H-D Aermacchi 39984-69P, Rear Chain Adjuster Assembly Threaded Shaft

What is a ‘39984-69P#Shaft#, REAR CHAIN ADJUSTER ASSEMBLY Threaded Shaft’?  Does this keep you awake at night?

The rear axle adjusters of Harley-Davidson / Aermacchi 350cc street bikes, along with the racing bikes of 250cc CRS/350cc ERS are called ‘39984-69P, REAR CHAIN ADJUSTER ASSEMBLY’ [1].  The mysterious ‘threaded shaft’ is part of an assembly.  When welded to the ‘adjuster body’ it becomes the assembly affectionately known as 39984-69P.  These sub-parts that make up the ‘39984-69P, REAR CHAIN ADJUSTER ASSEMBLY’ are called:

  • 39984-69P#Shaft#, REAR CHAIN ADJUSTER ASSEMBLY Threaded Shaft, and this decodes into:
    • Title for the drawing/BOM: REAR CHAIN ADJUSTER ASSEMBLY Threaded Shaft
    • Part number: 39984-69P#Shaft#
    • File name of CAD files: ‘39984-69P#Shaft#, REAR CHAIN ADJUSTER ASSEMBLY Threaded Shaft. SLDPRT’
  • 39984-69P#Body#, REAR CHAIN ADJUSTER ASSEMBLY Adjuster Body, and this decodes into:
    • Title for the drawing/BOM: REAR CHAIN ADJUSTER ASSEMBLY Adjuster Body
    • Part number: 39984-69P#Body#
    • File name of CAD files: ‘39984-69P#Body#, REAR CHAIN ADJUSTER ASSEMBLY Adjuster Body.SLDPRT’

Naming convention details are located here.

[irp posts=”441″ name=”Naming conventions for Aermacchi / Harley-Davidson project”]

The shaft has a 10mm thread. The DIN 934[2] standard gives good parameters for this thread.   This nut is 17mm across the flats, and this does not follow DIN.  You can read more about the nuts, in brutal detail, on another article on this site called ‘Harley-Davidson Aermacchi 7801P NUT, 10mm’.

Figure 1: 39984-69P#Shaft#, REAR CHAIN ADJUSTER ASSEMBLY Threaded Shaft

For the casual observer, if you looked at the assembled part, you would not immediately think “hey, this is 2 parts”. But Figure 1 clearly shows a shaft and a body welded together.  From the side view perspective, you would be tempted to think they just butted the 2 sub-parts together and welded.  On closer examination, the ‘body’ has a 6mm diameter hole drilled in the side of it, to accept the 10mm threaded shaft that is machined down to 8mm diameter and then further down to 6mm diameter.  See the pretty picture in Figure 2, below.

The hole in the ‘adjuster body’ is clearly visible in Figure 3.

We will update this article when the 3D CAD and 2D drawings are finished, with links to these CAD files.  Here is a small taste of the CAD, below. Note: All values are nominal values.

Figure 2: Chain adjuster for 350cc Aermacchi with nuts 7801P and 7801P#thin#.

Figure 3: Chain adjuster body, with 6mm diameter hole that accepts a press-fitted shaft.

Figure 4: ‘39984-69P, Rear Chain Adjuster Assembly’ from Harley-Davidson Sprint parts catalogue (April 1972)

[1] Harley-Davidson Sprint parts catalogue covering 1962-1972 (Issued: April 1972)

[2] http://www.fasteners.eu/standards/DIN/934/

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