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Cam Mounted Tach Issue

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1967 R502
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Joined: Thu Jan 25, 2018 11:47 am

Cam Mounted Tach Issue

Post by 1967 R502 »

Does anyone know how to correctly and easily mount a cam driven tach?

It seems the only way to do it is to do it blind, which is a completely random process.

1. The drive mounts to the front of the generator cover. It has a floppy tab on the inside which fits inside a 3mm slot in the head of a special bolt which also secures the advance and the magneto rotor.

2. The issue is that, not being Superman with x-ray vision, there is no way to see if the tab has fit into the slot on the bolt. If it’s not, the bolt could damage the drive tab one you start the motor.So you likely have one chance to do it right.

I find it hard to believe that the factory/dealers didn’t have another method for doing this.

Does anyone know how to mount the drive? Interested in hearing from anyone with experience.

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schrader7032
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Re: Cam Mounted Tach Issue

Post by schrader7032 »

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Re: Cam Mounted Tach Issue

Post by sherman980 »

Try using the end an old speedometer or tach cable. Cut the end off the inner cable that goes into the tach drive so you have a piece about 3 inches long. When you go to install your cover, remove the cable for the tach from the drive, and put the the short piece of inner cable you made into the drive. You can spin the drive with your fingers now using the short cable and can feel when the slot is lined up with the tab. About a 5 second issue.

Hope that helps.
Last edited by sherman980 on Sun Nov 28, 2021 1:26 pm, edited 1 time in total.
Thanks.
Chuck S

1967 R502
Posts: 143
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Re: Cam Mounted Tach Issue

Post by 1967 R502 »

schrader7032 wrote:
Sun Nov 28, 2021 10:10 am
I haven't done it, so not sure this will help. Duane has some pages about the tach on his site:

https://w6rec.com/wp-content/uploads/20 ... lation.pdf

https://w6rec.com/wp-content/uploads/20 ... update.pdf

https://w6rec.com/wp-content/uploads/20 ... ies-11.jpg
Thanks.

Well this is interesting reading, especially section 4. However, what they are describing is my setup.

It seems what they are saying is

1. Put the cover in without the drive
2. Insert the drive without the inner nut that holds it in, so you are hunting and pecking with the unit to find the slot on the drive bolt.
3.Once you’ve done that adjust the outer nut to give some clearance
4. Remember the position of everything.
5 take the cover off and install the inner nut while keeping everything in place.
6. Put the cover back on and hope it goes well.

1967 R502
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Re: Cam Mounted Tach Issue

Post by 1967 R502 »

sherman980 wrote:
Sun Nov 28, 2021 12:58 pm
Try using the end an old speedometer or tach cable. Cut the end off the inner cable that goes into the tach drive so you have a piece about 3 inches long. When you go to install your cover, remove the cable for the tach from the drive, and put the the short piece of inner cable you made into the drive. You can spin the drive with your fingers now using the short cable and can feel when the slot is lined up with the tab. About a 5 second issue.


I just threw away an old speedo cable Wednesday. No kidding! Perhaps I can try with an Allen key or similar,
Hope that helps.

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Re: Cam Mounted Tach Issue

Post by Flx48 »

Sounds like you've got your tach drive issue figured out.

I've never mounted one, the cam driven version always sounded a little sketchy, but again no experience.
Perhaps why they switched to the oil pump driven version (with the external pump cover) for the later years of the /2.

Here's a couple of pics of gen covers I took here that may be of some interest.

The '61 only gen cover with integrated tach mounting flat.
IMG_2461.jpg
And the Hoske mounting flange for stock gen cover.
IMG_2463.jpg
Best-
George

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Re: Cam Mounted Tach Issue

Post by jwonder »

Hello, the square drive driver bit is your friend while locating the drive pin.

As seen here and available at any hardware store:
1250064A-E422-428B-8485-4062C9E722FF.jpeg
Also this is a one year only generator cover with the tach drive mounted in it.
D1005EEF-441C-4029-806F-FA7594E0B1C2.jpeg
Thanks to Chuck for all your guidance and knowledge!!!
James Wonder
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vechorik1373
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Re: Cam Mounted Tach Issue

Post by vechorik1373 »

I have an orginal tach drive mounted on the front cover. I agree that to locate the hole precisely in the cover is tricky. What I did was to cut down an allen wrench, and grind the end to a point, by chucking it up in an electric drill and using a vertical belt sander, to make a pencil point on the end. I inserted that into the socket head cap screw that holds the advance on. I had to play around to establish what exact length it had to be, so when I installed the gen cover, I could start both of the nuts, and pull the cover down almost flush on the engine. I then tapped on the outside of the cover with a dead blow hammer, to make myself a "center punch" mark on the cover to locate where the hole for the gear box had to be. Once I had that mark, it was easy to drill the cover for the hole, exactly where it needed to be.

As for as installing the cover, the shaft on my tach gearbox has a hinged pin, which wanted to flop out of the center position during the cover installation. I solved that, by putting heat shrink tubing over the shaft and hinge, and shrinking the tubing. That kept the shaft much more centered, yet allowed it to wobble or flex just a little during installation of the cover or running if there was a slight misalignment.
Before I dreamed up the idea of heat shrink tubing to hold the shaft close to where it need to be, in order to get it engaged with the bolt that held the advance on, I drove myself nuts trying to get the drive to plug in...
Vech
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Re: Cam Mounted Tach Issue

Post by sherman980 »

Vech,
Interesting way to find the "center" of the cam bolt to machine a cover! I like that!

Best my experience tells me is that there were at least two different VDO cam driven tach drives and multiple front cover configurations supplied by the factory over the years (not including the oil pump gear box driven type). The earlier VDO "square" drive was designed to either mount to an aluminum mounting bracket that was bolted to the front cover or was mounted directly to the front cover through a hole with a locking nut on either side of the threaded portion of the drive. The OD of the threaded portion of the drive is approximately 23mm. The later cylindrical drive was designed to thread directly into the front cover but could also be mounted to a separate mounting plate that bolted to the front cover. The threaded portion of this drive that went through the cover has an OD of approximately 17mm and one lock nut was used on the outside to lock the drive in place. Lots of covers and mounting plates were machined/supplied outside of the factory to add these drives. I've included a couple of pics of Factory covers and drives (one NOS cover for the rectangular drive (with factory bulge for a vibration damper) - no threads, and one cover for the cylindrical drive - threaded) to help clarify. The pic of the cylindrical drive shows it attached to a mounting plate that bolts to the front cover, but if the mounting plate is removed, it will screw right into the cover with the threaded hole.

DSC07215.JPG
DSC07279.JPG
DSC07280.JPG
s-l1600.jpg
s-l1600 (3).jpg
s-l1600 (5).jpg
DSC07277.JPG
DSC07278.JPG
If anyone is interested in either of the two covers shown, PM me. I no longer have a need for either.
Hope that helps.
Thanks.
Chuck S

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