The original V8 GMC tach was actually a Stewart Warner electric tach. It
was two piece, a sending unit under the hood and the meter in the dash. It
had a 90 degree swing measuring (I think) 5000 RPM. The 6 cylinder tachs
were mechanical (like a speedometer) being driven from a special distributor
that has a gear on it's shaft and a place to connect the cable and had about
a 290 degree swing measuring 4000 RPM. I've never seen a stock tach on a
pickup, just on larger trucks.
I found a Sun Super Tach II at a swap meet for $15. If you pop the chrome
bezel off of the black bell-shaped case (it is glued on, but not well) you
will find that the case has a lip on it's open end. The lip is about 1/8"
too large to fit into the opening in the GMC dash. I took the guts out of
the tach and attacked the lip with a Dremel tool, sanding off just enough of
the lip to let it fit into the GMC's dash opening. A trial fit showed that
the glass rattled so I put 3 small dabs of clear RTV 120 degrees apart on
the face of the glass at the outside edge and let it dry. I then trimmed
off enough of the RTV to just allow the glass to be held securely between
the tach and the GMC dash bezel. There is nothing for the little plates
that GMC uses to hold the tach in place to grab, so I roughed up the plastic
on the tach and used a little JB Weld to build a ridge for the plates to
grab. I hooked up the tach's electrics and now had an operational 8000 RPM
tach in my dash. This is probably as far as you could go with a 6 cylinder.
The problem with an 8000 RPM tach in a truck with an engine that is hard
pressed to turn 4000 RPM is that the you only use half the swing. My
Hydra-Matic equipped V8 up shifts no higher than 3700 RPM and with a 3.08
rear end 4000 RPM in high gear would be about 110 MPH, much faster than I
would consider pushing my truck.
Tachs don't actually measure RPM, they measure pulses (point closures) per
second. Most after market tachs (including Sun Super Tach II) have a switch
on the back side to set 4, 6 or 8 cylinders. An 8 cylinder engine at 4000
RPM will have as many point closures as a 4 cylinder engine does at 8000
RPM. Using that same math, if you flip the switch on the back of the tach
from 8 cylinder to 4 cylinder and still hook it up to an 8 cylinder engine
you will now read full scale on the tach when the engine is turning only
4000 RPM. I set the switch to 4 cylinder.
Then I removed the face from the front of the tach (two small screws). I
turned the face over and painted the back flat black. The dash decal kit
from Heavy Chevy has a speedometer face for the 80 MPH speedometer. I used
an Exacto knife and cut just the outer hash marks in one piece from the 80
MPH speedometer and applied them to the tach face. I then cut just the
digits 0, 1, 2, 3 and 4 from the speedometer decal and applied them to the
tach face 0 at the beginning hash mark, 2 at the center mark, 4 at the end
and 1 & 3 between the others. A little model airplane paint (see below) on
the tach needle to match the GMC instruments and some green transparent
plastic around the tach's internal lamp (I used a strip cut from a plastic
7-Up bottle, Mountain Dew is too yellow) to match the GMC's dash lighting
scheme, button it up and I have a stock looking 270 degree swing 4000 RPM
tach. If you ever install a hot engine you can always flip the original
8000 RPM side face back out and select 8 cylinder.
By the way, when I recently installed a Pontiac HEI distributor in my GMC I
had no problem with the tach. Just hooked it up to the TACH connection on
the distributor cap.
A follow up to the clock message below. I considered hooking the clock to
the truck's electrical system, but it was just too easy to change the
clock's battery once a year or so. Plus I would have had to come up with a
circuit to regulate down to 1.5 volts.
-----Original Message-----
From: Blaine & Maggie Dumkee [mailto:bmdumkee@auroranet.nt.ca]
Sent: Wednesday, September 05, 2001 12:18 AM
To: Hanlon, Bill
Subject: Re: [oletrucks] GMC clock
Bill
Thanks for the advice. Sounds like a great way to get a clock.
My truck is a six cylinder, any ideas for a tach?
Blaine
-----Original Message-----
From: Hanlon, Bill <Bill.Hanlon@COMPAQ.com>
To: 'Blaine & Maggie Dumkee' <bmdumkee@auroranet.nt.ca>
Date: Tuesday, September 04, 2001 5:43 AM
Subject: RE: [oletrucks] GMC clock
>Blaine:
> I don't know how dedicated to stock you are, but if you can deviate
>a little you mght want to try what I did to my 57. The PO had cut a hole
in
>the middle of the blank panel where the tach or clock mounts. I used the
>hacked up panel as a templet and cut another out of 18 gauge sheet metal.
>Painted the new panel flat black. I purchased a set of gauge decals from
>Heavy Chevy and applied the clock face decal to the panel. I purchased a
>quartz clock movement from my local craft/hobby store. Cost was less than
>$10. The hands did not exactly match the originals, but were very close.
>Some model airplane paint from the same hobby shop closely matched the
>off-white of the GMC instruments. I drilled a hole in the middle of the
>panel and mounted the clock. I left the second hand off of the clock
>because the one tick per second quartz movement was a give away that it
>wasn't stock. A little more flat black paint on the mounting nut and it
>takes an expert to know that it isn't stock.
>
> I have since removed the clock and replaced it with what appears to
>the non-expert to be a factory tach. If your 59 is a V8 and you want to
>hear about my tach let me know.
>
>-----Original Message-----
>From: Blaine & Maggie Dumkee [mailto:bmdumkee@auroranet.nt.ca]
>Sent: Saturday, September 01, 2001 1:00 AM
>To: oletrucks@autox.team.net
>Subject: [oletrucks] GMC clock
>
>
>During my summer vacation I came across a 59 gmc longbox, stepside with a
>clock mounted in the dash. Now I know GMC clocks are rare, but does someone
>have picture they could email me or web page I could view to confirm this.
>The truck was in a wrecking yard, but the yardman only wanted to sell the
>complete truck, for a $1000 Canadian, or about $600 US. The truck did have
a
>deluxe heater but no radio. What is a GMC clock worth?
>
>Blaine Dumkee
>1959 GMC 9314
>Fort Smith NT
>oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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