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H-D Aermacchi 7801P NUT, 10mm

What do we know about this nut?  On Aermacchi bikes, it is used a few times around the front wheel/brakes.  The parts catalogue call it a ‘7801P NUT, 10mm’ [1].

Figure 1: 7801P Nut, 10mm. Also shown is the thin jam nut, chain adjuster for 350cc Aermacchi.

The general shape of this nut is hexagonal, accepts a 10mm threaded shaft with a thread pitch of 1.0mm.  Normal threads for 10mm shafts are 1.5mm, so these are unusually fine threads.  The width of the nut is 17mm measured between the flats, and the thickness is 7.9mm.  Upon visual inspection, the chamfer is quite small so we think it is a 30-degree chamfer and the nut’s mating surface extends all the way to the flats on the outside of the nut.

The DIN 934[2] standard for M10 dictates the distance between the flats is between 15.73 and 16.00 max.   This nut is 17mm across the flats.  These were made before there was a DIN and ISO standards.  Freedom.

There are 2 markings on this nut.  There is a S9 and a F.  We asked users on the Aermacchi Enthusiast Group,[3] hosted on Yahoo, if they know what these markings mean.  The Aermacchi group user ‘The Alpha Wolf’ said S9 could mean ‘Standard 9’. Nut property classes[4] range between 5-12 , so 9 is quite a “tough nut”!

Another Aermacchi Enthusiast Group user, ‘Mark in Wisconsin’, had a very good suggestion for the F marking. F = Fontana.  Back in the 1960s Fontana made lot of nuts, and they also made brakes.  The ‘7801P NUT’ is found several times on the front and rear wheel components of the Aermacchi, used primarily on brake related bolts.  Perhaps Fontana made the brakes for these Aermacchi.  Today, Fontana is one of largest players in fasteners, with 4,100 employees and 19 plants in Europe/USA, making 10 Billion pieces per year[5].

This nut is used on 250 cc and 350cc chain adjusters.  Each adjuster has 2 nuts.  The first nut is used to adjust the tension on the chain and the second as a jam nut.  But, on the 350cc ‘REAR CHAIN ADJUSTER ASSEMBLY’, the 2 nuts are different thicknesses, but the parts catalogue says they are the same.  The Aermacchi Yahoo group confirms the nuts are 2 different thicknesses.  The second nut is 6mm thick, but is identical to the thicker nut in all other properties including the markings.

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

Because we are producing the CAD files and drawings for these parts, we need to come up with our own part name for this thinner nut.  We will call it ‘7801P#Thin#’ with a description of ‘NUT, 10mm, Thin’.  We will update this article when the 3D CAD and 2D drawings are finished, with links to these files.  Here is a small taste of the CAD, below. Note: All values are nominal values.

Figure 2a: 7801P and 7801P#thin# nuts, 10mm.

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

Figure 3: ‘7801P NUT, 10mm’ from Harley Davidson Sprint parts catalogue (April 1972)


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


[3] Aermacchi Enthusiast Group with 2439 member

[4] Advanced Mechanical Engineering Solutions article/calculator on nut property class


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