mechanical tach question

what's a quick way to determine the ratio on a mechanical tachometer.

i thought about chucking it up in a lathe or drillpress with a known rpm to see if it reads actual or 1/2 speed. just wanted to know whether it's crank driven or driven off the distributor.

the needle moves by turning the outside of the collar with my fingertips. to get an accurate reading, do i need to use a shaft or can i get a true reading off the outside of the collar?

it's a late 50's stewart warner that matches this speedometer. i'd like to find other large logo gauges and an old panel to set them in.

 

Most are 1/2 speed .Drive with a piece of old cable inner on the lathe.

 

I'd just use a piece of speedo cable and a cordless drill, set to reverse. compare rated rpm of the drill to what it displays on the tach.

 

You can file a piece of wood or br*** square to duplicate a speedo drive.


Ago

 

at this moment, i don't have a cable to hook up to it.

 

then chuck the collar in the drill, and see what happens. Try to apply very little pressure to the collar, if possible.

maybe use a piece of rubber tubing or something to isolate the drill from the tach

 

The vast majority of tachometer are a 2:1 ratio to be run off a distributor drive. If you have the matching tach to the speedometer pictured, you're 2:1 standard. Also, if the unit should deviate from standard calibration, it usually has a stamping on the housing indicating its ratio.

 

hooked the tach up to a drill that revs to 1000rpm and the tach read 2000 rpm. does that sound right?

 

Yes, the distributor turns half as fast as the crank, so that's what you want.

 

Yes if driven off a tach drive dist. as dist runs 1/2 engine speed.

 

so let me get this straight, that must mean the crank goes around two times for every one time the cam does. huh, imagine that.

i feel like pinto in animal house right about now.

can i buy some pot from you?

 

You can't be serious.

 

yeah, i don't smoke pot