G5 Crankshaft Rebuild - SORTED!

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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G5 Crankshaft Rebuild - SORTED!

Postby Cosmic-Wizard » Sun Jun 10, 2018 10:19 pm

Good evening all :)

I hope you maybe able to help. I have just bought a Matchless G5 with a dismantled engine so this will probably be the first of a few questions.

I have reassembled the crankshaft with a new big-end and small-end in the con-rod.

Upon reassembly, I've noticed the con-rod isn't totally centre of the flywheels. The problem (if it is one) is the con-rod forging, it's running very close to one of the flywheels. Should the con-rod be fitted in a specific orientation? Am I correct that there should be a thrust washer either side of the con-rod between the weights? The parts list does say '2 of' but the exploded diagram shows one.

Finally, the rod does grab every now-and-then when turning slowly, it doesn't run tight, just a little grab. Is this something that will sort itself out while the engine is running-in?

Thanks in advance for any advice and I hope to start a rebuild thread soon.
Regards
James
Last edited by Cosmic-Wizard on Thu Jun 14, 2018 9:11 am, edited 2 times in total.

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Re: G5 Crankshaft Rebuild

Postby Ozmadman » Mon Jun 11, 2018 8:47 am

Hi

Normally a crank rebuild is a job for a professional workshop that has the required press to press the crank halves together, you cannot get way with just tightening up the large nuts in the flywheels they do need to be pressed properly together, maybe this is where your problem lies.
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Re: G5 Crankshaft Rebuild

Postby SPRIDDLER » Mon Jun 11, 2018 9:20 am

CosmicWizard wrote:Finally, the rod does grab every now-and-then when turning slowly, it doesn't run tight, just a little grab. Is this something that will sort itself out while the engine is running-in?

Welcome, James.
I've not worked on a L/W engine so will defer to those who know better with details. Certainly, as Ozmadman said, assembling the flywheels/big end is a specialist job.
However, in general terms the big end shouldn't grab or have tight spots. It's a hardened roller bearing so unlike a bush it won't ease when running in. If it does then you're wearing away the hardening on the rollers or the crankpin. :(
If the flywheel assembly is fitted in the crankcase could the tight spot be in the main bearing/bush?
I cannot imagine that a washer would be fitted only on one side of the big end assembly. Is there space on your flywheel/big end assembly for the second washer? The spares lists aren't infallible but if it shows two washers then I'd say two should be fitted.
If you're not already aware, Steven Surbey at AMC Classic Spares has much experience of L/W models and stocks one of the best range of L/W spares. He's not in business to spend time on just giving advice but is happy to help and supply parts.

http://www.amcclassicspares.com/
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Re: G5 Crankshaft Rebuild

Postby Cosmic-Wizard » Mon Jun 11, 2018 1:38 pm

Thanks for the replies guys :)

@ Ozmadman - the crank was put together and trued-up the correct way. I did read that about the nuts in the workshop manual. I can see why some would do that.

@ Spriddler - I thought the same regarding the number of washers and I know I've read a post or article somewhere that says not to forget fitting them. Can I find it now though? No! Lol

I was given a spare crank with the bike, but this too is in bits. However, there are another two washers. The only thing I keep thinking about is a keyway that is in the washer. I know one side of the big-end pin is keyed into the weight, the other side doesn't have a key, but there is no 'sided' washer listed, just 2 washers of the same number.

The grabbing has only occurred since fitting the other weight, I'm almost certain it's to do with the washers as the big-end bearings are a nice snug fit and rotate beautifully with the crank in bits. It's my first Matchless engine build, so just want to make doubly sure I'm putting the bits back in the right places.

Thanks again for your help :D

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Re: G5 Crankshaft Rebuild

Postby clanger9 » Mon Jun 11, 2018 9:26 pm

Hi James,

I'm a bit confused. What "washers" are we talking about here? To the best of my knowledge, there are no washers either side of the big end (and I cannot imagine why there would be...)
Untitled.jpg

Lightweight cranks are difficult to line up correctly. When the flywheels are pressed together, they can end up not-quite-parallel. The gap opposite the crankpin can close up slightly, possibly causing the nipping up that you've noticed.

I don't know of any solution other than getting a professional engine shop to build up the crank for you.
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Re: G5 Crankshaft Rebuild

Postby Paul R » Mon Jun 11, 2018 9:51 pm

I might be able to throw some light on this . The exploded drawing I believe shows a cast iron crank . I pressed one apart recently and there were no thrust washers in there . The steel crank for my M14 that I have just pressed together however does have the 2 thrust washers as mentioned . Both are cut away for a woodruff key but this is only used on the drive side . The other timing side washer is the same but only locates with the outside small flat section in a relief milled into the inner sde of the timing side pin to stop it revolving when the whole crank is pressed up . It is a fidddly job to make certain that is is correctly lined up before finally pressing both flywheels together . If not done correctly the washer will be slightly cocked over and may well be the cause of the rod catching when rotating . I may be wrong and other with more experience may well chime in shortly . If it helps I do have a G5 crank with a cream crackered big end that should have both washers still in there but would have to press it apart to see if they are in decent nick . Good luck

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Re: G5 Crankshaft Rebuild

Postby Duncan » Mon Jun 11, 2018 9:52 pm

Spot the difference:

Extract from LW engine part 4.PNG



These are usually referred to as thrust washers, certainly fitted on heavyweights, I have no experience of lightweights however but cant see why they would not have them as the fly wheel would be taking the thrust and wearing, they also keep the oil in the bearing.
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Re: G5 Crankshaft Rebuild

Postby SPRIDDLER » Mon Jun 11, 2018 10:10 pm

The 1960 Spares List shows two crankpin washers for the G5. Presumably they are the thrust washers either side of the big end assy and not for fitting under the crankpin nuts as in the exploded drawing no washers are shown under the crankpin nuts.

Lightweight flywheel assy 1960 (640x213).jpg


http://archives.jampot.dk/book/Spares_l ... s_List.pdf
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Re: G5 Crankshaft Rebuild

Postby Cosmic-Wizard » Tue Jun 12, 2018 6:43 am

Thank you for all your help.

@ clanger9 - I will have a look at the gap between the weights. I ran out of time at the weekend, but I agree, that is possible. Cheers :)

@ Paul R - Many thanks for your detailed reply. I'm happy now I know both washers are definitely the same. I will pull the weights apart and double-check the fitting of that washer. I was very careful when fitting it up, but I know how easy it is to disturb something with a moment's lapse in concentration. I'll let you know what I find.

I suppose a wonky washer could also cause an uneven gap in the weights as clanger9 suggested above.

@ Duncan - You echoed my thoughts there ;)

@ SPRIDDLER - This was another thought I had, but they are too big to sit under the crank pin nuts. If it had all gone together and ran sweet as a nut, I wouldn't have thought anymore about it. But, things like that woodruff key cutout play on my mind lol.

Many thanks again for all your comments and ideas, it's been a great help. I will keep you posted.

Cheers
James :D

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Re: G5 Crankshaft Rebuild

Postby clanger9 » Tue Jun 12, 2018 5:14 pm

Thanks also for the clarification regarding the steel flywheels vs the cast iron ones (shown in my post). It makes sense that the big end assembly would need thrust washers in this case. I think they're integral to the Alpha replacement big-end assembly, so I didn't notice them when I handed the parts over to the assembly shop...
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