Basic Tachometer Info Needed

Hi, I plan to install a cable-driven tachometer. I have a Delco distributor with the cable drive option for my 392 V8. There are two different types of cable-driven tachs available from Isspro. I noticed for every full revolution of the distributor's rotor the distributor's cable drive would make one full revolution. Isspro's tech support could not tell me which tachometer to use. One version states .5:1 ratio and the other version is 1:1 ratio. I'm not clear - does .5:1 mean that the tach shows half the rpm's or shows double the rpm's because it expects to see the drive to spin at half the rotation speed of the rotor? I'm so confused I don't even know how to ask the question properly, especially taking into consideration the rotation of the crank in relation to the distributor. Which version of tachometer should I use and why?
Thank you.

Cummins 4bta with TH475 and NVG 241c transfer case. Dana 4 axles.

My experience with tachometers has been with vintage IHC drives. The drive is half the speed of the engine crank. I do not know what to make out from the description you give. If this were my project, I would be looking for an original IHC. I have several from the 1950s and another I just got off eBay for the earlier IHC "K" trucks.
I machined the tach drive in my shop.
I intend to run the tachometer off a Cummins 4bta in a 1952 L110 4X4.
https://youtu.be/87WuczmOI6s

Actually this is an IHC Delco distributor. It's used in some mid to later '60's Loadstars with a governed 392 V8 with a cable drive tach. The drive clearly spins one complete revolution with one complete revolution of the distributor's rotor. I don't have the part numbers handy, but I could look it up if it would be of any help. I don't have the tools or experience to modify a non-IHC dizzy to work in an IHC. I like the IH Delco over the other IH distributors because the cap, rotor, points, weights, springs, vacuum advance etc all interchange with GM making fine tuning easy for me. (Actually I found this dizzy as NOS in the obsolete section of a large ag tractor parts supply house, with the IH part number attached).

I've had several discussions with Isspro's tech support. We've determined the proper cable to go between the distributor and their tachometer. (Two different types in three different diameters of cable are possible). But they have no info on using their tach on an old International V8. And that's about all they know - what their book says to use for each of the big 3 manufacturers' engines.

Just trying to figure out which ratio of tachometer reads properly. If the crank spins twice for every one rotor revolution, that means for one complete revolution of the rotor I would have 720 or two rotations of the crank shaft. That means I would need the distributor's cable drive to rotate twice per one revolution of the rotor. Or have one cable rotation seen as two rotations by the tach. I would assume I need a .5:1 tach for a V8 because the cable is only going half the speed of the crank, or a 1:1 tach for a 4-cylinder engine? Does that make sense?

Yes, you want the .5 drive one. 1:1 are generally used on 2 stroke engines like old Detroit's. Even there you have to check what drive is being used.
4 stroke engines generally are driven off the cam or off a drive that is driven at cam speed (1/2 crank speed) like distributors.
So the cable turns 1000 rpm to show 2000 rpm on the tach
If somehow you end up with the wrong one there are adapters to correct for it.
number of cyl has no effect on the drive speed, the dist turns at 1/2 crank speed on a 4 cyl just as it does on a 6 or 8

Be aware, some distributor mounted governors mount the control on the dist and can not be converted to drive a tach cable. I think there are also distributors that have drives for both. IIRC the Delco unit on the Loadstar can drive both. The one for the 269-282-308 was one or the other.

Thank you! That makes sense. Why couldn't Isspro's tech support tell me that?
I just never stop learning with these old IH's.

Nice looking tach's you have there. I would not be converting everything to Isspro gauges if I found IH gauges that look as good as those. Eventually I'll put all my old gauges on eBay if anyone wants them. The only gauge somewhat rare is a mechanical IH oil pressure gauge marked with a scale of 0 to 75 psi. I'll probably keep that one until I finish my projects.

Checked my Fleetstar for that oil gauge, Nope, 0-120 which is odd esp for a 2 stroke powered truck, where oil pressure all but goes away at idle. Sorry can't help.

Why couldn't Isspro's tech support tell me that? If I had to ask that question, my hand would be on the EXIT door handle. That is something the sales should know. If the person does not know, get some help ASAP.
I know there are Old IHC tachometers out there waiting to be discovered.
It has been my experience that I had better know what I want and need before I go to buy something. I used to f"k up the counter know-it-all. I would ask for a part by part number or detailed description. The dummy would ask for the vehicle make and which engine. An R120, 4X4 with a 6.9 Diesel and C6 automatic always set off a fire storm of confusion. I miss that truck. Check eBay.

https://www.ebay.com/itm/265751550039?_ ... %3A2047675