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Old 12-03-2019, 11:02 AM   #1
busterwivell
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factory tach

Has anyone figured out the issue with these? I have an HEI in my 402, and the tach is way off. I know I've got a 4:10 rear end, but the tach reading would indicate about a 7:50. Can they be calibrated? A friend suggests it needs a filter?
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Old 12-03-2019, 11:35 AM   #2
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Re: factory tach

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Originally Posted by busterwivell View Post
Has anyone figured out the issue with these? I have an HEI in my 402, and the tach is way off. I know I've got a 4:10 rear end, but the tach reading would indicate about a 7:50. Can they be calibrated? A friend suggests it needs a filter?

First off, are you sure you don't have a 6 cylinder tach instead of a V8? There should be an ink stamp on the back of the tach that tells. You might be able to see it with a mirror and a good light.

There are also part numbers for each. This was posted by Member Ray Mcavoy:

GM #6468321 is for the 6cyl gas engine tach.
GM #6468228 is for the 8cyl gas engine tach (except medium duty trucks with transistor ignition).
GM #6468320 is for the 8cyl gas engine tach with transistor ignition.

All of the stock ones I have used have been pretty acurate with the HEI.

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Old 12-03-2019, 11:45 AM   #3
busterwivell
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Re: factory tach

Hmmm........truck is an original big block truck, but maybe someone changed out the gauge cluster? Thanks for the numbers. I'll check them. Are the numbers stamped/printed on back of tach?
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Old 12-03-2019, 12:27 PM   #4
custom10nut
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Re: factory tach

Short answer is, they can be adjusted a little bit. If you do have the 8cyl tach, itís most likely your electronic ignition is sending too much signal. There are conversion kits out there to fix your issue. Go to the search and look for threads on this subject.
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Old 12-03-2019, 12:41 PM   #5
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Re: factory tach

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Originally Posted by busterwivell View Post
Hmmm........truck is an original big block truck, but maybe someone changed out the gauge cluster? Thanks for the numbers. I'll check them. Are the numbers stamped/printed on back of tach?

Yes, the numbers are stamped on the back but they can be hard to read sometimes.

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Old 12-03-2019, 02:08 PM   #6
TBONE1964
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Re: factory tach

I can test and calibrate your tach if needed.
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Old 12-03-2019, 02:45 PM   #7
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Re: factory tach

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I can test and calibrate your tach if needed.
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Old 12-05-2019, 01:02 AM   #8
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Re: factory tach

Same issue , Put a complete Petronix in my 72 and tach is out, reading higher than it should, 4/500 RPM at 70mph. Called Petronix and they suggested getting some resistor which I did and it still is out.
Thanks for those part numbers Leon as I have to pull my dash and will check which tach I have because tach was assembled by Paul ( PTs70 CST) with original parts. It was also reading high like 7/800 with the points distributor when I installed it in the truck.
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Old 12-05-2019, 01:23 AM   #9
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Re: factory tach

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Same issue , Put a complete Petronix in my 72 and tach is out, reading higher than it should, 4/500 RPM at 70mph. Called Petronix and they suggested getting some resistor which I did and it still is out.
Thanks for those part numbers Leon as I have to pull my dash and will check which tach I have because tach was assembled by Paul ( PTs70 CST) with original parts. It was also reading high like 7/800 with the points distributor when I installed it in the truck.

No problem. Let us know what you find out.

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Old 12-05-2019, 10:08 AM   #10
clay68c10
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Re: factory tach

There is a hole in the back of the tach to access the adjuster screw (potentiometer)
I was able to get mine close. These aren’t super precise, but you may be able to get it close enough.
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