuilds almost just like the M40. We'll note the few differences for you as we go. [M40-only procedures are contained in brackets.] M41-only procedures are denoted by italics.

You will need:


Parts: what wears out?

Beyond that, clean and inspect everything that comes out of the gearbox, particularly the polished surfaces on steel parts. These are "case hardened," meaning they received a heat treatment when they were made that toughened the outside of the part -- but only a thin layer. Perceptible wear on those surfaces means the soft underlying metal is exposed (or about to be exposed). You likely won't encounter this -- it's more in the "damage" category than "wear," but look for it on bearing surfaces, rollers, etc. Don't even think of reusing such damaged parts.

Taking it apart:
Before we do this, drain any oil out of the gearbox (and OD). Clean off the large amount of dirt that's probably coating the unit. M41 only: Undo the eight nuts where the silver OD unit bolts to the red gearbox adapter and separate the two. Mop up the mess where a bunch more oil just gushed out of the OD. Wrap the OD in clean newspaper so dirt can't get inside and put it away somewhere until the end of this article. Place what's left on your clean, tidy work area.

Again, what follows might seen like an excessively long list, but take my word for it: once you have your tools and parts in place and have had a little practice, this can be done in a few hours. If this is your first time, allot a liesurely weekend to the process. It really can be done.

First, we're going to take it apart about this far:

Here we go.

Take a break! If you've never done this before, this has taken a while and you've accomplished a lot. If you've done this a number of times, this has taken you about half an hour, so you have time to spare.

At this point, you are in a position to evaluate the condition of the main bearings, even though you can hardly see the rear one in its housing -- do they spin without notchiness? Is there play in them? Also clean and inspect all the rollers and their bearing surfaces inside the input shaft and countershaft, its spindle and the forward tip of the mainshaft.

I presume you took this thing apart this far for some reason other than that you like taking things apart; if it was because of noisy running, you should now be able to locate the cause. In any case, don't despair. We're not done taking things apart quite yet.

Mainshaft disassembly:

You should now have something like this (except with the main bearings loose and no labels):

Unless there's some damage to the reverse gear idler still in the case or the sliding plate in the top cover, that's as far as we're going to take this apart. In fact, there's nothing much left to take apart. Clean up all the parts that aren't already and inspect for wear / damage carefully. If you'd like to paint the case, have it pinstriped, media-blasted, powder-coated or whatever else it is you like to do to iron parts to show your Volvo how much you love it, now's the time.

Putting it back together:
First, we'll reassemble the mainshaft. Put everything together with a light coating of oil between the parts (whatever oil you'll use in the finished tranny is fine).

Decision point: realistic thinking required here! The next step is to press the new main bearings; you might be able to do this yourself or you might have it done for you by a shop with more equipment (there is no shame in this). It is possible to install the rear bearing in its housing with a large vise and it's also possible to pull the front bearing onto the input shaft, but it's crucial to apply force only to the outer race of the rear bearing and the inner race of the front bearing, so you'll need suitable arbors in between your vise / puller / press and the bearings. You don't want to damage your pricey new parts by doing otherwise, do you? I know for sure that you would never even dream of hammering on them, aren't I right?

Now, we need to load everything back into the case. The first step is to make the dummy countershaft spindle -- this will hold the roller bearings and spacers in the countershaft while we put it through some gyrations. Your life will be much easier if you get something that's just the right diameter. You need to cut it to the exact length of the countershaft or a fraction less (not the length of the "real" spindle!). The places where we're going to apply grease should not have oil on them -- they must be clean and dry or the grease won't stick.

M41 people: The OD just slides on. Make sure you align the oil pump ring with the eccentric. Use a new gasket and tighten the eight nuts evenly in small stages.

A few installation notes:

  1. If the transmission came out of a salvage yard or otherwise acquired external rust, look at the input shaft. The tip will have to slide into the pilot bearing on the car and the splined part through the clutch disk; it might be nice to clean it up a bit.
  2. Look at the cylindrical surface of the front bearing cover. The clutch throw-out bearing is going to slide back and forth on that. I would make sure it's pretty shiny.
  3. [M40 only: There should be a small oil seal in the outer tip of the speedo drive fitting. This falls out easily until the speedo cable is hooked up. I recommend you save installing this until the tranny is bolted into the car. But remember it!]
  4. If you have the short cover (for the long shifter), it's easy to fill the tranny with oil through the shifter hole. If you have the extended cover, it's easiest to fill the tranny before putting the top cover on. Volvo recommends 90W gear oil for the M40 and 30W motor oil for the M41.
  5. Brooks Townes notes that the plastic bushing at the base of the short shifter in the extended cover can cause all sorts of problems when worn out, including popping out of gear. It is inexpensive and easy to change out. Take a look at yours, if that what you've got.

Well, that's about it, folks! We're done