Wednesday, December 28, 2022

1948 Norton Manx 30M 499cc

This 1948 Gardengate Manx came to New Zealand from California as a basket-case as part of a swap with noted Norton restorer Paul Adams. The bike was originally supplied to McGill, the Canadian and North American Norton importer. It spent most of its life owned by the famous Californian Norton tuner Clarence Czysz, and was probably raced at Daytona by Czysz’s rider Tex Luse. Unfortunately no records exist.

The bike has been restored by Ken McIntosh to the more common Isle of Man rather than the American Class C” specification mainly because the “Class C” requirement of a working kick-starter meant the exhaust pipe would be rather low. Although no problem at Daytona Beach with only left turns, it is a major problem on New Zealand circuits.

The Manx specification remains fairly constant in the 1946-49 period, with the main change for 1948 being the introduction of welded steel oil and petrol tanks, replacing the pre-war type soldered tanks with their nice “pie-crust” edges.


The 79.6mm x 100mm SOHC motor has magnesium major castings, with the square finned aluminium cylinder head being cast around a bronze skull instead of having inserted valve seats, and the aluminium cylinder has the cast iron liner, also cast in. The bronze skull is a feature of all Manx engines until 1951.


The close ratio 4 speed “Dolls-head” gearbox has no kick starter and a very high ratio (1.77 -1) first gear, meaning over 70 mph at maximum (6200rpm) revs with Isle of Man gearing.


The 3 plate clutch is special to a Manx and runs dry. The primary and rear chains are eachlubricated by an adjustable vintage type drip oiler from the 1 gallon oil tank. The bike uses pure SAE 50 Castor oil. This bike is fitted with an “O” ring primary chain which can happily run dry.


The correct BT-H (British Thompson-Houston) “TT Magneto” is fitted along with the correct Amal T10RN (remote needle) carburettor and Amal bottom feed float bowl.

The Smith 8000rpm RC83 “Chronometric” tachometer is correctly rubber mounted on the left fork leg, with the rather vulnerable brass drive gearbox mounted low on the timing cover. This is because the large AndrĂ© steering damper mounted in the centre of the fork stem precludes the later central mounting of the tachometer

The single leading shoe 8” conical iron front hub has magnesium shoes and brake plate. Fitted with Ferodo AM4 linings it works well, when combined with the very heavy cable and long front brake lever. The bike came from the USA with the original woven asbestos linings fitted and they hardly worked at all!


All the handlebar levers are special to the Manx with the brake lever being extra long and the shorter clutch lever having a long throw to give enough lift for the clutch. The lever bases are made of brass and are much more solid (and rigid) than the standard road type. The pressed steel air and magneto levers are only used on the racing models (and possibly lawn mowers), and being made of sheet steel rather than cast metal, they are very light weight.

The Dunlop steel wheel rims (alloy rims were an optional extra) were normally painted black, with a WM1 (1.65” inside width) x 21” front and a WM2 (1.85”) x 20” rear. The bike is fitted with a 19” rear tyre so an Avon GP race tyre can be used. The slight reduction in diameter appears to cause no problems.


The Terry spring saddle and Norton-made hinged rear seat are peculiar to the post-war Garden Gate Manx models and are set rather high giving a strange riding position. At first it feels rather odd but actually turns out to be quite comfortable. You certainly feel “on top” of this bike.


The front forks are unusual in having a lower yoke with a bigger off-set than the top, adding about 3 degrees to the fork angle, giving the bike a very long, raked-out appearance. This geometry means the bike has quite small amount of trail (compared to a featherbed Norton) and relies on the 21” wheel for stability. These models have a reputation for bad handling when compared with the later Featherbed, and even the pre-war girder forked models, with weaving and pitching combining to unsettle all but the bravest of riders. After riding this bike, the famous journalist Alan Cathcart expressed surprise at how well it handled, having ridden similar models before.


The answer lies in the front fork damping. The pre-1950 Manx models use the same damping system as the standard post-war Norton road models. This is really only a hydraulic bump-stop in both directions, and offers no damping at all for the normal fork travel. Hugh Anderson advised me during restoration of his experiences with plunger frames and early Nortons, and said much of the criticism levelled at the frame and plunger suspension was actually the lack of fork damping. When I suggested that originality was important, he added that so was not falling off! The answer was to fit the piston type damper that Norton first used in the 1950 Gardengate Manx, and later in every Featherbed and Commando model. This does not change the appearance at all and transforms the bike into a nice handling machine even at well over 100mph.


The bike has a very nice motor with almost no vibration, and is happy to rev over 6000rpm without complaint. If ridden on the open road, the high first gear means a fairly brisk push to get the engine started. Once running, the high first gear is quite easy to live with and does not require too much clutch slip.

The RN carburettor has a reputation for being troublesome, but has been found to be near perfect with no flat spots at all. Its one strange characteristic is if the bike is leaned towards the left when idling, it will stop dead, and if push started from the left side it is reluctant to start unless flooded (to raise the fuel level). Raising the fuel level in the float bowl has not cured the problem. It is quickly learned it is essential to only lean the bike to the right when warming up, and people who push start on the right-hand side are at an advantage!


Manx Nortons were often run on alcohol fuel in Australia and New Zealand, and some non Championship UK events. The normal Amal RN was classed by Amal as unsuitable for alcohol because of its convoluted fuel passages. The TT carburettor as used on International models was highly suitable with the huge jets and high feed float bowl available for this very purpose. Forged pistons were available from Norton giving up to 14:1 compression ratio to suit any type of fuel. The “Pool” petrol (70 octane)  available for a few years after WW2 was used in Grand Prix and TT racing and would only stand about 7:1 ratio without terrible, destructive detonation. Norton supplied suitable forged ‘flat top’ pistons.


As part of the AMA “Class C” regulations used at Daytona, 500cc OHV engines were restricted to 8.5:1 compression ratio to help the Harley-Davidson and Indian Side-Valve 750’s which struggle to achieve 7:1. With the 100 octane available in the USA, the Manx would have run happily at well over 10:1, giving a few more BHP.


I think the Garden-gate Manx is the last “Vintage” motorcycle made by the British Industry and is the final development of the pre-war bikes, before being completely surpassed by the Featherbed models in 1951.