In an attempt to reddy up the 74 convertible for her upcoming LS2/4L60e transplant, one area I knew I'd have to address is the tachometer. The speedometer can be made to work with a mod to the 4L60e, but the mechanical tach simply wouldn't in an engine without a distributor. And while the interior is gutted what better time.

In research, I learned I could just get a tachometer from a 75 up which were the first year for the electronic tach, and volo! Problem solved, right? Or so I thought anyway. What WILL work is the 75 up tachometer WILL fit just where it's supposed to, with all the same mounting and all. So what's the big problem?

This is.

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Looks fine right? Pay attention to the number font and compare it to the next picture.

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And finally, a side by side.
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Clearly, this might have worked but would have left the two biggest in my face gauges looking like an apple and an orange and not the matched set they were intended to be. I needed them to match, and look better than new or at least as good as original. I ordered shiny new reproduction speedometer and tachometer gauge faceplates ( the 1971 glow in the dark ones) and went to work on the conversion. When I drilled the rivets to remove the needle can on the 76 tachometer and transplant the needle can to my shiny new, (still in the plastic face protector) 71 glow in the dark faceplate I found the pre-drilled mounting holes in a mechanical tachometer faceplate aren't as wide as the holes in a 75 up electronic tach. Matter of fact it would put the mounting screw VERY close to the needle coil windings, something I wasn't comfortable with, especially having to spring for a new tach board and the rest of this conversion mess.
What I decided to do was to carefully position the needle can EXACTLY where it needed to be on the back of the new mechanical tach faceplate with tape.

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I then JB Welded the needle can to the back of the faceplate.

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If you use the 12 volt mounting posts as a level reference for where the needle can needs secured to the faceplate it wont line up with the rest of the mounting guts. It secures slightly off kilter as can be seen here.

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Now with everything secured back in the main tachometer can, you can clearly see the two left over unused mounting holes for the tach face. Looks kinda ghetto and I cant leave it like that.

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My solution was these tiny black plastic hole plugs with an oval head.

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I needed to shave them down for a flush fit.
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Once I secured them to the new faceplate, one would be hard pressed to even notice. I plan on placing two more over the speedometer faceplate mounting screws as well for a perfect match.

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Pleased with my work, I went to place the tach needle from my original 74 tach onto my newly converted tach. This way the needle matches my 74 speedometer needle as it should (must). When I went to fit the needle, even before zeroing it, I quickly found that the tachometer needle for a 74 mechanical tachometer will fit the needle can of a 76 electronic tachometer like a wet noodle. The 74 spindle opening is much larger than the needle on a 76 can. As if this tiny little project couldn't get any worse, right? For one, early 70's needles aren't easy to come by. Period. Even if you found one, it wouldn't fit anyway. I feel another obstacle coming on.

After a visit to my buddy at Doc's Corvette in Tulsa, we found a tach needle from a late 90's Corvette. It was white plastic and didn't even come remotely close to the nice metal needle on an early 70's Corvette. What it DID have was a spindle that fit the needle can in my converted tachometer. I NEEDED that spindle, which was easy to remove as it was the ONLY metal thing on the 90's tach needle. The rest was plastic. I removed the stainless steel spindle with ease.

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The spindle on the original 74 mechanical tach was VERY different. It was part of the counterweight as well as the actual spindle and didn't look like it could be removed from the needle without damage . In other words it was one piece brass and part of the needle to the point I feared destroying it by doing what needed to get done. Running out of options here. Since it was one piece brass, the first thing I needed to do was carefully grind the brass spindle down flush, then carefully line up and center the drill bit used to provide the required recess for the stainless steel spindle.

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I continued to drill deep enough to press fit the stainless spindle flush so the needle didn't poke out too far, checking several times. I didn't want to pop through the brass counterweight into the needle.

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Here it is with the stainless spindle inserted into the 74 brass assembly and the chrome needle cap installed

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Finally, here is the finished product, ready for the LSx required electronic tach while looking sheepishly stock. SWEET!
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Next project. Converting the 3 speed automatic shifter to a 4 speed auto shift for the 4L60e.....BUT there IS a kit for that conversion!

 

Nice work, that only a true corvette lover can appreciate! . I converted my '70 to electronic tach as well, using a '76 tach. I went the easier route and had the '76 tach refaced by Roger at Corvette Instrument Service. Came out great and you'd never know it's not a factory '70 piece.

 

Nice Work! Impressive creativity!

 

Looks good! I went a slightly different way on mine. Using tin snips I cut into the coil can right at the mounting ears. I was then able to bend them inwards until they lined up with the holes in the face plate.

I used the screws from an old speedo. The holes in the mounting ears were a bit large so I "crushed" them a bit making them oval. That was enough to make the screws bite. Ended up looking nice, with the screws matching the speedo.