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Harley-Davidson 74 OHV Magneto Front Drive

    The OHV panhead factory racing motor (Catalina, hillclimb, TT, etc.) was equipped with an Edison-Splitdorf magneto driven from the generator position (similar to the WR). The magneto was available as a complete kit under 29501-49R, using the same magneto as the WR. Unlike the WR, the drive was composed of five separate and interconnected elements, four of them unique to this application:
      1. 21 tooth (std.) pinion gear (24010-39).
      2. 42 tooth idler gear (part of R0626-48)
      3. 12 tooth magneto drive sprocket (part of R0626-48).
      4. Magneto drive chain (R1831-48)
      5. 12 tooth magneto driven sprocket (R1805-48)
    The pinion gear drives the 42 tooth magneto idler gear at the correct ½ engine speed. The magneto idler has the same tooth count as the original ignition drive gear (25850-36) and the 1930-48 VL & UL flathead big twin idler gear 25775-30), but is not the same part.
    Since the magneto idler gear is the same size as the ignition drive it replaces the stud location and gear pitch distance in the right case and cam chest cover are not disturbed. The magneto drive sprocket is attached to the rear of the idler gear, and therefore also turns ½-speed. This ratio is continued by the chain to the driven sprocket, and rotates the magneto counter-clockwise at the correct speed.

    As can be seen in the drawing, various minor clearancing operations are needed to the interior of the right crankcase half. Click the picture for a much larger version on a separate screen.
    As in all magnetos of this type, ignition timing is fixed. There is no automatic (centrifugal) advance mechanism or retard feature for starting.
    There does not appear to be any method of timing the ignition advance without removing the cam chest cover, magneto, etc. This is unlike the subsequent WR front drive system, in which two vernier plates permit very small adjustments to be made through an access cover. However, this system had not yet been introduced in 1948.
    There is also a later kit (29505-50R) for the Wico magneto (29504-50R, which is the same as the WR) with different components. There are more parts listed for the magneto driven gear, which leads me to suspect that there may be an adjustable timing function built into these newer parts. I have no pictures or other details.

Harley-Davidson OHV Magneto Front Drive
Harley-Davidson OHV Magneto Front Drive

    Please note that these parts will not fit the 1937-48 U-Series motors, as the pinion gear tooth count is different (20 vs. 21), along with many other dimensional differences.
    In short, despite the advertising literature there is no “bolt-in” front drive magneto for the OHV big twin. The Morris etc. units that install in the generator opening require the original (or equivalent) drive assembly to function at all.
    For those with a suitable magneto but none of the original drive parts, the whole thing could be faked with no actual H-D parts since the important gear is the ignition drive, which is already the right size (42 teeth), or you could use a UL idler as a donor.
    You may be able to find an ignition drive gear with a broken spiral (which you don’t need, although it could be used to drive a tachometer from the original circuit breaker opening) and modify it. Add a small sprocket along side the tooth row, and the same size sprocket goes on the end of the magneto shaft - the size here doesn’t matter as long as they’re the same. The two pitch diameters plus the chain length must be very close to the pitch distance of the original gears though, or you won’t have a tension adjustment.
    The Norton twin uses a similar drive for the camshaft, and bits from it might work - 2 sprockets and a small chain, more info here: Norton twin chain drive.
    There were also other racing parts listed for the special OHV motor and frame:
  »  22017-48R Magnesium (Dow) piston, 7.5-1 compression.
  »  25522-51R Cam Gear; this is probably the “Victory” cam, part of the high-performance FLH option in 1955.
  »  61211-51R Right side oil tank.
  »  62502-50R Special TT 1-3/8” shorter oil tank “used when raising transmission 1” higher for more road clearance” (designed to be used in frame, below).
  »  Frame, described in Special Bulletin August 3, 1949: “same as Part No. 2801-40, except left side of front oil tank bracket is removed for transmission clearance”.

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