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The design of the frame changed in 1973. 1972-earlier used a heavy rough casting where the top tube and the vertical rear tubes meet.
The 1972 and earlier frame tubes are canted at an angle of 10° where the 1973 frame tubes are at an angle of 9°45'. Not a great difference but still different.
1973 did away with the casting.
And used what looks like lightweight welded strap metal with an angular bracing running from the rear engine mount up to the end of the fender supports.
But a greater change (apart from the casting elimination) is the distance between the head tube and the rear vertical tubes. 1972 = 28.42“, 1973 = 29.04”.
Even though the 1972 frame head had a greater rake angle (30°) the
1973 rake angle was less (29°35') the distance to the rear tubes is greater. ¹⁾
73-78 tanks are notched on the flange between the top bolt holes. This notch allowed HD to raise the tank position.
Raising the tank meant that both the upper and lower mounts also changed.
In 75, a ridge was added to the inside of the filler neck to retain the filler cap better.
The 73-78 bottom mount is taller for the raised tank. ²⁾