Beck TD, Part 59: An M41 Shifter

I'm writing this on April 14th, 2020 - almost exactly a month into the COVID-19 quarantine. It was Friday, March 13 when we decided to cancel our Sunday worship services at Lancaster Church of the Brethren, where I work. By the next week, we were doing online church, and the building was basically closed. What a change! All those changes have kept me away from the Grant St. Garage, because I was the person with the tech skills to get us online. It has been a wild four weeks!

But... as I came to understand some things about video production, audio editing and live streaming, the pressure has eased a bit and I made it back to the shop this afternoon. The project of the day: the shifter for the M41 overdrive transmission I laboriously rebuilt beginning in Part 51, extending for multiple posts. When I got that rusty, seized transmission, it came with a coveted remote shifter, but it was totally broken - something heavy had fallen on it and destroyed it. The upper photo is of the remote shifter currently installed in Beck TD, and the lower is of the broken shift extension from the one I rebuild. In addition to that irreparable break, the entire top plate of the shifter had warped and cracked. I needed a new shifter!

If you expand the photo above, you'll see the annotations on the top picture. There are two switches at the front of the shifter. The driver's side one is for the overdrive - it ensures that it is engaged only in fourth gear. That was a Volvo decision, because other manufacturers would allow operation in 2nd and 3rd as well. The important thing that switch does is to ensure that the overdrive is never engaged in reverse, while would damage the overdrive unit immediately. The second switch is for the reverse light, and different Volvos had that in different places.

Earlier shifters didn't have those switches, but many of them had cast-in "bosses" in the spot where the switch should go. I had an earlier shifter, but it was so old that it didn't have the bosses, so I asked Joe Lazenby to source me a shifter that had the right boss. I specified the long-levered non-remote shifter, because they are cheaper and I have this crazy idea that I'm going to build a rally-style PV544 someday. Anyway, here's what I got - no threaded hole for a switch, but the location clearly defined by the boss, and enough "meat" to make several threads.

I did some research, and found that was one odd thread - M16 x 2.0, which is a metric thread, but not a typical pitch (the second number). It's common enough that a tap was readily available on Amazon. Oddly, two taps were only $2 more than one, so I even have a spare now!

Of course, I didn't even think about the drill bit necessary to tap that hole. 16 millimeters is quite a bit more than half an inch, and all my drill sets max out at 1/2". Another oddity - the specified tap drill size for M16 X 2.0 is not metric, but another oddball size - 35/64". I found that on Amazon too.

Today, I had time to give it a try. First things first - tap a test hole in scrap, and see if this size is even right for that overdrive switch. It is!

Next up - precisely align the hole in the new-to-me, relatively expensive shifter. I turned a close-fitting plug on the lathe, and used it to precisely align the milling machine table centered on the hole. Note that I'm using the large machinist's jack I made in this post to hold the far end of the plate.

Once aligned, I used successively larger drill bits to drill out the boss. This is the final 35/64" bit - maybe the only time you'll ever see one!

After that, it was time to tap. One more learning experience - my tap set only went to 1/2" also, so a new, larger tap wouldn't fit in my tap wrench! I used a pointed pilot to align the tap, and turned it with a wrench. Big holes like this are hard to tap, so after it was started I had to use an extension on my wrench for leverage.

And... the switch fits!

Time for a test - I reassembled the shifter, and installed it on the transmission. It shifted easily in to the gears, and then I attached a multimeter to the terminals on the switch. Sure enough, when I put the trans into fourth gear, the switch closed and I had continuity.

Friends, this was marvelous news, because it meant that work I did way back in Part 51 was done correctly. Back then, I needed to locate and precisely drill an angular hole in the 3-4 shift rod, to locate a tab to actuate that switch. Here's a photo from that post:

The rear shaft is the original, which I had to cut in two to get out because the trans was so rusty. The parallax error of the camera makes it hard to see, but I did a lot of careful work to ensure that hole was in the right place. It is there to locate the tab in the photo below - as the annotation notes, it's what actuates that overdrive switch.

Today's work proved that I got that right, and I now have the correct shifter to use the overdrive safely. Result! There's only one more thing to procure, and that is the correct speedometer gear. Different Volvos have different gears, and I need the same one I currently have in Beck:

Volvo color-codes those gears, so it has to be that precise color. There's a darker yellow one that is totally wrong. This has 16 teeth, and the teeth have to "lean" that direction. I can actually buy a new one at this link, but it would be north of $60 with shipping. I'm hoping for a cheap used one! If you have one for sale, let me know. Or... if you have one you're willing to let me measure, maybe I could duplicate it in brass and make my own.

Continue on to Part 60....