1959 650 rebuld help

Information relating to the Matchless G12 or AJS Model 31 650cc twin
SOHC
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1959 650 rebuld help

Post by SOHC »

Hello, I am rebuilding a 1959 650 twin for a friend, I have not done one before as I have only ever worked on the AJS/ Matchless singles, anyway the bike was sold to him as having a rebuilt motor but when I pulled it apart it had

new +60 pistons in old bores, has about 001" lip

Freshly reground crank but it looks to me like it has BSA 500/650 rod bearing shells as they have an oil hole in the top and bottom, they seem to have alot of clearance like 003"? I hope they haven't shaved the caps to make them tighter.

I noticed the crank was hard to turn by hand with all timing gears, head, barrels removed, I loosened all the nuts on the center main support and let it center it self in the cases then nipped the bolts up but when I bolt the crankcase halves together it becomes stiff and hard to turn, I think the LH case is too tight and is rubbing on the side of the crank.

What would be the best way to look at fixing the crank case problem?
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Rob Harknett
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Re: 1959 650 rebuld help

Post by Rob Harknett »

Are you referring to a lip at the top of the bore ? This should be removed.
SOHC
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Re: 1959 650 rebuld help

Post by SOHC »

Rob Harknett wrote:Are you referring to a lip at the top of the bore ? This should be removed.

Yes, it has been honed by the last person but this didn't remove it,

it would be impossible to totally eliminate it with out a rebore but I don't think I can find pistons bigger than +60?
TrevorCSR
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Re: 1959 650 rebuld help

Post by TrevorCSR »

Sound like you have a lot of work to do.
001" is NOT a lot. First thing to measure is the ring gap at the top of the bore where the rings do NOT go. Then compare that with the gap where the rings DO go. You'll see the lip, where the pistons (and rings) change direction. Simple arithmetic will indicate how much the bore is worn. Not unusual to fit new pistons in 'slightly' worn bores. Definitely no need to remove the lip if it's only 001".
+060” is probably maximum you'd want to bore, and what little wear you have will last ages, maybe longer than you'll ever use the bike. In the day we'd just get it bored and fit new pistons as they were readily available. Not so much the pistons, the barrels are the valuable parts.
Crankshaft and cases need careful inspection. As you have the rods off, try re-assembly and check for binding anywhere.
Assume nothing on these engines that have had 60 odd years of abuse, both on the road AND in the hands of ‘mechanics'. Lots of ‘bodgers' out there, just get it running and sell it on with a profit, KNOWING it's bad.
SOHC
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Re: 1959 650 rebuld help

Post by SOHC »

Cool thank you, I understand the bike was rebuilt a long time ago by a bodger, I measured the crank today and it has been freshly reground a fraction too small for -030 bearings witch is a pain but I might be able to get some -040s

I have been playing around with the crank in the cases and found the inner part of the main bearing on the timing side rubs on the case, I gess I can have that releaved a fraction?



One big question I would be happy if someone could awncer is on the drive side case there are 2 plugs for the ends of the cam bushings and the front one has that thing they call an oil distributor that drives off the end of the cam, SHOULD THERE BE TWO OF THEM?
Groily
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Re: 1959 650 rebuld help

Post by Groily »

Crank can go right down to -60 if it has to and if the radii are good.
Barrels can be resleeved to standard if they're really worn, but it sounds as if they'll still serve for the moment.
Not sure why the main bearing inner race should foul, cos it shouldn't of course. Is is going fully home on the shaft? If it isn't, could it also be responsible for tightnesss of assembled bottom end? But it can take several dry runs to get the centre web etc all set nicely and the crank spinning easily. I think new thrust washers are also now available from the Club, which might help too?
If the machine has been run with sloppy big ends or if the PO has messed with the rods in any way at all (as in shaved the caps maybe), I'd have the rods' eyes checked for ovality (and invisible cracks). It's a point of weakness anyway, and a potentially catastrophic/expensive one. I've found it's a 50:50 whether they are true.
There should be the oil distributor thing on the end of the exhaust cam (two sorts, one with a flat, one with a hole, and interchangeable), and there's nothing but the blanking plug on the inlet side. The reason there's a gap between the inlet and the bung is that the earlier 500s had a breather driven off that camshaft, which was then ditched when breathing was done via the crankshaft on the drive side.
All depends how much you want to put into this engine - it can end up quite expensive to get a messed up one right. But it's worth it I think, as they're lovely engines when done properly and they can hang together for a good long time.
SOHC
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Re: 1959 650 rebuld help

