Harley-Davidson Aermacchi frame tube sizes

During the process of reverse engineering Aermacchi CRTT, SS, H, early CRS, late CRS and ERS, we kept track of the common tubing sizes used on the frames.  The result is the handy table and diagrams, below.

Take a look at the first column of the tube thickness table and you will see the letters A-L, and these letters correspond to the letters on the CAD frame rendering. If you scroll further down this article, you will see all the major frames have a computer rendering of the frame, and a key on the picture to show you which tube it is, in the table. Please note we make no warranty to the accuracy of this tubing gauges, use “as-is” and with no warranty expressed or implied.

The biggest problem we had was figuring out the wall thickness.  I have an ultrasound machine that is very accurate, to 0.010mm (0.0005 inch). Ultrasound transducers are designed to measure materials that a flat on both sides, and a tube is exactly the opposite of flat.   Some of the tubes had various layers of paint, and that complicated it quite a bit.

The most challenging tubes to measure were the small tubes.  Smaller tubes have greater curvature, therefore exacerbating the problem with the ultrasound machine.  The smaller the diameter of the tube, the harder it was to get a reading – and sometimes I could get no reading at all.  In the table, there is a column labelled “As-built (mm) Wall”.  This is the data reported by the ultrasound device, often rounded for convenience.  If I measured the same tube in different places or on different bikes of the same model, I would get a range of readings.  So please take the data I have measured as ‘indicative results’ and not ‘absolute results’ that can only be obtained by destructive methods (like drilling or cutting tubes and taking physical measurements).

There is an Ebay seller of Aermacchi parts called Wilto54 (http://www.ebay.com/usr/wilto54) that I have purchased from in the past.  I sent him a message to ask if he had any scrap frames that he could saw through the backbone, so I could get a definitive measurement of wall thickness.  He agreed and a few days later he said a 1972 350cc had a backbone wall thickness of 0.18inch (4.572 mm).  That is off the scale!  The thickest wall I measured with the ultrasound was 4mm (0.157 inch), and I thought that was excessively thick.  Many custom builders today use DOM 0.120 inch (3.0mm).

We have one CRS that had the seat rails cut off it, so we could measure the tube thickness with a gauge.  It turns out to be quite thick, at 3mm.

In the table, there is a note regarding the 350cc swingarm.  In that model, they put one tube inside another tube, and then pressed it into the final shape.  We will show more details on this feature in a future article. In the photo of the below, you can see the difference between the 250 and 350 swingarm.

Here is a list of all the frames we measured with our 3D scanner and ultrasound thickness gauge.

The example frames are below.

We do hope this helps with frame builders and restorers of Aermacchi and other classic bikes.

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