Beck TD, Part 37 - A Temporary Speedometer

Back in Part 32, I told the tale of how I installed a "temporary" Auto Meter tachometer to replace Beck's non-working unit. Now Beck has a matching "temporary" speedometer!

This project has references all over this blog. In addition to the Part 32 tachometer post, it was way back in Part 17 that I got the original speedometer working via a custom cable that matched the Volvo transmission to the MG speedometer. That worked well until Part 34, when Cor Engelen and Troy Nace helped me upgrade the rear gear to a 3.9 gearset from an MGB. That made the speedometer wildly inaccurate, so Phil Oles agreed to help me make a gearbox to correct it.

Before Phil and I could even get started, the 66-year-old speedometer decided that enough was enough. I started hearing a noise that at first I thought was the starter Bendix dragging. I later realized it was emanating from the speedometer, accompanied by wild jumps of the needle. I unhooked the speedometer cable before the speedometer could destroy itself, and removed the speedo from the dash.

I made a little chart to help me know my speed, based on engine RPM. I used a gear calculator to do those calculations, one that I first described in Part 4, when I first was researching that rear gear change.  I taped the chart to the dashboard so I could refer to it while driving.

That worked, but it was pretty cumbersome. I started researching the Auto Meter Sport Comp speedometer that matched the existing tachometer. It seemed promising. First, I needed to see if it would read correctly, or if Phil and I would still need to make the gearbox to correct it. The specs told me that the speedometer reads 60 miles per hour when the cable was spinning at 1000 RPM. 

I asked my buddies at Make717, our local Maker's group, if they had a good way to measure RPM at the cable. They told me that a laser tachometer was just the ticket, and I had my choice of two, since both the makerspace and Phil had one. All I needed was a way to mount a reflective sensor. I made a round widget with a square hole that matched the cable. The black line is what I hoped would trigger the laser tach, and the reflective tape is what actually did.

I recruited Wes Bischel from Make717 to help with the testing. It was chilly and Beck has no top, no side curtains and no heater, but Wes was hearty! Here he is holding the laser tach, surrounded by good gear at Make717:

Using the gear calculator, I determined that Beck's tachometer should read 3092 RPM at 60 MPH. That's essentially impossible to read on an analog tach, but I figured 3100 RPM was plenty close enough for this test. We were hoping for 1000 RPM at the speedometer cable. We got out on the highway, and held the tach at 3100 and Wes took a reading. 991 RPM! That is an astonishing coincidence, because the 3.9 gear in Beck's rear axle is not the same as the Volvo transmission was designed to use. Somehow I got really lucky.

So, on to the next step: hooking the cable to the speedometer. There were four pertinent data points:

1. The Auto Meter speedometer had a male fitting threaded 5/8-18.
2. The Auto Meter expected a square cable end, sized 0.104".
3. The MG speedometer (and therefore my custom cable from Part 17) had a male fitting threaded metric 12mm x 1.0.
4. The MG expected a square cable end, sized 0.120".

Now, by far the easiest thing to do would be to contact the guy from Part 17, and have him make another custom cable with the correct end for the Auto Meter. But where's the fun in that? Besides, that would cost another 70 bucks, and I'm cheap. I set out to make an adapter. I needed a housing to match the cable thread to the Auto Meter thread, and a driveshaft that would accept the 0.120" square, and terminate in a 0.104" square.

Making the driveshaft, barely an inch long, was the hard part. In fact, making the square hole to accept the MG cable was complicated enough that I split that part into a separate blog post. If you're curious how I did it, click here.  I turned a reasonable part for the housing, adding a Delrin liner to the part that the driveshaft would spin in, to reduce friction.

Now I just had to add internal threads, 5/8-18, to the hole, and external thread, 12mm x 1.0, to the narrow end. I visited the ever-useful Phil Oles, who reached into his toolbox and extracted a 5/8-18 tap. That made quick work of the inner threads.

The external threads were harder, because 12mm X 1.0 is not a standard size. Phil used his lathe, set up for metric threading, to create the threads using a single-point cutter.

Phil and I were eager to try it, and the threaded parts worked perfectly. However, we learned that there was a fatal flaw in the driveshaft I made (as detailed in the post linked above). It took three more tries, but today I got the driveshaft done. Here are two views of the tiny shaft, showing the 0.120" hole and the 0.104" square drive, and two views of the shaft in the housing.

And here's the housing mounted on the speedometer:

Early in the project, I made a matching set of mounting hardware, as detailed in Part 32. It was ready to go:

All that was left was to install everything and give it a try. I put Beck on my lift so I could spin the wheels, and got this result:

If you refer to the dashboard chart pictured earlier in the post, you'll see that 2300 RPM in 4th gear equaled 45 MPH. I held the tach at 2300 RPM, and that sure looks like 45 MPH to me. As Edd China used to say on Wheeler Dealers, "Result!"

So let's take stock here. I have a high-quality "temporary" tach that is highly accurate, as verified by comparing with the digital tach built into my timing light. And, I have a high-quality "temporary" speedometer that is mathematically accurate based on gear calculations. Maybe someday I'll spend $$$ to have the stock instruments refurbished and modified to work with my modified setup, but... this is feeling sort of "permanent" to me. The MG tach and speedo are carefully packed and stored. I may just leave them for the next guy!

Continue on to Part 38...