Post by SOHC »

Groily wrote:Crank can go right down to -60 if it has to and if the radii are good.
Barrels can be resleeved to standard if they're really worn, but it sounds as if they'll still serve for the moment.
Not sure why the main bearing inner race should foul, cos it shouldn't of course. Is is going fully home on the shaft? If it isn't, could it also be responsible for tightnesss of assembled bottom end? But it can take several dry runs to get the centre web etc all set nicely and the crank spinning easily. I think new thrust washers are also now available from the Club, which might help too?
If the machine has been run with sloppy big ends or if the PO has messed with the rods in any way at all (as in shaved the caps maybe), I'd have the rods' eyes checked for ovality (and invisible cracks). It's a point of weakness anyway, and a potentially catastrophic/expensive one. I've found it's a 50:50 whether they are true.
There should be the oil distributor thing on the end of the exhaust cam (two sorts, one with a flat, one with a hole, and interchangeable), and there's nothing but the blanking plug on the inlet side. The reason there's a gap between the inlet and the bung is that the earlier 500s had a breather driven off that camshaft, which was then ditched when breathing was done via the crankshaft on the drive side.
All depends how much you want to put into this engine - it can end up quite expensive to get a messed up one right. But it's worth it I think, as they're lovely engines when done properly and they can hang together for a good long time.

Are they making reproduction bearings? the crank radius are questionable, I don't know much about that but I would have liked to see more than this, Yes inner race is fully home but rubs up tight on the case, I think if I removed 020" from the inner part of the case it would be happy. crank seems true,

I will have the rods crack tested and honed if possible, I can see someone has been rubbing the mating face of the cap with sandpaper.

Everyone tells me they are a horrible engine full of problems with bad vibration, but I don't see why it should fail if I fix all the problems witch the last guy installed
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ajscomboman
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Re: 1959 650 rebuld help

Post by ajscomboman »

New shell bearings for the crank and rods are available from the club. Once the engine is sorted there should be no major issues at all. Contrary to what you've been told these engines are very usable, reasonably smooth, certainly no better or worse than Triumph or BSA but certainly better engineered. I've a G15 twin a Norton Atlas engine and it's a crude bloody engine and a bugger to work on.
Groily
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Re: 1959 650 rebuld help

Post by Groily »

I bet the big end eyes are out-of-round then, and need cleaning up. Not worth sucking and seeing on this, you're right. New rods can be had in the last resort (but not a cheap option inevitably).

If the radii on any of the journals are 'questionable', it will really pay you to have them checked. The dimensions are absolutely mission-critical here (the Club publishes them, or used to, in the Spares Lists) - because lack of radius or damage to them can lead to breakage. Radii beyond limits = no go.

It does all sound scary, but as ajscomboman says these are great engines when sorted. They're not at all horrible - that's a bar-room myth. Very nicely made, long-lasting and reliable, not too many vibes despite high revving ability compared to almost all contemporaries, cranks don't break often (contrary to folk-lore), the oil can be mostly kept on the inside, and the top-end design is elegant (ask any pre-unit Beesa or Norton-engine owner without double-jointed fingers about that!) Separate barrels and heads aren't maybe ideal at 650cc, but work OK - and more oil-tight than a typical large capacity Royal Enfield with similar separate bits. But the bottom end is a one-off, yes. A bit more complicated than your average, but the benefits include equal oil supply to the big ends, no crank end-float issues thanks to the centre main and its thrust bearings, no bronze bush wear issues on the timing side, big rollers both ends which seldom wear. Excellent quality castings. Much under-rated I have always argued, but then I and other long-term owners here are naturally a bit biased!
SOHC
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Re: 1959 650 rebuld help

Post by SOHC »

Thank you very much for the enthusiasm and encouragement, I took the parts to show a couple shops, everything is bad they sed, but it might be possible to repair, re sleeve, submerge ARC weld crank and hone the rods


I need to send some info on grinding the crank to the shop, is this book ok for an AJS? https://oldthumpers.files.wordpress.com ... manual.pdf

It has all the info on the rods and crank but its for a Matchless
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