Norton Atlas Tach Question

wilkey113 · Jul 25, 2011

Wondering if anyone can help:

I have an Atlas motor. I know that the tach drive is a 2-1 ratio, and runs a 2-1 ratio tach.
I recently acquired a nice brass 1-1 ratio tach drive that I would like to use. Does anyone know if I can use that with the Atlas motor? What ratio tach would I need to use? I haven't seen a 1-1 ratio Smiths tach.
Are the brass ones used with the chronometric tach?

And help, advice or suggestions would be greatly appreciated.
Thanks
Jeff

grandpaul · Jul 25, 2011

Google Joel Levine in Atlanta.

He can rebuild any meter to any spec.

Like new.

BillT · Jul 25, 2011

a 1:1 tach drive would run a 4:1 tach.

I have a 1:1 drive on my Ranger and a 2:1 tach, so my tach registers 1/2 the actual RPM.

Atlas, Mercury and Hybrid motors ran the 2:1/2:1. Early Commando ran the 1:1/4:1

Atlas tach drive should be stamped 1508/05 on the body, and Commando should read 1508/06. Matchless G80 and G12 ran the /05 box, same as the Atlas.

Hope this helps

wilkey113 · Jul 25, 2011

Grand Paul,
Thanks for the information. I emailed Joel Levine, and he said that he was unable to do anything with the tach drive. Sounds like he only rebuilds gauges.

Bill,
Thank you also for the advice. That actually works great. Grey Face Smiths tachs are fairly readily available, so acquiring one of those shouldn't be a problem. And that's the gauge that I was hoping to be able to use.

Stupid question: Does the first number in the ratio stand for the drive side of the unit? And the second number indicate the cable side?
Also, does the motor turn at half the rate?
Just wanted to have an understanding of it all.

Basically, this is all good news as I definitely like the brass tach drive. So being able to use that on the Atlas motor with a grey face tach is a perfect scenario for me.

Thanks again,
Jeff

BillT · Jul 25, 2011

As I understand it, the first number is the output ratio, so with a 4:1 tach, the drive cable spins at half the speed of a 2:1 tach.

The tach drive runs at half engine speed, since it runs off the cam and the cam runs at half crank speed

k7k · Aug 25, 2012

Late to this post, but I wanted to jump in because I believe the information posted by BillT is not correct. The normal tach gear drive/gauge pairing on these bikes goes like this: a 1:1 drive spins a 2:1 tach, and a 2:1 drive spins a 4:1 tach. The first number in the ratio of the drive is the INPUT speed, thus a 2:1 drive REDUCES the output cable rotation by half. Since the input of the drive is run off the cam, which BillT correctly points out is already running at half the crank speed, the cable going into the tachometer is spinning at a quarter of the crank speed. The first number in the ratio of the tachometer is the OUTPUT speed, thus a 4:1 tach registers an RPM reading that is 4 times the RPM of the cable. Therefore the tachometer must be a 4:1 ratio to register the correct crank rotational speed for a 2:1 drive gear.

As you can see, a 1:1 tach drive is simply replicating the cam RPM, which is running at half the crank RPM, thus the tachometer must be a 2:1 ratio to double the RPM of the input cable.

My understanding is that the pre-Commando bikes all came with 1:1 drive and 2:1 tachometers. The Commandos typically came with 2:1 drives and 4:1 tachometers. What is the advantage of the 2:1 drive/4:1 tachometer setup? It is better suited to high-RPM bikes because the cable is turning at half the speed of the 1:1/2:1 setup, and therefore there is less chance of cable breakage or drive bearing failure.

I believe the ratio of your tach should be stamped on the tach housing somewhere. Using the guide above you can tell which drive ratio you need (or vice versa if you have the drive but not the tach).

Johnnymac · Aug 2, 2015

k7k said:
Late to this post, but I wanted to jump in because I believe the information posted by BillT is not correct. The normal tach gear drive/gauge pairing on these bikes goes like this: a 1:1 drive spins a 2:1 tach, and a 2:1 drive spins a 4:1 tach. The first number in the ratio of the drive is the INPUT speed, thus a 2:1 drive REDUCES the output cable rotation by half. Since the input of the drive is run off the cam, which BillT correctly points out is already running at half the crank speed, the cable going into the tachometer is spinning at a quarter of the crank speed. The first number in the ratio of the tachometer is the OUTPUT speed, thus a 4:1 tach registers an RPM reading that is 4 times the RPM of the cable. Therefore the tachometer must be a 4:1 ratio to register the correct crank rotational speed for a 2:1 drive gear.

As you can see, a 1:1 tach drive is simply replicating the cam RPM, which is running at half the crank RPM, thus the tachometer must be a 2:1 ratio to double the RPM of the input cable.

My understanding is that the pre-Commando bikes all came with 1:1 drive and 2:1 tachometers. The Commandos typically came with 2:1 drives and 4:1 tachometers. What is the advantage of the 2:1 drive/4:1 tachometer setup? It is better suited to high-RPM bikes because the cable is turning at half the speed of the 1:1/2:1 setup, and therefore there is less chance of cable breakage or drive bearing failure.

I believe the ratio of your tach should be stamped on the tach housing somewhere. Using the guide above you can tell which drive ratio you need (or vice versa if you have the drive but not the tach).

Sorry to dig up the old thread. ... but could one replace the stock 1:1 Tach drive on an Atlas with a 1:2 drive then use a 4:1 tachometer? I can't find a 2:1 tach and it might be easier to get a 1:2 drive and a plentiful 4:1 tachometer.

BillT · Aug 2, 2015

Johnnymac said:
Sorry to dig up the old thread. ... but could one replace the stock 1:1 Tach drive on an Atlas with a 1:2 drive then use a 4:1 tachometer? I can't find a 2:1 tach and it might be easier to get a 1:2 drive and a plentiful 4:1 tachometer.

Yes, you could.