Beck TD, Part 68: A New Speedometer and Tachometer

 Beck TD has had an interesting variety of instruments in the dash since it came to live at the Grant Street Garage in July, 2017. When Beck arrived, it had the original MG TD instruments, consisting of speedometer, tachometer, a dual water temp/oil pressure gauge and an ammeter. The small gauges worked, kind of, but the speedo and tach did not. But they had that gorgeous old look - in 1952, MG was still using the domed glass:

Restoring those instruments is north of $1000, but I still have them - we'll see if I ever win the lottery. In 2018, Part 32 and Part 37 of this blog detailed converting to "temporary" instruments from Auto Meter that had the very great benefit of working correctly.

That setup worked well for two years, but just at the end of the 2020 driving season, the speedometer adapter that I had laboriously machined went "flaky" - sometimes worked, sometimes didn't. In addition, I now have in my possession a 4.3 ratio rear end to replace the 3.9 rear end we added in Part 34, and the M41 overdrive trans I completed rebuilding in Part 57, both of which will render the Auto Meter speedometer hopelessly inaccurate. 

What I needed was a speedometer that used the modern "speed sensor" instead of the old-fashioned cable hookup. At first, I was going to buy the Auto Meter 3-3/8" electronic speedometer that looked just like my old one, plus their speed sensor, for about $350. But then I found the company Marshall Instruments, which has made-in-USA, 5" instruments to match the size of the original MG instruments, and I was able to get both speedometer AND speed sensor AND tachometer for that same $350! The instruments use precise stepper motor technology to ensure that they are accurate at all parts of the dial. Sold. This is what I bought - the speedo is part number 2054 and the tach is 2053.

Yes, I know that Beck TD will never see the second half of those dials, but it matches the retro-racer vibe. Because I wasn't at all sure I could make the speed sensor thing work, I bought just that at first from Amazon for $41 - it's part number 9220. This is the GM-style sensor. The rest of this post tells what I did with it, but you should also know that there's also a GPS sensor available that directly drives the speedometer and never needs calibration - waaaay easier!

If you look at the business end, you see that same square axle that speedometer cables have, in the industry-standard 0.104" size. I have no idea why they couldn't have just done 0.100" - it's not a standard metric size either. Anyway, when you spin that, the sensor sends out a series of pulses, the frequency of which depends on speed. This one matches another industry standard: 1000 RPM = 60 MPH, and at that speed it matches yet another standard: 16,000 pulses = 60 MPH.

Here's what the Volvo parts look like. The bit with the gear is called the "pinion" and you can see the indentations in the hollow shaft to make a square end that accepts that 0.104" driveshaft. That gear goes into the "pinion housing" and is terminated by the black rubber "pinion seal."

In a perfect world, the sensor would simply thread onto the pinion housing, and I would be done. Not so easy, however. The sensor is threaded to accept Unified threads in 7/8-18 (which is even finer than "fine thread") and the Volvo housing is metric, M18-1.5. My friend Jake in California spent a lot of time advising me and measuring his Volvo parts to double-check my measurements. At first, I was thinking I would try to duplicate the entire pinion housing with 7/8-18 threads, but I decided to try this first - an adapter. Yes, it's 7/8-18 on the male end and M18-1.5 on the female side.

This is what you get when you put it all together:

And here's what it looks like installed in the M40 transmission. Incidentally, that plug and pigtail are included with the sensor from Marshall.

The rest was pretty easy. The wiring I had previously done for the Auto Meter gauges was right for the Marshall ones too, although I had to add a couple more connections. Of course, I had to make a long cable from that pigtail to the speedometer, but that was straightforward too. The only other fabrication was a tiny bracket to hide the reset switch for the two built-in trip odometers.

And it all works! The car is still significantly apart (you can tell by the clear view that there's no steering wheel, for instance) but since I have the driveshaft out, I was able to "drive" and see both instruments in action.

There's one more step: calibration. Basically, I will put the speedometer in calibration mode, drive an exact mile as measured by the mile markers on a divided highway, and use the reset button to say "done." It is easy to verify using a GPS unit with speedometer, and redo if needed. When I change to a different rear gear or transmission pinion ratio, just redo the calibration, no wrenches necessary.

So, if you are a casual reader of this blog, you're done! The rest of this post is for the machinists in the crowd - my work log for making the adapter, in case I ever want or need to make another. 

Read on, or continue on to Part 69....

1. Starting with a 2" long piece of 1" diameter aluminum (this is 7000 alloy), plow a wide groove for threading tool clearance, 1/2" from the end, to a depth of 0.120".

2. Move down another 1/2 inch, and plow a narrow groove to define the eventual hex, to a depth of 0.140".

3. Drill 3/8" to approximately 1-1/2" deep - enough to clear the second groove.

4. Turn the first 1/2" segment to a diameter of 7/8" = 0.875".

5. Set up the lathe to thread at 18 TPI, and install a hard stop (not pictured) to ensure you can return to the same starting point without drama. I threaded left-to-right in reverse rotation, which required the tool (a standard "E" 60-degree carbide insert tool) to be mounted upside down. The tables claim the compound infeed will be 0.034", and that will be close.

6. Sneak up on the final fit, to the point that the sensor will completely thread onto the adapter.

7. To minimize, but not eliminate the gap between the sensor and the hex part of the adapter, I also trimmed the threads to get a good fit.

8. Once the threading is done, reverse the adapter, gripping by the eventual hex portion, and turn the remaining stub to 0.750" to fit an ER32 collet.

9. I would normally have cut that hex in my large milling machine using a spin indexer, but I was snowed out of my workshop. I did it at home on the little Sherline milling machine, using a hex collet block.

10. Once the hex is done, return it to the lathe, and part off the stub that was used in the ER32 collet.

11. I used a 9/16" end mill in the drill chuck to define a flat-bottomed pocket, 0.480" deep. The flat bottom is important for the pinion seal to seat against.

12. Using a boring bar, increase the diameter of the pocket to 16.5 mm, the tap drill size for M18 thread.

17. The final step is to tap the adapter M18-1.5, but there's an important note. The threads need to extend ALL the way to the bottom so that the pinion housing can seat and compress the pinion seal against the face of the adapter. To accomplish that, I bought a pair of M18-1.5 taps from Amazon. The set included a taper tap and a plug tap for only $16, and I carefully ground the taper tap to a flat bottoming tap. I held the tap in a square collet block to keep it from tilting, and I used a vertical belt sander for grinding, cooling frequently in water. Incidentally, the color difference in the photo below is because the right hand photo was taken with the flash on. Left and right are the same two taps.

18. Threading big taps like this takes a lot of torque, so I had to wait until I could get back into Grant Street to use the Logan lathe for that. I used a 3/8" steel rod to align the adapter while tightening the chuck to keep it square, and then started the threads with the M18 tap in the drill chuck. Once started, I locked the lathe by engaging back gear, and used a wrench with a extender handle to turn it. Oddly, the wrench that fit the square on that tap was a Whitworth size - a good homage to the MG heritage, I guess.

And that's it! I could probably make one from scratch in a couple of hours including machine setup time, or could make multiples in about an hour and a half each. 

Continue on to Part 69....