750 Atlas Tach Timing Cover & Tach Drive x Commando?

pierodn · Dec 8, 2014

Hi.
Can it works a 750 Atlas Timing Cover & Tach Drive on 20m3 ........ 1968 early Commando engine?
Thank you.
Piero

commando6868 · Dec 8, 2014

They are not compatible. You can buy a new tach drive for the 1968 Commando from Andover Norton. I bought a NOS tach drive for my 68 Commando from Rabers of California, last year.
I don't know about the timing cover.

MF

madass140 · Dec 8, 2014

yes you can. the 68 used the Atlas engine which had the timing cover tach drive, the same parts.

Matchless · Dec 8, 2014

You need to check there is a drilling in the timing cover to feed the pressure relief valve. If the cover is the earlier type, you will need to plug the tapped hole in the crankcase with a 1/4 BSW grub screw or the oil feed will run into the timing chest. I'm sorry I can't show you a photo of this because it would make it easier to understand. Maybe someone else can provide pictures? The later set up returns excess oil to the feed side, where the earlier system returns it to the timing chest.
The rev. counter gearbox you need is 2:1 ratio to suit the Commando magnetic clock.

Martyn.

Stephen Hill · Dec 8, 2014

Tach drives are not the same Atlas to early Commando. The reason is one has a 2:1 head and the other a 4:1 head. I believe the info below is sound.
Stephen Hill

Reply with quoteRe: Norton Atlas Tach Question
by BillT » Mon Jul 25, 2011 11:17 am

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.

pierodn · Dec 9, 2014

madass140 said:
yes you can. the 68 used the Atlas engine which had the timing cover tach drive, the same parts.

Hi,
But the 68 Commando had the new head with oil pipe on the side and not on the top like Atlas?
The barrell had the larger studs thread or were smaller like Atlas?.
Thank you.
Piero

madass140 · Dec 9, 2014

maybe Piero, but to answer your original question, the cover will fit, just check what Matchless said about the oil feed
and the Tach drive ratio as what Stephen said.

Matchless · Dec 9, 2014

Piero,
The rev. counter gearbox you need IS 2:1 ratio to suit the 4:1 rev. counter. ALL Commandos used this ratio. Remember that the cam. runs at half engine speed.
Martyn.

dynodave · Dec 9, 2014

Atlas timing cover 25186 is for atlas 20M
Commando timing cover 25186 AB1 is for commando 20M3

20M3:
different from atlas to mention a few...
crankshaft
flywheel
barrels
head
crankcase both timing&drive side
timing cover and yes tach drive

Rohan · Dec 9, 2014

dynodave said:
Atlas timing cover 25186 is for atlas 20M
Commando timing cover 25186 AB1 is for commando 20M3

So how does 25186 differ from 25186 AB1 ?
Or are they 2 versions of the same thing....

We've seen Jim Comstock with a Model 7 timing cover on a Mk3 850,
so with some suitable slight mods they are all closely related ??

htown16 · Dec 9, 2014

Make sure the oil drillings match between the timing cover and case. I bought an Atlas cover to use on a very early 68 Commando engine and they did not match. Not something you want to mess up as the engine will go bang very quickly.

pierodn · Dec 9, 2014

dynodave said:
Atlas timing cover 25186 is for atlas 20M
Commando timing cover 25186 AB1 is for commando 20M3

20M3:
different from atlas to mention a few...
crankshaft
flywheel
barrels
head
crankcase both timing&drive side
timing cover and yes tach drive

Hi and thank you.
My timing cover has stamped 25186 WM with tach drive BG 1508/06.
The tach drive would be good for Commando (as Stephen Hill said before) but the cover???? It is not a AB1 as you wrote!
Ciao
Piero

acotrel · Dec 10, 2014

On most bikes the drive on the end of the cam is 2 to 1, and the tacho head is 4 to 1. However the direction of rotation is important. I use a Suzuki RG250 tacho and the direction of rotation can easily be reversed by simply undoing the two screws holding the gearbox onto the back and turning it over. The unit is similar to what was used on Italian racing bikes in the fifties, right down to the rubber 'O'-ring mount.