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Early Commando tach drive ratios

Discussion in 'Norton Commando Motorcycles (Classic)' started by Stephen Hill, Jan 10, 2012.

  1. Stephen Hill

    Stephen Hill

    Joined:
    Oct 28, 2009
    Someone has a Smiths tach drive BG1508/06 for sale. Says it is a 1:1 ratio and correct for early Commando.

    I thought that the BG1508/06 is correct for the early Commando, with a 2:1 ratio.

    I thought the BG1508/05 is correct for Atlas, with a 1:1 ratio.

    On this site, in archives, one post says that the BG1508/06 is 2:1, another post says that BG1508/06 is 1:1.

    Can somebody please shed some light on this?

    Stephen Hill
    Victoria, BC
     
  2. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
    The BG1508/06 tach drive gearbox for the '68 Commando is 1:1 ratio (Reversing) according to the Smiths list, however, the tach listed for that year is RSM 3003/10 4:1 ratio which would require a 2:1 drive gearbox.
     
  3. Norton-Villier

    Norton-Villier

    Joined:
    Dec 4, 2009
    I have a smiths motor cycle equipment book 1962 - 1975 It shows a pic of the gear box Below it states BG 1508 Series (Replacing 70549 Series)

    These gear boxes reverse the rotation of the shaft to which thay are attached
    The rotation is determined by looking immediately on the end of the engine
    spindle with which the gear box or flexible drive is ingaged

    Plain phosphor bronze bearings Ratios 1:1 and 2:1

    Phil
     
  4. Stephen Hill

    Stephen Hill

    Joined:
    Oct 28, 2009
    I'm still confused.
    If the correct tach drive for the '68 Commando is a BG1508/06 1:1 ratio
    And the 4:1 tach calls for a 2:1 drive,
    WTF.
    Stephen Hill
    Victoria, BC
     
  5. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Joined:
    Nov 20, 2004
    There is the possiblity that the BG15806/06 ratio was listed incorrectly as 1:1 (I think the RSM3003/10 is 4:1)?

    Perhaps our 20M3 Commando owning members can add some information?

    Is your tach (RSM3003/10?) marked 4:1?

    Edit:
    http://www.burtonbikebits.net/Rev-Count ... rboxes.htm
    http://www.britishonly.com/webcatalog/15.txt
     
  6. frankdamp

    frankdamp

    Joined:
    Oct 7, 2005
    On the two prototypes we had trouble with the wrong gearing for the speedometer drives, but the tachometers were fine. The speedos showed 1.6x actual road speed, so were off the top end a lot of the time we were riding (actual 75 mph showing as 120).
     
  7. Ron L

    Ron L VIP MEMBER

    Joined:
    Feb 27, 2004
    All Commando tachometers are 4:1 ratio. All P11/N15/G15/Atlas tachometers are 2:1 ratio. BG1508-06 tach drive is 2:1 and used for 4:1 tachometer. BG1508-05 is 1:1 and used for all 2:1 tachometers.
    Tachdrives are driven at cam speed (1/2 crank speed) so final ratio must be 2:1, thus a 4:1 tach needs to be reduce by a 2:1 drive, but a 2:1 tach is driven by a 1:1 drive.
     
  8. Bluesman3000

    Bluesman3000

    Joined:
    Mar 12, 2012
    Five years later...restoring my 1968 Commando, I found that the tach was reading very high. The drive gear on the bike was the BG1508/05, a 1:1 drive, while the tach is a 4:1. I got the correct 2:1 drive gear from Andover Norton and now it reads correctly.
     
  9. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    If the Tacho read-out is 4:1 the drive ratio off the camshaft must be 2:1, because the ration between the crank and the timing gear is 2:1.
    I recently got involved in a debacle over a tacho for a racing Triumph. I offered my friend a 4:1 tacho off an RG250 Suzuki which is what I use with my 850 motor. There are two ratio Triumph tacho drives - the one which fits the generator hole is 1:1, the one in the '59 Bonneville timing cover is 2:1.
    So two different ratio tacho read-outs are used at the handle bars. My friend wanted to use the drive in the generator hole, but he needed the 2:1 drive. An answer would have been to fit an early Commando 2:1 drive onto a standard timing chest , driven off the exhaust cam to use the 4:1 read-out, but it became all too expensive.
    All he has done is buy a cheap 2:1 read-out and a flexible mount intended for a manx Norton - problem solved !
     
  10. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    If you are changing the tacho read-out. the direction of rotation is important. The advantages of the RG250 Suzuki tacho are that the direction of rotation can be reversed by simply undoing two screws on the back and turning the helical drive over, and vibration won't kill it. The tacho read out is usually much cheaper to change than the drive mechanism at the motor.

    [​IMG]
     

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