Crank

Information relating to the Matchless G9 or AJS Model 20 500cc twin
Stuart Jenkins
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Posts: 50
Joined: Fri Mar 16, 2012 6:18 pm
Location: EAST SUSSEX UK

Crank

Post by Stuart Jenkins »

Hello All,
I've had an AJS m20 engine in storage for a number of years, and recently dug it out to see if any bits are salvagable for my next project!
It was all loosely together and the engine cases say it's a '52, but of course it could be a right bitsa thrown together!
The crank is in general good condition, with what looks like a fresh grind to -30 and the main bearings seem unused. My concern is the drive side spline area as there are chips/damage to the end and wondering if the shaft is shorter than it is supposed to be? Is this the case?
Thanks
Stuart
IMG_2660.jpg
IMG_2662.jpg
IMG_2664.jpg
IMG_2658.jpg
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g3l 1941
BMW r80G/S '82
Yamaha FZ750 '90(Project)
Groily
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Re: Crank

Post by Groily »

Looks a bit 'ooer' to me. Here's a pic of a '53 crank, G9/M20, in engine.
You can see there's a bit more there (4.75cm outwith the c/case, oil flinger & seal mod etc in place). And the end of the thing isn't moth-eaten.
Whether you have enough there to be able to clean up and re-use it safely I don't know. Cush drive AMC clutch maybe, and lock the sprocket up solid? Be a shame not to be able to something with it if the journals and radii are decent . . .
Your splines taper away a bit differently from mine, which may just be the model year (can't swear mine's truly a '53, just that the cases are.)
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SPRIDDLER
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Re: Crank

Post by SPRIDDLER »

Here's a couple of pics of a '52 Model 20 crank if it's of any help.

Crankcase assembled with con rods.jpg
2018-04-26 Engine assembly 012.JPG
2018-04-09 002.JPG
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Stuart Jenkins
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Joined: Fri Mar 16, 2012 6:18 pm
Location: EAST SUSSEX UK

Re: Crank

Post by Stuart Jenkins »

Thanks Guys,
Yes, It would be a shame not to use it as the journals are nice and clean with radii all present.
After a bit of digging I'm wondering if it's from a later model as the outer webs aren't round but wedge shaped? The diagram from a '55 seems to have a shorter shaft.
Spriddler- very nice, do you want to swop it for a box of rusty bits?! :lol:
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g3l 1941
BMW r80G/S '82
Yamaha FZ750 '90(Project)
SPRIDDLER
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Re: Crank

Post by SPRIDDLER »

Stuart Jenkins wrote: Thu Oct 21, 2021 6:47 pm Spriddler- very nice, do you want to swop it for a box of rusty bits?! :lol:
Sadly not mine, Stuart. Every bit was done in his shed by my chum Resto Ray, turning this incomplete example in about 4 months.....
2017-11-14 Rays 1952 model 20 001 (640x480).jpg
into this........
Finished (640x480).jpg
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Mick D
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Re: Crank

Post by Mick D »

Hi

I have a 53 G9 in kit form at the moment and can confirm my spline is identical to that of Groily.

Regards Mick
Stuart Jenkins
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Posts: 50
Joined: Fri Mar 16, 2012 6:18 pm
Location: EAST SUSSEX UK

Re: Crank

Post by Stuart Jenkins »

Thanks Mick,
Has anyone out there got an amc gearbox fitted to theirs, does this eliminate the need for the drive cam and spring?
Cheers
Stuart
g3l 1941
BMW r80G/S '82
Yamaha FZ750 '90(Project)
56G80S
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Re: Crank

Post by 56G80S »

I know this is about a twin not a single but I did have an AMC box and clutch but retained the s/absorber spring etc.

Be aware that for singles with Burman / AMC boxes there is a difference in thickness between the spacer on the drive side axle inboard of the s/absorber arrangement depending on which box is fitted. Otherwise there is a small impact on primary chain alignment.

Others will comment if this is appropriate for twins, of which IKBA.

Johnny B
Mick D
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Re: Crank

Post by Mick D »

Hi

The major change was triggered by the move from mag ignition to coil. The shock absorber was replaced with a rotating magnet for the alternator and the primary drive shock absorbing function was moved to the clutch via rubber blocks.

Can't help with the dimensions of an alternator axle but I'm sure others can.

Regards Mick
Groily
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Re: Crank

Post by Groily »

And I'll just add I ran a 500 dynamo crank engine - the '53 G9 motor in the earlier pic in fact - without the shock absorber spring, with an AMC box & clutch with cush drive, for ages in what was meant to be a '61 M31. It was fine, just had to make a spacer to lock everything solid, while retaining the standard breather etc.

The complication may be the outer chaincase though.
I have a modified 'non-alternator non s/absorber' alloy one for an AMC box that will go over a dynamo crank, with or without the s/absorber functioning. See pic. The chaincase is still in service, but with the correct 650 motor, dynamo version, being used. Matching numbers bike - jeez, hard to believe round here! I call the whole thing my 'Farmyard Special' because it suits me, if no-one else.
I haven't tried to graft an AMC clutch onto a GB52 Burman though so not sure what other messing around that would entail to get the chaincase to fit.
(The extreme cobbled-up rearsets in the pic are long gone, they were good until I got to about 60 and went all creaky, but the Norton flat bars are still there.)

I think the club has the alternator extension shafts available, or did. I had one, but let it go to a deserving home with a reground pre-noddy 650 crank that T&L pronounced 'good' but I didn't need any more owing to having a couple of Club new ones, Praise Be.
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