Hood Tachs

Specifications

Pontiac used two different physical sizes of hood tachs on GTO's. The taller 67 hood tach and the smaller 68-72 hood tach. The shell on the hood tachs were painted the color of the car and the front bezel insert was left a low gloss black. The hood tachs came with a decal on the underneath side of the tach. The upper half of the decal read "DO NOT EXCEED 180 DEG WHEN PAINTING" and the part number of the tach was ink stamped on the lower half of the decal on 68-72 hood tachs. Each of the 10 different versions of hood tachs had a different P/N. All Pontiac hood tachs had “Pontiac” on the faceplate. There is a small tab mounted to the faceplate that hides the center pivot point of the pointer. This tab had no markings in 67 but 68-72 had the letters “RPM” on it.

1967 was the first year for hood tachs, It was available only as a dealer-installed option up until April of 67 when it became a regular production option. The 67 hood tach stood 3" tall on the hood and was 8 5/16" long. It mounted to the hood via 2 studs and 2 pop rivets. 67 was the only year that Pontiac used a rubber gasket between the base of the hood tach and the hood. The tach only had one lamp on the base of the tach. The face plate used in the 67 GTO/Lemans/Big car tach was “steel-blue” background with white letters and division lines, the metallic blue was used on all Pontiac faceplates from 63-67 (except the 67 Firebird). The redline started at 5100 and swept to 8000 on all V8 tachs. The redline and pointer was a fluorescent red in color. Firebirds used the same tach but the face was a different color combination (black background with yellow-green lettering and division lines). 6 cylinder tach faces had a red line that swept from 6500 to 8000 in 67. The face plate in all year hood tachs always matched the background color of the interior gauge/speedo faces. Installation of the 67 Hood tach was via a 3.5 inch hole in the hood.

The 68 to 72 hood tachs were physically the same size but there were several variations between the different years. The 68- 72 tach stood 2 1/8" high on the hood and was 7 3/4" long. Hood tachs used from 68-72 were available as regular production options on all Pontiacs. The 68-72 hood tach mounted to the hood via 2 studs and 1 pop rivet. The pop rivet was reported to be theft deterrent and is not required to hold the hood tach onto the hood. The 68-72 hood tachs had 2 lamps installed in the base of the tach. Installation of the 68-72 Hood tach was via a 3.75 inch hole in the hood.

There were two different types of faces used in 1968, early production and late. The early production faces were not used for very long in the 68 model year. The latest date I have seen on an early style face plate is August 25, 1967 and the earliest date that I have seen on the later style face plate is September 27 1967. The background color of all 68 faces was the Steel Blue with white lettering and division lines. Both face styles were available in 2 different redlines depending on application 5100 and 5500. The early style face had division lines that were short in length and in a circular shape and the pointer was very short and reported to be hard to see which prompted the change to the later style face with the division lines more in the shape of a football with a longer pointer. See pictures below. Early style faces used 5100 redline for all V8’s and 5500 redline for 6 cylinders (The RAII engine was not yet available). The later style face used 5100 redline for all V8’s except RAII, 6 cyl. The RAII cars had a 5500 redline. The only difference in 6 cylinder and 8 cylinder tachs was the size of the capacitor on the circuit board, 8 cylinder circuit boards used a .33uf cap and the 6 cyl. used a .68uf cap.

The same hood tach was used in 69 and early 70. The only difference between this tach and the late 68 was the face background color was changed to black, the numbers and division lines were unchanged from the later 68 type face. Again the pointer and redline were a fluorescent red.

I have no documentation of when in 1970 the hood tach was changed but there were a couple changes. The redline was changed from a fluorescent red to a flat red/orange which was less susceptible to fading. GM also added a tube to the base of the tach which was attached to a fitting on the heater assembly via a vacuum hose. The idea was to blow warm air up into the tach to fight moisture. I have had a 70-72 NOS hood tach that had a non-glare lens made of plastic rather than the glass lens used on all production 70-72 tachs that I have seen come in for repair.

Buick used Hood tachs on their GSX cars. The face plate was different than Pontiacs. The division lines turned red at 5000 RPM and there was no redline in 70-71. In 72 the same face design was used but division lines were all white and their was a red line that swept from 5000 to 8000. There is a small tab that sits in front of the face plate that hides the center of the pointer. This tab is labeled BUICK on Buick tabs.

AMC used a hood tach on their Rebel Machine. It used the same faceplate as the 70-71 Buick but had the AMC red/white/blue emblem on the tab.

What causes a Hood Tach to fail?

Do not spray any type of oil or lubricant into the meter movement. Do not spray compressed air into the meter movement when trying to dislodge debris; it will bend those fine hair like springs beyond fixing. It is best to replace the circuit board with one of my reproductions. If you fix one cracked trace, it or another trace will fail. The little black spots on the bottom of the circuit board are carbon film resisters and affect calibration.

Things not to do if you are trying to get your hood tach working.

Do not spray any type of oil or lubricant into the meter movement. Do not spray compressed air into the meter movement when trying to dislodge debris; it will bend those fine hair like springs beyond fixing. It is best to replace the circuit board with one of my reproductions. If you fix one cracked trace, it or another trace will fail. The little black spots on the bottom of the circuit board are carbon film resisters and affect calibration.

75% of the NOS tachs sent to me to be checked out did not work or was off by over 1000 RPM at 6000. How is that for reliability?

Hood tach P/N’s:

Note: GM service Bulletin 3D-5 lists many different P/N’s for 70-72 Hood Tachs (almost like a different P/N for each color car however all 70-72 tachs sent in for repair that had legible decals were P/N 5657097)

Hood Tach face P/N’s:

(stamped on the back of the hood tach faceplate as well as date produced. The 67 face plates did not have P/N’s stamped on them.)

References:
AC Service Bulletin 3D-5 – AC Electronic D’Arsonval Tachometer Repair