Legacy Stables

I just rebuilt my Speedo and Tack in a 72. They use a spring and magnet to translate input to visible output. I plan on testing these with a digital dremel and need to know at what RPM being inputted, the needle should show outputted up to 10,000 RPM. I can adjust the spring to get them working correctly.

I need to know what speed my Speedo is supposed to show at what RPM (clarify RPMs being input into the housing not the RPMs on the tack)
I am assuming that my tachometer is supposed to show the RPMs inputted into the housing.

Pleas don't say send it to a professional. If you have the info please post. If I get what I need I will post picks of the rebuild and a basic how to.

...Roger...

I'm sure the info is available,I'll go look at the Willcox site and see if I can link you to it.

...Roger...

OK I didn't find the vid I was looking for at Willcox but I bet Ernie can give you the info your looking for.
I did find a post on a forum and the guy states the SAE standard is 1000 rpm for 60 mph.
http://www.garagejournal.com/forum/s...ad.php?t=82744

ragtop00

UNSC
I'm doing the same thing on my 72 Tach and Speedo... would love to see step by step(with pics) on the entire rebuild/testing process.
Mike

Alan 71

Hi,
I'd be interested in this, too.
There are often posts about the distributor gear and shaft, and the cable to the tach, but nothing about the tach itself.
Regards,
Alan

Legacy Stables

took me a while but I managed to dig up a pdf with SAE J678 (thanks for the idea ...Roger...) which covers speedo tack and od. there is no mention of flex cable rpms indicating speed but it does state that 1000 turns of the cable should equal 1 mile on the odometer which leads me to believe 1000 rpm should be 60 mph. it does state that the tachometer should indicate twice the rpm of the flex shaft and that the flex shaft is driven at half the speed of the motor.

I have no idea whether GM used this to calibrate the c3 tack and speedo. please chime in if you know for sure

Mike Ward

I know that rebuilders use the 1K RPM = 60 mph for speedo calibration. The odo is a straight gear reduction so no calibration is needed or possible.

For the tach, I believe the distributor gear drive is a 1:1 ratio so 1K input speed to the tach should indicate 2K RPM.

Legacy Stables

the tachometer in my 72 uses an identical magnet and coil as the Speedo. the spring needs to be adjusted if it fatigues or the magnet looses strength

Legacy Stables

thanks every one so far on helping me. As soon as the rpm laser gauge come in the mail I will try the posted RPMs and see what I get

if anyone has more info don't hesitate to post

it's not a how to but here are the pics of the rebuild of my Speedo and OD.
http://s764.photobucket.com/albums/x...0OD%20rebuild/

If I can get both the tack and speedo calibrated correctly I will write it all up and post it.

ragtop00

UNSC Great pics!
1. What is the rpm laser gauge?
2. What cleaners did you use?
3. What lubricant (if any) did you use?
Thanks
Mike

Legacy Stables

1 rpm gauge basically its works by attaching a reflective piece of tape or paint on an object. as the object rotates a laser detects the reflection and records and displays how fast an object is rotating. I plan on using it on a drill for my calibration tests.\

2 remove rust with evaporust. a gallon will submerse the largest parts. dont submersed more than an hour at a time to reduce the chance of the paint lifting. rinse in cold water after removal, same reason. do not clean the odometer wheels. the paint will fade and lift easily. On the face of the gauge only use water, avoid the white paint it streaks and may lift off. clean bare metal parts however you want. The plastic parts will react with strong solvents. I melted one of the wheels on my trip od with one particular cleaner.

3 light mineral oil. done use any kind of lube that will get gummy over time it will mess with the drag of the magnetic cup or the odometer. Originally there wasn't any lube

As a side note I like to dip metal parts that aren't getting painted like the magnetic cup assembly in Metal Ready because it leave behind a zinc residue which will help prevent rust. Just Google the name.

...Roger...

Is Metal Ready like Ospho ?

Legacy Stables

Yes, but MR is used in prep for POR-15 and leave behind zinc to help rust prevention. Looks like Ospho will leave behind a layer of film which should be iron phosphate and whatever acid it is. so it probably will work to prohibit rust. You don't want to etch the metal just try to keep rust from coming back on the magnet and drum.

ragtop00

Thanks UNSC

0Willcox Corvette

If you guys have time.. I'll add my 2 cents worth on this.

