Early and Later Centre Bearing Widths?

Information relating to the Matchless G9 or AJS Model 20 500cc twin
peterg123
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Location: Norway NORWAY

Early and Later Centre Bearing Widths?

Post by peterg123 »

Hi, I am wanting to build a G9 1953--without an engine....but can only find a 51 Crank and cases ( which appear to be in good condition). I have read that there is a difference either in the web or the actual centre main width between early twins and later, but more specifics were not given.

I'd like to know more about the years that are involved and if a centre web from a 55 Model 20 will work with a 51 G9 crank? Many thanks
Groily
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Re: Early and Later Centre Bearing Widths?

Post by Groily »

This is from the Technical Articles section on jampot.com, heading 'Twin Changes'. The key thing is the engine number, and the year of the change . . . 1951. So I guess your bits might be either sort maybe. Measurements etc aren't given here - maybe they are somewhere in the Archives, haven't scoured them to see.

"Engine changes after engine No. 7000:-

A changed crankshaft incorporates a new narrow centre bearing with separate thrust washers and a reduced centre web. A groove is machined in the face of the crankshaft to enable the use of a wedge-shaped extractor for pulling off the main bearings.
Cylindrical central cam tunnels fitted.
Oil deflector plates deleted from crankcase.
Oil pump had improved gears.
Crankcases modified to suit the above.
The pressure relief valve in the oil system, which was a spring loaded ball, is replaced with a plunger-type unit.
The non-return valve, which was a spring and ball, retained in the cap for the filter housing, by a circlip, is now fitted with a detachable cap to enable easy dismantling and also enables fitting of a pressure gauge.
The small timing pinion is reduced from 15/16" to 23/32" but given a spacing washer between it and the crankshaft."

I do not know whether the c/case changes were to accommodate the different web,the cam tunnels or any other / all of the above. Nor do I know if it's possible to mix 'n match with a bit of ingenuity . . . has the whiff of 'tricky' about it perhaps. But certainly there are differences, as you say. Good luck!
peterg123
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Re: Early and Later Centre Bearing Widths?

Post by peterg123 »

Thank you Groily...This engine is 51/G9/2226 and would this be a crank with the notches for a "wedge shaped extractor" that you mention? It's in USA but looks in good shape

I can't seem to upload an image so I have sent the link

https://www.ebay.com/itm/383610417468?h ... SwNK5e-S4j
ajsph
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Re: Early and Later Centre Bearing Widths?

Post by ajsph »

yes you can use the later web in the 51 crankcase, but the oil relief valve is different in 51and the crankcase release valve is also different in 51,and the cam follwers are also different, so it is not the best solution to use a 51 crankcase
Poul
peterg123
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Re: Early and Later Centre Bearing Widths?

Post by peterg123 »

ajsph wrote: Thu Nov 11, 2021 7:11 am yes you can use the later web in the 51 crankcase, but the oil relief valve is different in 51and the crankcase release valve is also different in 51,and the cam follwers are also different, so it is not the best solution to use a 51 crankcase
Poul
However if I have a real 51 crank and a pair or 51 crankcases, then a 55 wider Centre web would fit? The cam collowers are not a problem I think Poul they are not part of the "heart" of the engine
Groily
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Re: Early and Later Centre Bearing Widths?

Post by Groily »

peterg123 wrote: Thu Nov 11, 2021 6:53 am would this be a crank with the notches for a "wedge shaped extractor"
I honestly don't know Peter I'm afraid. It does seem to have recesses for a puller onto the inner races though, doesn't it? Whether they are for a 'wedge-shaped' weapon though I simply don't know. When I think 'wedge-shaped' I think 'knife-puller' type of thing, whereas the recesses there seem more like those used with a two-legged puller.
The only two 500cc (later) cranks I have are in engines that are assembled, so I can't do a visual comparison very easily. But if it's a '51 crank like the seller says, it presumably has the earlier centre main etc etc.
Someone here who's got models from the various years will no doubt be able to spot the differences quite easily ? ? ? ?
peterg123
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Re: Early and Later Centre Bearing Widths?

Post by peterg123 »

Thanks Groily....I do hope another member can chime in....someone whose got comparison.....
SPRIDDLER
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Re: Early and Later Centre Bearing Widths?

Post by SPRIDDLER »

ajsph wrote: Thu Nov 11, 2021 6:04 pm the crank in the pictures is not from a 51 it is later
Poul
Reply deleted to avoid any confusion. Pics were of the wrong year.

Thanks Poul.
The bike was registered in '52 but the dating cert says '51 so I guess the engine must have been later than the frame.
I took the pics and titled them during a pal's restoration but I didn't check the engine number. It has been sold now so I can't check the date on the c/case.
Last edited by SPRIDDLER on Thu Nov 11, 2021 8:06 pm, edited 5 times in total.
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ajsph
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Re: Early and Later Centre Bearing Widths?

Post by ajsph »

the crank in the pictures is not from a 51 it is later
Poul
peterg123
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Location: Norway NORWAY

Re: Early and Later Centre Bearing Widths?

Post by peterg123 »

Dear AJS PH, the vendor is offering this crank as from G9 no 2226 in USA, either a 51 or 52??? But you say this is incorrect? How does onetell it is from later....?? Vendor says it was from a running stock bike.
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