Trip Odometer How to Add

[oldgold70c10]

To read the original thread and discussion on this topic go here- LINK


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.

[oldgold70c10]

Part 2

Sheet metal Modifications

Dial Face

A new dial opening will have to be made to accommodate the relocated odometer. This opening is the same size as the stock opening, and centered the same left-to-right. The bottom edge of the new opening is 0.700" above the top edge of the stock opening. A very simple way of making this opening is to use a factory trip odometer face & attach it via the dial mounting hole with small screws & nuts on top of the truck face. Since my donor speedo did not come with a face, I used a 78-81 Firebird trip sppedo face as just described as a template for the cutout. I drilled three small holes & then filed with jeweler's files until the openings matched. After smoothing the edges, be sure to use some paint on the edges of the opening to prevent rust.


The other modification will be the back tin. You will have to drill a hole for the reset cable to come out. Use a gutted out speedo frame & drill with a small bit to locate the initial hole, then use a step bit to enlarge it to final size. Because of the angle that the cable comes out, you may have to file the opening so the cable will thread freely into the back of the speedo as shown below. You may want to make a gasket to seal around the cable after installation.


The final modification to be done is drilling the bezel for the reset knob end of the cable. The cable is held to the bezel by a 3/8"-24 threaded barrel & nut (comes with cable). Mark the location as shown below, then drill first from the back side to one size under with a step bit & final drill to snug fit from the front.


Attach the cable to bezel & put the knob on. Test the completed assembly before putting the speedo face back on as shown below.



If everything checks out to your satisfaction, your almost done.
You may want to make light baffles from 1-1/2" long by 3/8" wide strips of thin metal or black-painted paper glued to the backside of the speedo face running up and down at right angles to the axis of the odometer reel. I attached mine with loctite superglue. If you don't make these baffles, the first time you drive at night you'll understand why.

Once everything is to your satisfaction, re-attach the speedo face & put the hand back on & re-assemble the complete cluster & install in the truck.

Using your trip odometer is very easy. As you might have guessed, to zero, push the knob in and turn clockwise till zeroed. You can also advance the trip odometer to any value you wish by turning it counterclockwise.

Good luck & I hope you enjoyed this project!