Trip Odometer How to Add

[oldgold70c10]

This is a guide on how to add a trip odometer that looks factory to your stock 67-72 spedometer:


I did this conversion twice. First in a speedo out of a 1967 truck cluster I had, and again to the stock 1970 speedo out of my DD after seeing how far off the 67 speedo was from calibration. I noticed some differences between the two, as you'll see, and having the older speedo actually helped the conversion a bit.

To do this conversion, you will need:
-A donor trip odometer speedometer From what I can determine looking at repro parts availiable, 68-77 Corvette trip odometer speedos or 60's and maybe early 70's Impala or other full-size GM cars with a trip odometer built into the speedo. I used a 68-77 Corvette Speedo purchased off ebay.

-64 late to 67 Corvette trip odometer reset cable I bought mine here.

-Your stock 67-72 speedo assembly

There are alternative substitutes for some parts, and these will be pointed out as needed.


Let's Begin.

Disassembly
Remove your gauge cluster from the truck and take out the 8 screws that hold the outer tin to the plastic bezel. Remove the speedometer by removing the 2 screws that hold it to the rear tin. You may want to strip everything off the rear tin at this point, because you will be drilling it.

Remove the speedo hand by twisting it counter-clockwise past 0 and gently pulling. It is a friction fit, but simply pulling it straight may damage the hand.

Remove the 2 screws that hold on the face.

Remove the odometer reel. Do this by pulling the retainer clip at the left side, slide the reel left, lift and move it to the right. Set it aside.

Remove the light baffle from the speed bridge by pulling it straight up, as shown below.



Modifications to the Frame Assembly

At this point look at your speedo frame and look at the picture below.



On the right side of the frame, their should be a hole in the side 0.140"-0.145" in diameter. If your frame lacks this hole as the 67 frame did, drill one 0.815" from the edge of the casting as shown.

Next, let's address the odometer "key". Since the regular odometer will now go on top, you'll have to add one there. This is really the only part of the modification where a donor speedo is mandatory, as all the other parts are available as reproductions. The keys are riveted to the center frame as shown below.



The bottom key was already in place on this 67 speedo. The top key was the trip odometer key removed from my 68-77 Corvette donor speedo. Note that neither one are the tamper-proof style. The tamper-proof style will have a portion cut out of the key so that a fragile plastic clip will have to be used to keep the tabs on the odometer reel in line. The top & bottom keys of the non-tamper-proof style are the same. The top & bottom tamper-proof keys are backwards of each other in shape. A source of the top tamper-proof key is slide-rule style speedos where the pivot of the speedo hand is hidden & the odometer is above this, such as 73-77 Malibu & El Camino Speedos.

To remove the key from the donor, simply pry it off gently with a screw driver. Be careful when removing the tamper-proof keys, they are easily bent.

The rivet studs will be present on the center frame for the top key. On the 67 they were not peened over and all that had to be done was put the key in place & stake them with a small Philips screwdriver. On the 70 they were peened & had to be straightened to install the key. A few small dabs of Loctite superglue helped also.

The third point to look at on the speedo frame assembly itself is the idler gear, as shown in the picture below.



The stock idler is very thick. This will cause problems with the trip odometer not disengaging to be reset, as shown in the picture below. You have two options to correct this: File the gear, as I have done on the middle speedo, or remove the idler from a factory trip frame. I ended up doing it both ways, filing on the 67, & carefully removing the idler stud & re-riveting it to the 70 frame with a dab of loctite superglue for measure. Either way works.


The final point to take care of is threading the frame for the reset cable. The reset cable has 3/8"-24 threads at the speedo end. The hole will have to be tapped at an angle matching the angle of the reset pinion shaft. The picture below shows the reset cable end and the threaded hole. This hole has been present on all the speedo frames I have seen.


The angle can be easily determined by temporarily installing the reset assembly and marking the angle on the frame with a pencil, as shown in the picture below. Carefully tap the hole at the angle. This angle is critical, and not getting it lined up right will cause the cable to bind when it is pushed in and released.


Here is another view, at right angles to the view above. This angle should be straight, though it can be slightly angled to provide clearance between the casting and the flats on the cable end without ill effect.




After all these points have been addressed, you can assemble the speedo head with the reset assembly, the trip odometer reel, and the odometer reel. Install the reset cable and test operation. The reset cable will seem to long when straight, but when bent into a U-shape as it will be in use, as shown below, it will be the correct length.


When not pressing the reset knob, the reset gear should be fully retracted as shown below.


Press the reset knob, the reset gear should extend and shift the trip drive gear & engage with it, as shown below. While pushing in on the knob, you may now turn the knob clockwise until the trip odometer is zeroed.


You've now done the nitty-gritty mechanical modifications needed to add a trip odometer. In the next post, I'll go over the modification to the rear tin and face.