Model 20B Crankshaft ID

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Joined: Fri Jan 01, 1999 12:00 am

Re: Model 20B Crankshaft ID

Postby matchless » Thu Dec 13, 2018 1:53 am

I do not have a direct answer for you, but can elaborate on the variety of 500/600 cranks that I have found.
In particular, the first image you posted is of one of the very first twin cranks (1949 - 1951). In 1952 they went o a heavier crank with a narrow centre journal.
I have always assumed the crank with the round outer flywheel was the '52 type? But reading your post it makes more sense that this, heavier crank, would be used with the heavier 550cc pistons.
In Australia, I have found that most of the 500cc cranks from 1952-on, had the triangular styled outer flywheel.

In summary:
1949 - 1951 : Lightweight Crank using lightweight section conrods
1952 - 1955 : Crank with triangular outer flywheels
1956 - 1957 : Heavier 550 crank (possibly with round outer flywheels)
1956 - 1958 : 600 crank (same as 500 crank with different balance factor of 60%)
1959 - 1962 : 500 crank with or without alternator spigot.

P.S. I note that the G45 crank is similar in weight to the early 49-51 cranks.
This may not be 100% accurate, but it shows the variety of cranks available. You must also remember that there are at least 4 different sets of con rods (all with the same part number) and these must be matched to the crank as well.


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