1 of 50 made worldwide

[Ixion]

AMC (AJS/Matchless) owned Norton,not the other way around,so as a way to update the AJS/Matchless line Norton motors were fitted to Matchless frames,the scrambler version was pretty common,because the Norton frames were no good for off road work,there was one on TradeMe not long ago with no takers.A factory Cafe version of the same bike would be pretty rare.

'Twas mainly cos the AJS/Matchless 650s kept snapping crankshafts. Whereas the Norton 650/750 mill was (despite all logic) reasonably robust. There was a hybrid cafe racer (Atlas engine) , though I'd agree it would be rare in this country. The one of Trademe, I don't think was a factory hybrid it was a AJS 650 frame that someone had dropped an early 50's Norton 600 twin engine into. Prolly when the AJay blew up.

[Motu]

The one on TradeMe was a piece of crap alright.

Pity about the AMC twin,it was a beautiful motor,and the 650 had the most stump pulling grunt of all the twins - but is as often the case,by doing it right,they got it wrong.

The British Twins - there were six of them,BSA,Triumph,Norton,AMC,Ariel,Royal Enfield,any more? - were often critisised for their big central flywheel and two main bearings.AMC with the identical AJS/Matchless twins fixed that by having a centre main bearing,but it became a weak point,and as a 650 would break at the centre bearing,fixed in later models,but the damage was done - it was a crank breaker,don't touch it.Obviously the design wasn't able to stretch to 750 safely.

It was a well engineered motor - twin cams with roller lifters,big gear oil pumps on each camshaft,the top end was split,each barrel and cyl head seperate,,so good cooling air flow around them.The best bit about them was the rocker gear - under a skull cap held down by two allen bolts was the valve and rocker,and the valve clearance was adjusted with an eccentric rocker shaft - just loosen the locknut,turn the shaft and then lock again.In all the motors I've worked on in my life I have never found an easier motor to set tappets on.

I had a 1953 Matchless G9 - the 500cc twin,the worst bike I've ever owned,pity,it had so much potential.

[Jackrat]

Looks like a w650 but

Not this time mate.

[Ixion]

...
The British Twins - there were six of them,BSA,Triumph,Norton,AMC,Ariel,Royal Enfield,any more? - were often critisised for their big central flywheel and two main bearings.AMC with the identical AJS/Matchless twins fixed that by having a centre main bearing,but it became a weak point,and as a 650 would break at the centre bearing,fixed in later models,but the damage was done - it was a crank breaker,don't touch it.Obviously the design wasn't able to stretch to 750 safely.

...

'Twas all becasue the Briddish industry either could not, or would not (not sure which) switch from vertically split crankcases to horizontal split.

This meant that they couldn't do the simple thing for main bearings, and use split shell bearings (which they already were for the big ends). Or, for that matter , use bronze bush bearings at each end. Because it was not possible, with vertical split cases, to get the two case halves aligned accurately enough for plain bearings. Ball races could take up the slight misalignment. But plain bearings, the misalignment would have flogged the bearings.

And a three bearing crankshaft with ball races (or roller bearings) had problems - the inherent play in ball races meant that the long crankshaft would be too whippy. And it meant a split crankshaft , with all the issues that implies.

Problem with the AMC design was that once there was a bit of wear in the outer bearings (inevitable after a while), the crank ended up effectively being supported by the central , close clearance , bearing alone. Which, understandbaly, broke the shaft. Central plain bearing needed plain outers - but they couldn't do that becasue of the verticallly split cases. The later shaft, they just killed the problem with brute strength, the later shaft was strong enough to take the flexion load and not break.


They were a nice design, very clean. And yeah, that tappet adjustment was cool. Very easy to work on , too.

[Motu]

I didn't think about it that way,I thought twisting,but yeah,the flex with a centre plain brg would do it.That's why Norton had those superblend roller bearings in the 850,the standard bearing didn't flex and that caused the crank breakages.They just let it do it's thing and solved the problem.

It was still pre unit days and horizontal split didn't really come in until unit construction....the idea just hadn't been picked up then.Horizontal split makes a long motor though,one shaft behind the other,stacking shafts would take time again.And yet the Cub and early BSA unit singles (same design really anyway) had a plain big end,not successful either.

Here are some 650 AMC heads,you can see they are seperate and see the valves are really acessable,the eccentric rocker shaft just goes through the pedastals,which were prone to breakage.

And here is a yoof of dubious moral standards playing the fool,but he didn't need to pretend - at his feet you can see a couple of 650 cyl heads.