Engine strip

Information relating to the Matchless G12 or AJS Model 31 650cc twin
Peteg
Posts: 32
Joined: Tue Jul 11, 2017 9:00 pm
Location: YORKSHIRE UK

Engine strip

Post by Peteg »

Hi all
Bought a 63 650 csr none runner for restoring
I'm in the final stages of stripping engine and found deep gouge marks in chain case side con rod looks like it's been catching crankcase opening
I'm sure I've read some wear that con rod should have a chamfer on one side but neither of these look to have
A couple of questions
1. Does anyone have a photo of the chamfer
2. Are all twin con rods the same
3. Is it acceptable to fit one new con rod or should they be fitted in pairs
4. Should the oil scraper piston ring have the same gap as the other rings
Any help appreciated
JEAN-NOEL
Member
Posts: 601
Joined: Wed Oct 12, 2005 10:45 pm
Location: FRANCE

Re: Engine strip

Post by JEAN-NOEL »

Hello Peter,
2. No, conrods are the same in 500 & 600, but slightly chamfered on 650, because the stroke was increased from 72 to 79 mm. 650's conrods are suitable for 500 & 600. The chamfers must be oriented towards outside.
3. No new conrods are available (except some expensive substitute made to order) : I think it is easy to find one, or a set of second hand items from AMC parts dealers. But in either case i would recommand to have them checked for cracks, balanced, and he big end eye checked for ovality in a machine shop knowing well our twins.
4. Instructions manuals state the same gap for all the pistons rings.
kind regards.
Peteg
Posts: 32
Joined: Tue Jul 11, 2017 9:00 pm
Location: YORKSHIRE UK

Re: Engine strip

Post by Peteg »

Cheers Jean Noel
Do you have any idea what the chamfer looks like
zwarts
Posts: 514
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

Re: Engine strip

Post by zwarts »

Hello Peter. As you look at the big end eye of the rod you will see that the cap studs are retained by threading into trunnions housed in machined bores at the shoulder of the rods. It is on one side of this shoulder that the chamfer is located (on each shoulder of course). If using 500/600 rods in a 650, the chamfers are easily filed to shape.
Regarding replacement rods. There are secondhand rods around but many of them have problems with big end ovality which requires a specialist engineering solution to repair. Then there is the age of the rods to consider. Alloy rods do fatigue with age and cannot be expected to last forever. When/if a rod 'let's go' the consequences for the rest of the engine can be dire!
Fortunately there is a simple solution. New rods, of very high quality are available. Contact 'Thunder Engineering' and explain exactly what you require, they will quote and deliver within a very reasonable time scale.
They list the rods as being made for the G45 engine, but these are dimensionally identical to the other AMC twin rods. I fitted a set to a friend's G12 two years ago, they are almost too good to hide away in an engine.
Website is here
www.thunderengineering.co.uk
Cheers, Bob.
JEAN-NOEL
Member
Posts: 601
Joined: Wed Oct 12, 2005 10:45 pm
Location: FRANCE

Re: Engine strip

Post by JEAN-NOEL »

Hello Peter,
I agree with Bob. I think that in your engine the near side (drive side) conrod has been fitted wrong way...Yes conrods from "Thunder Engineering" are very nice, but quite expensive. I was in touch with them when I ordered several years ago a brand new crankshaft, as I wanted brand new conrods to suit. Finally, before becoming too old, in 2014 I carried my engine in the UK, to get a specialist's advice. As a result :
Derrick, from T & L Engineering said me :
1. your crank is perfectly true (it was grinded @ minus 30, big ends, minus 40 centre web)
2. Conrods are perfect (only the studs and nuts were replaced). From memory reaming the big ends (not necessary in my case) was quoted £ 25 each.
3. Boring + oïl tunnel O ring conversion.
My invoice was around £ 300 vith shells, and 1 roller bearing. A lot of money saved !
And I was money refund for my deposit for the new crankshaft.
To ride on the lanes Under legal speed limits I think that I made a good choice.
But if your crankshaft is in very bad condition, the option of a brand new crankshaft (waited on the shelves of AMOC parts) + Thunder conrods is perfect to have a bullet proof bottom end !
Cheers. J.N.
Peteg
Posts: 32
Joined: Tue Jul 11, 2017 9:00 pm
Location: YORKSHIRE UK

Re: Engine strip

Post by Peteg »

Thanks for all your replies I'm still hoping someone might have an old one even a scrap one they could photograph so I can get some idea how much to machine off
JEAN-NOEL
Member
Posts: 601
Joined: Wed Oct 12, 2005 10:45 pm
Location: FRANCE

Re: Engine strip

Post by JEAN-NOEL »

Hello Peter,
in 2013 Don Madden posted some pictures of 650 conrods (broken), but i am unable to find them...
The link was : http://www.jampot.com/forum/viewtopic.p ... 23#p111092
Just a thought : if there are no damages on the off side (timing side) conrod it is the good one ! So you can compare the two conrods to see what is wrong. If there is an important play, and a badly ovalised big end eye an interference with the crankase may be possible. Also a wrong conrod, or a conrod fitted wrong way (chamfer oriented towards Inside).
Cheers. J.N
JEAN-NOEL
Member
Posts: 601
Joined: Wed Oct 12, 2005 10:45 pm
Location: FRANCE

Re: Engine strip

Post by JEAN-NOEL »

Hello Peter,
Finally I have found the pictures you are looking for :
Go to "Photos" in Jampot.com
Menu Pictures.

1. Bikes. Classic Road Machine, page 25 - 16 sept 2013 - broken AMC rod

No chamfers on this side.

2. Parts pictures. Post war twins - page 2 - 30 jan 2011 - Carrillo twin rod vs std.

You can see the two little chamfers.

I hope these pictures help you !
Kind regards. J.N.
Peteg
Posts: 32
Joined: Tue Jul 11, 2017 9:00 pm
Location: YORKSHIRE UK

Re: Engine strip

Post by Peteg »

Hi Jean-Noel
I'm struggling to make out the chamfers I've added some numbers to the photo which one is next to chamfer
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JEAN-NOEL
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Posts: 601
Joined: Wed Oct 12, 2005 10:45 pm
Location: FRANCE

Re: Engine strip

Post by JEAN-NOEL »

Hi Peter,
the chamfers are only under"1" and "2", exactly at the level of the trunnion's center, where the width is maximum.
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