Understanding G80 Crankshaft End Float

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Understanding G80 Crankshaft End Float

Postby Iron Head » Wed Sep 26, 2018 7:36 pm

Apologies here for some potential stupid questions or lack of understanding as I haven't seen a G80 crank out of an engine in person.

Several books and forum discussions have talked about crankshaft end float and how it is taken up by tightening up the primary drive sprocket/shock absorber assembly (or 'sliding member' as I see the Ariel club call it with great mirth?!) My '48 primary sprocket/shock absorber is off and, from what I can see, all it bears against is the machined step on the crankshaft which, to me, makes sense.

I can't see how tightening up the sprocket/shock absorber pulls up end float unless what I am, in effect, seeing is the inner race of the outer crankshaft main bearing which would make some sense but would be a weird roller bearing with an extended (width) inner race protruding from the crankcase. However that concept also then brings into play how tight the inner race is on the crankshaft and the effect it might have on influencing crankshaft end float, or lack of, depending on inner race to crankshaft fit.

I'm sure I am probably missing something here in my understanding. I'm also interested in what (in an ideal) set up allows the end float and what moves in the system to provide it and would be very interested to hear from the experts.

Iron Head

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Re: Understanding G80 Crankshaft End Float

Postby clive » Wed Sep 26, 2018 7:50 pm

If your driveshaft is a sliding fit in the bearing (see the forum thread "crank case reassembly" where I have been whittering on) then the dished cap bolted on the end of it is the start of the process. The spring pushes the sliding cam against the drive (and dynamo sprocket) this should have a spacer behind it to hold the the whole lot against the inner race of the outer main bearing. I suspect what you are seeing is the spacer rather than a "weird roller bearing with an extended (width) inner race".
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if it ain't broke don't fix

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Re: Understanding G80 Crankshaft End Float

Postby Iron Head » Wed Sep 26, 2018 8:04 pm

Thanks Clive, so do I take it that in the other (very useful) thread Pharisee's second picture of his crank inserted into the L/H crankcase is showing spacer rather than crankshaft because I have exactly the same amount protruding from my case.

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Re: Understanding G80 Crankshaft End Float

Postby Pharisee » Wed Sep 26, 2018 9:46 pm

Iron Head wrote:Thanks Clive, so do I take it that in the other (very useful) thread Pharisee's second picture of his crank inserted into the L/H crankcase is showing spacer rather than crankshaft because I have exactly the same amount protruding from my case.

Iron Head


There is no spacer shown in the second picture. If you look at the top photo, you can see a slight 'step' in the axle diameter about 1/4" above the outer bearing. In the lower photo, that step is still visible. The spacer is about 1/4" thick and fits over that step and as Clive said, butts up to the inner ring. The outer diameter of the spacer is just a little smaller than the crankcase aperture. This photo shows the spacer in situ on the crankshaft.

18S_2793-small.jpg
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Re: Understanding G80 Crankshaft End Float

Postby Iron Head » Thu Sep 27, 2018 9:35 am

Many thanks Pharisee (and Clive) now I fully understand it! My spacer looks exactly like the picture but, with a bit of metal ‘flow’ from use, looked like part of the crankshaft.

Iron Head

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