69 Hurst

Hey guys, looking for some assistance. I've always felt the tach on my H/O was off, so I bought a Bosch tach and hooked it up this past weekend. Well, I was correct and surprised how far off the factory one is. At idle, they are close (factory just above 1000, Bosch at 750) but the faster the engine revs the further apart they get. I took her out for a drive, at 60mph the factory tach is around 3500 and the Bosch around 2500.

So my question is...can the factory tach be recalibrated? Thanks for any input.

joe_padavano

Are you sure one tach isn't calibrated for 6 cyl and not 8 cyl? 750/1000 and 2500/3500 are dangerously close to 6/8. Just sayin'.

It would be the higher reading tach that would be set for 6 cyl

Koda

I know my 72's tach is stupid high. There's a method to adjust them somewhere. I remember you take it out the front, and you must ground it else it will ground you, and you adjust with screwdriver on the fly.

69 Hurst

Hey Joe,
I made sure the Bosch is set to 8cyl. The higher reading tach is the factory one, hopefully it is set up for 8 as well!

joe_padavano

You know what happens when you assume.

Like I said, the difference is 6/8, within production tolerances.

69 Hurst

So if its the higher reading tach that would be incorrectly calibrated, are you saying the Rally Pac tach could be configured for 6cyl??

matt69olds

They can be calibrated, but I question how accurate you could get them just by adjusting the calibration screw. Your description is the same as many others have complained about, at idle, it’s pretty close. As the engine speed increases, so does the degree of inaccuracy.

If you want modern accuracy, your probably going to have to have the tach rebuilt. There is only so much you can do with 60 year old electronics. These tachs weren’t very accurate when new, they almost certainly haven’t improved with age.

There was a article in hot rod magazine a few months ago about rebuilding old gauge clusters with modern gauges. They look stock, but are basically new gauges in the old housings. I’ll see if I can find the article.

matt69olds

Here it is.


https://www.hotrod.com/articles/hrdp...custom-gauges/

69 Hurst

Thanks, really good to know.

allyolds68

The foil tape covered hole in the back is the adjustment screw. It's next to impossible to try to adjust it while it's in the car and people that have tried to adjust them say the slightest turn changes the reading a lot

69 Hurst

Just a quick update... I was able to peel back the foil tape and get to the adjustment screw without having to remove the tach. I used a phillips driver bit and snaked my hand up behind the unit. As some have said, the slightest turn results in wide variations. The factory unit now reads within 100 RPM of the Bosch unit I temporarily installed throughout the range, much better than the 1000 it was off before.

Thanks for all the input on this!

Koda

Excellent. I need to do this. Did you just trial and error it multiple times till you were happy? And you snaked that bit up there installed?

69 Hurst

Koda,
Yes I snaked the bit up there with the tach in the dash and the car running (you have to be careful not to drop the bit while lining it up with the adjustment hole in the back of the tach as it can get lost quickly!). It took a number of tries as I would make a small adjustment on the screw and then rev the engine watching both tachs to see how that adjustment affected the reading. The total travel lock to lock on the adjustment screw is less than 180 degrees. Once the readings were the same, I took the car out to insure they stayed true while under load. Its quite a bit of getting up and down and I feel it a little in my back today... but patience paid off.

My car is non-AC, not sure how AC would impact this process.

Koda

Ok I shall try all that.