600 Twin Problems

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Re: 600 Twin Problems

Postby saltbox alf » Sun Aug 25, 2013 5:42 pm

Looks like you have found the answer :) but I would replace both pistons as a precaution, they should be matched. If you can get a single piston it would have to be the same type/manufacture and you would need to make sure the weight was the same.
With new rings you need to make sure there is no ridge at the tops of the barrels, other wise your top rings will break, so if there is a fine ridge a hone will hopefully clear it up. If not a re bore will be called for!
There used to be a tool for removing ridges, your friendly engineering shop might still have one hidden away under the bench if it proves necessary to do.

When you get to put it together make sure the threads are clean and any tapped holes are cleaned out too. other wise you will be tightening up on a lump of groc and not get a proper torque setting.

Yep an air leak would cause it to run weak on that side , lucky the piston did not melt it's crown.
Allons-y, amis de Matchless et AJS

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Re: 600 Twin Problems

Postby kbs1240462 » Fri Sep 13, 2013 4:17 pm

Deep joy!!

I stripped all the engine down as I detected some "float" on the big ends - upshot is wear on the crank pins and they measure over 60 thou under already.

So has anyone had experience of metal spraying as a reclamation process for these cranks and has anyone used a company in the Midlands (Hereford/Worcestershire border).

Regards,

Kevin

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Re: 600 Twin Problems

Postby SPRIDDLER » Fri Sep 13, 2013 5:05 pm

Loads to be found on here Kevin if you enter the search words Metal spraying in the Forum 'Search' facility, e.g. the thread below is one of many.......

viewtopic.php?f=11&t=7753&hilit=metal+spraying
Last edited by SPRIDDLER on Fri Sep 13, 2013 7:42 pm, edited 1 time in total.
I poke badgers with spoons.

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Re: 600 Twin Problems

Postby MartinAmes » Fri Sep 13, 2013 6:12 pm

An alternative is to buy a second hand crank with less wear.
Have a look at:
http://www.amcclassicspares.com/
on the second hand spares page.

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Re: 600 Twin Problems

Postby kbs1240462 » Sat Sep 14, 2013 5:47 pm

Hi ,

Having read a number of the technical articles I see that Mini shells have been used in the past on crank regrinds up to 80 or 100 thou undersized - does anyone have any experience of this?? It looks like it might be a better way than metal spraying? I would also think that hard chrome plating would not be an option for me seeing that the journals are already 60 thou over and one of these has "picked up".

I would value people thoughts.

Kevin

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Re: 600 Twin Problems

Postby 1608 » Sat Sep 14, 2013 9:50 pm

I should have thought metal spraying was a better as you will not lose any of the strength. Going to -80 or -100. will just make the crank even weaker. They do turn up on e-bay occassionally, I bought two on standard grind last year. There may even be more available in the near future what with the new ones seemingly rather popular.

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Re: 600 Twin Problems

Postby Groily » Sun Sep 15, 2013 5:28 am

The new ones are for 650s only (with their longer stroke), but 2nd hand 500/600 ones are out there. I ran a 500 at -80 for several years a long time ago without mishap, and have known others do the same, even on far more powerful 650s. However, after I'd had it metal-sprayed to standard, it promptly snapped owing to the radii on the crankpins having been ground off by the people who did the work. If it were me, I'd start by getting someone competent to look at the radii on yours Kevin to see what state they're in. If they have disappeared, you've got a large metal doorstop. If they're good, then either you could go down another size, or you could use metal spray. Metal spray doesn't restore strength though as far as I understand things - it just gets you back to a larger diameter. T&L Engineering, who advertise in the Jampot, have been metal-spraying these things with success for some time they say. A second hand one might be the best option if you can get one near to standard to recondition. The only difference between the 500/600 items is in the balance factor so there ought to be a good few out there that would serve. Good luck, Bill

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Re: 600 Twin Problems

Postby kbs1240462 » Sun Sep 15, 2013 7:39 am

Thanks guys,

I have some considerable concerns on metal spraying
1. The journals have to be ground to roughen then up to provide a "key" for the metal spraying - this from a stress raiser on the crank is not a good idea and may explain why some of them break. Also as already said I will need to ensure they do not grind out the corner radii for exactly the same reason.
2. Metal spraying involves heat and again this gives opportunity to introduce stresses and the issues that go with those.
3. After metal spraying a stress relieving heat treatment would seem to be a good idea and what does that do to the life of the crank?

It seems to me another crank - does anyone have one? Or going down to what is likely to be -90 would seem to be the way to go. I would anticipate that provided the work is done well on going down further on the crank pin size then there should not be to big an issue. Given that I do not thrash the bike then I am sure there will still be many years of life in the crank, and in the meanwhile if I come across a better crank I can go that way later.

Kevin

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Re: 600 Twin Problems

Postby Don Madden » Mon Sep 16, 2013 4:50 pm

Another possibility is to convert to 1958 G12 specification. The only differences from the G11 were the long stroke crankshaft & special pistons with higher location of the gudgeon pin to offset, cylinders & heads are the same. Carb will have to be re-jetted, too. You may find some of the original or Robbins pistons still about as many ordered them for later engines, then found they did not fit. You may not have any cheap options to get it going. Cheers, Don. 8-)

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Re: 600 Twin Problems

Postby kbs1240462 » Sat Oct 12, 2013 4:06 pm

Hi,

So I have sourced another crank (thanks Alex), big ends 40 thou' and good - so life left there. Centre main is 10 thou' - which is the bit that gets interesting. AMOC spares do not have, and have not have for some while, 10 thou' shells!! So, given I do not fancy having the centre journal ground to 20 thou' when it is perfectly good at 10 thou', a couple of questions:
1. Does anyone know where I can source 10 thou' shells for the centre bearing?
2. Anyone have experience of standard shells being opened up to 10 thou' as I have access to some very good machinists who could do this work.

Other than that everything ready for the engine rebuild.

Thanks an anticipation,

Kevin

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