The speedo is adjusted by the spring tension lever on the side of the speed cup. Called a retainer in the picture below.

And as a re-builder, I think its great you guys are trying this at home. But there are things you need to check before putting them back together.

The end play clearance of the first worm and magnet when installed. If excessive it can drag and catch the speed cup.

The OD of the first worm and magnet where it rides in the Oyllite bushing. Unusual to have an issue here.

The play in the jewel bushing in the middle of the magnet head. If you look at the magnet picture with the 1 on it.. you'll see the jewel bushing in the end. If this is worn out, the needle will wobble. The same goes for the bushing in the jewel plate. This is a common problem.

Lastly the condition of the actual first worm oylite bushing which leads to about 95 percent of the failure issues in speedometers. Symptoms for this are a fluttering needle, a odometer not working, a tinning noise and just about any other associated issue with the speedometer. Unless you have access to a lathe this bushing is not a standard size and can not be purchased out right.

You kind of have to read the issue of the gauge to determine which way to go with it.

If the odometer is not working but the gauge is working, then the issue is more times than not the first worm and magnet bushing. We get tons of people that want to purchase the second worm plastic gear and usually this is only a band aide.

If you look at the picture below, B is the second worm gear. If this gear is shot and made of metal there is a very high chance that the first worm gear teeth A will also be shot from mis-alignment. The cause.. the first worm bushing.

One thing that can happen and just did the other day to us.. is that everything can be bon bon and cherries in the speedometer, it can be tested for 4 miles and operate flawlessly! Only to fail when installed after a brief moment! Why did it fail? Each wheel in the odometer and the trip odometer has a gravity gear in it and this gear bound up... I mean it bound up solid and took out the second worm. No way to really check for this and it is unusual for it to happen but this problem has happened more than once. These cars are getting so old nothing would surprise me at this point.

Just my opinion, but the best calibration machine you could get is on your car. You can use the tachometer to calibrate both the speedo and the tach by using a dwell meter and reading the rpms off the coil while moving the adjustment lever.

Yes indeed, 1000 rpm's is 60 miles per hour +-/ 3mph at any given point.
The the tach calibrates at 1000 rpms to 2000 face reading.

And last of all.. NEVER grease a speedo or tach cable and do not lubricate the inner workings of the speedometer. The speedo cables are reverse wound and will pull the grease in to the head. When this happens all the lint and dust will collect in the head and cause pre-mature failure. I have a page on cable lubrication on the web site for those interested.

One other thing the popped in my mind.. the Jewel plate screws should always have loc-tite on the treads when installed... They were that way when new and this is usually one way we know that someone else has been in there before.

IMHO.. Willcox

0Willcox Corvette

UNSC, Great pictures for sure... and that plastic second worm gear is a life saver. What is nice about the plastic gear is when the first worm bushing wears out.. it will only strip out that plastic gear and save the teeth on the first gear. With the metal gear it will take out both the first and second worm... and always the bushing is the culprit.

I will have to say that not many would venture in to the workings of the odometer..

Kind of neat how they work huh.

Legacy Stables

thanks I that's more info than a week of Google searching and scouring of books got me. I am glad you guys chimed in with this I didn't mess with the first worm at all. How do you properly remove it and what are the clearances I should be checking for.

0Willcox Corvette

Ok.. now your putting the heat on me.. Let me say that I did look at all your pictures and if the second worm (plastic) is the one you removed from the gauge.. you'll be fine. I consider the clearances to be trade info and I'm sorry... But I kind of thought you might ask me that question...

But if your first worm and magnet were out of specs.. it would eat up that plastic gear so fast your head would spin.. For you.. put a drill to it.. and listen real closely for a tick nose between the two gears... you hear one.. you got a problem.. you don't.. run with it forest...

Willcox

Legacy Stables

I understand. at least I have the answers I am looking for. I will know soon if that gear will last. if it doesnt I will likey send it in. I know when some things are beyond me but since I make 20k a year its imperative I do the work. 90 percent of every thing works after 40 years, just need to disasemble derust and calibrate to keep it going.

0Willcox Corvette

Understand fully. Is that the gear you took out of the speedo?