factory tach - The 1947 - Present Chevrolet & GMC Truck Message Board Network

busterwivell · 12-03-2019, 11:02 AM

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?

**LockDoc** · 12-03-2019, 11:35 AM

Quote:

Originally Posted by busterwivell

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.

LockDoc

busterwivell · 12-03-2019, 11:45 AM

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?

custom10nut · 12-03-2019, 12:27 PM

Short answer is, they can be adjusted a little bit. If you do have the 8cyl tach, it’s 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.

**LockDoc** · 12-03-2019, 12:41 PM

Quote:

Originally Posted by busterwivell

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.

LockDoc

**TBONE1964** · 12-03-2019, 02:08 PM

I can test and calibrate your tach if needed.

**Sheepdip** · 12-03-2019, 02:45 PM

Quote:

Originally Posted by TBONE1964

I can test and calibrate your tach if needed.

^^^^^^^^^^^^^^Excellent Service^^^^^^^^^^^^^^^

**Already Gone** · 12-05-2019, 01:02 AM

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.

**LockDoc** · 12-05-2019, 01:23 AM

Quote:

Originally Posted by Already Gone

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.

LockDoc

**clay68c10** · 12-05-2019, 10:08 AM

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.

12-03-2019, 11:02 AM	#1
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	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? __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

12-03-2019, 11:45 AM	#3
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	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? __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

12-03-2019, 12:27 PM	#4
custom10nut Registered User Join Date: Apr 2015 Location: East Tn (In the heart of the Smoky Mtns) Posts: 1,846	Re: factory tach Short answer is, they can be adjusted a little bit. If you do have the 8cyl tach, it’s 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.

12-05-2019, 01:02 AM	#8
Already Gone 70+ ( Old Skool Club ) Join Date: Nov 2003 Location: Saskatoon, Saskatchewan ,Canada Posts: 9,073	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. __________________ 1972 C10 Custom/Deluxe 613 Highlander 406/700R4 1999 White Tahoe LS 4x2 with Z56 Police Package 1992 K1500 GMC Suburban Members I have personally met: MusicMan70 - HeavyD - ChewyChevy67 - StingRay -71SWB4x4 - 67 Burb - DeadheadNM - too much stuff - bc65 - das601 " Circumsatances Do Not Change Responsibility " " The Sky is not the Limit, Your Mind is." Marilyn Monroe .. RIP Charlie Watts 1941 - 2021

12-05-2019, 10:08 AM	#10
clay68c10 Senior Member Join Date: Aug 2004 Location: Centennial, CO Posts: 2,882	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. __________________ ClusterTruck: 68 C10 Bought in 89, wrecked in 03. Slow low $ rebuild started '17. 6.0 & TKO http://67-72chevytrucks.com/vboard/s...d.php?t=788602 93 K1500 Short Step, 350, NV4500, EBL flash ECU. Vortec heads & roller cam someday... 05 Jeep Wrangler Unlimited. 3” lift & 6-speed 02 E320 the insurance total “free car” 13 Tahoe LTZ

12-03-2019, 02:08 PM	#6
TBONE1964 Cluster King Join Date: Nov 2012 Location: Junction City, OR Posts: 5,263	Re: factory tach I can test and calibrate your tach if needed. __________________ Contact me on all of your gauge cluster needs. I specialize in restoration, repair and parts sales for 67-72 Chevy and GMC trucks. email me at tbonegarris@yahoo.com I am also a dealer for Counterpart for gauge cluster parts only. Also see my facebook page, CG&C



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