72 AJS Norton Stormer

by **frankdamp** » Mon Aug 08, 2011 7:17 pm

For the two seasons I was there, we raced the 250 and a "360" so we could race in the "over 350" class. Nobody ever checked, but the "360" was actually only 345 ccs!

The bigger capacity engines were real problem items, I understand, (I'd emigrated before they came along) because the cylinder liners got very thin and the bored-out crankcases weren't very stong. I suspect that the fuel/air charge into the cylinder, from what was basically a 250 crankcase, may have seriously limited the power of the big engine.

by **Cheesy** » Wed Aug 10, 2011 2:46 pm

swooshdave wrote:
frankdamp wrote:I spent the last 8 months of my N-V career working on the Stormer program. Two things to watch for, if they hadn't been fixed in your model year are problems with localized overheating which damages the cylinder liner and frame cracking inmmediately aft of the headstock stiffener (if yours doesn't have the tapered top tube). Taking a closer look at your full-bike photo, what appears to be a weld line along the center of the top tube suggests you have the tapered set-up. Your bike is dated four years after I left for America. The "new" ones being built in a chcken shed were built by one of the retired Moto-cross mechanics from parts he'd bought when the company went broke. if you go to the AJS Owners news group there's some info on him.

The engine overheat was a part-throttle detonation issue. The Starmaker woud run WOT for days without complaint, but in stop & go, light throttle settings, the liner would get red hot around the area of the exhaust port and you'd get instantanous welding of the top piston ring as it passed by. This would set up an oscillation as the piston travelled on and circumferntial ripples would develop. Initially, we would see this in less than 1000 miles on a new liner. I tested the bike riding to and from work on one that had lights.

Initially the fix was to use Castrol "R" in the fuel, but it doesn't dissolve in gasoline, being a vegetable-based oil. After about 6 hours of not being run, you had to give the bike a good shaking to re-mix the gas and oil. As I left (June '68) they were working on a more user-friendly fix.

The early M-X bikes had frequent frame failures caused by a stress raiser where the original design of heastock reinforcement ended. I propsed splitting the top tube into two semi-circular pieces and welding a long triangular piece of flat steel in between on each side. the resulting top tube went from being an obround cross-section almost the full depth of the headstock down to the original circular section at the back. I think this made it into production.

One of the cleverest bits of design on the bike was the eccentric plate for chain adjustment, It shows very well in your pic of the gearbox. I've since seen an identical set-up on (I think) a KTM, so maybe the AJS design was sold off when N-V went Tango Uniform.

I was waiting for you to chime in with your experiences.

I'm sure with a modern two-stroke oil like Royal Purple the overheating issue should be minimized.

Didn't they also use the eccentric adjuster on the B50?

I wouldnt bet on new oils being better, I did my masters thesis on two stroke engine liner/ring/oil wear systems, one of the interesting side outcomes I came across was that the Castrol A747 (cant remember exactly what it was called) had the best scuff resistance of any of the oils I tried over a range of ring and liner materials, this included some rather expensive synthetic 'race' oils. That said scuff resistance is only one aspect of it.

by **Carbonfibre** » Thu Aug 11, 2011 12:27 am

A747 is a synthetic/castor blend. 747 has now been superseded by XR77 which is a fully synthetic oil, which among other advantages provides accurate plug colour readings, when modern fuels are being used.

by **Rohan** » Thu Aug 11, 2011 7:53 pm

Cheesy wrote:
I wouldnt bet on new oils being better,

I would. Engine seizure in GP125 and GP250 racing is almost unknown these days - although the engines are watercooled. And producing beyond 400 bhp/litre.

by **Cheesy** » Fri Aug 12, 2011 2:08 pm

Rohan wrote:
Cheesy wrote:
I wouldnt bet on new oils being better,

I would. Engine seizure in GP125 and GP250 racing is almost unknown these days - although the engines are watercooled. And producing beyond 400 bhp/litre.

They also wont be using cast iron liners or chrome plated rings either..... There are a lot of variables involved, of the oils I tested the A747 had the best scuff resistance, I cant remember what all the other brands were off the top of my head though. Of the rest of the oils the ones that were TWC3 rated oils were the worst, the rest had different rankings for different ring/liner material combinations, the ranking wasnt of scuff resistance but the wear rate of the seals and the liner material. When a particular material couple and oil did scuff it was a surprisingly repeatable test.

Also when I say liner it is really a plasma sprayed coating that is applied directly to the cylinder wall and then ground, the ring materials were generaly PVD coatings on nitrided SS rings, almost all of these materials had wear rates that were orders of magnitude better than cast iron and chrome.

I hadnt remembered that the A747 was a synthetic blend either, I thought it was older than that as it had already been discontinued before I tested it so you are right about newer oils being generally better.

72 AJS Norton Stormer

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