Crankshaft Grinding

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Joined: Sat Sep 23, 2017 12:08 pm
Location: Paris

Crankshaft Grinding

Postby Jamesone » Sat Sep 23, 2017 12:40 pm

Hello, im David , i live in France ( sorry my english :-) ) and im trying to find a way to fix my matchless g12´s crankshaft . One of the crankpins is worn , the crankpin diameter is under the 3th regrind size 39,85 mm :-( .

Thanks for all !

David

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Posts: 364
Joined: Wed Oct 12, 2005 9:45 pm
Location: FRANCE

Re: Crankshaft Grinding

Postby JEAN-NOEL » Sun Sep 24, 2017 9:21 pm

Hi David,
you have 4 solutions :
1° to find a second hand crankshaft with journal diameter allowing a regrind (difficult).
2° regrind @ - 80 thou and use modified shells from Austin Mini.
3° have your undersize journal metal sprayed : T & L Engineering - Elstow (Bedford) have succesfully resprayed AMC crankshafts
4° get a brand new crankshaft from AMOC PARTS when a new batch will be available.
You may join the Club to have a full acess on this forum, to the Spares Scheme, and other services, including JAMPOT JOURNAL monthly.
There is a French section of AMOC : Mr Alan Smith - france@jampot.com
Kind regards. J.N.

Posts: 3
Joined: Sat Sep 23, 2017 12:08 pm
Location: Paris

Re: Crankshaft Grinding

Postby Jamesone » Tue Oct 03, 2017 11:25 am

Thanks for your answer Jean Noel,

It is possible to replace a G12 cranckshaft by an other model , like G8 , G9 ? . When i check the technical data in the book it seems that the same datas about the cranckshaft , right ?

The second solution sounds good , but i don't know if it is possible with my crankpin diameter ... around 39,95 mm

It is dangerous to regrind to 0.80 ?

In the MAOC Parts service i see many plain bearing ( central for the example) :

CENTRE MAIN BEARINGS .010 ( It is for the first regrind at 0.10 ? )
CENTRE WEB PLAIN BEARING; +020 ( It is for the second regring at 0.20 ? )
CENTRE WEB PLAIN BEARING; +030 ( It is for the third regring at 0.30 ? )
CENTRE WEB PLAIN BEARING; +040
CENTRE WEB PLAIN BEARING; +050
CENTRE WEB PLAIN BEARING; +060

Thanks for your support the team :-)

David

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Joined: Wed Nov 18, 2015 7:43 am
Location: wiltshire

Re: Crankshaft Grinding

Postby g5wqian » Wed Oct 04, 2017 5:52 am

hi david ;
G9 and G11 crankshafts are not same as G12 , the stroke length is different .

G9 and G11 are same stroke length but cylinder barrel for G11 is 7 fin and G9 barrel is smaller bore .

G12 barrel is 8 fin except some early G12 which had 7 fins and special pistons [ some 1959 models]

there are dynamo cranks and alternator cranks , the dynamo has short snout on the primary chain side ,with an internal threaded hole in the end and the alternator crank has a long snout and keyway with an external 3/4" thread on its end , to hold the alternator rotor .

if you have a tin timing case with a spring loaded cush drive and sprocket on the primary side it is a dynamo crank , if you have no cush drive and use an alloy chain cover and alternator then it is and alternator crank

some G12 cranks are cast and some forged i believe , the nodular iron cranks make a ringing sound if you tap them and the cast ones make a dull thud noise , i expect you are not worried which type it is if you want a spare crankshaft , although the nodular iron type is the preferred type .

maybe in the future the club will have new cranks available from the spares scheme or may have a number of them made up for members again ...

you can find used cranks sometimes but make sure you get the type you need , ie alternator or dynamo type .

you can use an alternator crank in a dynamo engine but need a dummy rotor/spacer and engine sprocket and spacer and breather etc .

i think T & L engineering could repair your crank .

i have had UPS send a crank for my AJS model 30 to me recently , 18kg i think it all weighed .

if you do send it for repair make a wooden box for it to be supported inside and roll it up in a lot of bubblewrap .

Posts: 3
Joined: Sat Sep 23, 2017 12:08 pm
Location: Paris

Re: Crankshaft Grinding

Postby Jamesone » Fri Oct 06, 2017 11:11 am

OK , thanks i think there was the same , for me the stroke length was determinate by the rod length

So for me the only solution is to find a other G12 GRANCKSHAFT or respray crankshaft by a specialist ? Right ?

Member
Posts: 364
Joined: Wed Oct 12, 2005 9:45 pm
Location: FRANCE

Re: Crankshaft Grinding

Postby JEAN-NOEL » Sat Oct 07, 2017 9:17 am

Hello David,
It is important to know what year was made your engine (not the 1rst registration but according your engine number). On a std G12 your crankshaft must be an alternator one, but material could be Mehanite, before 1962, or nodular iron on late models.
Yes it possible to regrind Under 0.080", using Austin Mini std bearing shells. Conrods need in this case to be slightly modified, as the tangs are wrong way on mini shells. Not very recommended on a Mehanite crank...
I agree with Ian. You can send your crankshaft to T & L to have it checked for radii, cracks, then metal sprayed and grinded. You may also send your conrods to check big end eye ovality, et c.
AMOC Parts have now in stock the thrust washers.
Cheers. J.N.

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