H-D Aermacchi 39984-69P, Rear Chain Adjuster Assembly
Rear axle adjusters come in many flavours. Today we are going to look at the Aermacchi Harley-Davidson adjuster that came on the 350cc street bikes, along with the racing bikes of 250cc CRS/350cc ERS.
The parts catalogue calls it ‘39984-69P, REAR CHAIN ADJUSTER ASSEMBLY’ . Easy, right? Well, this part is an assembly of 2 smaller parts. These 2 parts are welded together into an assembly we know as 39984-69P. These sub-parts need names, so let’s call them:
- 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.
The shaft has a 10mm thread. The DIN 934 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’. Someone got carried away writing about a simple nut.
Figure 1: ‘39984-69P, REAR CHAIN ADJUSTER ASSEMBLY’. Also shown is the normal and thin jam nut 7801P.
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: ‘39984-69P, REAR CHAIN ADJUSTER ASSEMBLY’ from Harley Davidson Sprint parts catalogue (April 1972)
 Harley Davidson Sprint parts catalogue covering 1962-1972 (Issued: April 1972)