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Mitchell Barnes in Australia sent these photos of a 1927 Rex-Acme with a Blackburne engine. Can't tell yet whether it's a 250cc or 350cc, but given the size of that TT Amac carb, I would guess 350cc.Rex-Acme were the little factory that could, winning the Isle of Man TT in '25 (Lightweight and Junior) and '27 (Junior), winning second and third places in '22, '23, and '26 - quite a
record for such a small concern. Their 1927 TT-winning machine (Wal Handley aboard; see photo of him in the '27 Junior TT - special thanks to ace web researcher Dave Martin) would have looked very much like this motorcycle, as factory 'specials' of the period amounted more to carefully tuned standard bikes, than unavailable-to-the-public racers which were shortly to appear on the scene, and change European motorcycle racing in the 1930's (as had happened on US board tracks in the 'teens).These Blackburne engines were surprisingly robust, belying their delicate appearance. Look at the period photos of a dismantled engine, which show the sturdy castings and well machined mating faces... no wonder these little engines did so well against much larger factories in the mid and late 1920's.
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The b/w photo of the opened crankcases shows the interesting flywheel assembly; they're not simply a pair of discs, but shaped and polished, and the mainshafts are huge. It looks like they're still using crowded roller main bearings though, which aren't the best at higher revs (and unpleasant to assemble...). The cases themselves are cast very thick, with hefty bosses for the engine bolts. The crank discs are surprisingly robust for a machine which also has an EXTERNAL flywheel; that's a lot of spinning mass to dampen vibration, and Blackburne engines are renowned for smoothness.
The color photo of the flywheel is from The Classic Motorcycle (Nov. 1983) and shows a very similar machine, although if you look closely, 'our' machine has a an oil tank rather than a toolbox, and that TT carb, both signs of racing heritage. The gearbox is a 3-speed Burman item, the front forks are double-damped Webbs, same as used on Nortons and Velos of the period.
The cylinder head is also an interesting case - the valves are so large they break the bounds of the combustion chamber and the top of the barrel. It was clearly necessary to chamfer both the edge of the cylinder barrel and the spigot recess in the head for proper gas flow. A consequence of a long stroke (the 250cc engine is 60mm bore x 88mm stroke) is a small
combustion chamber, as for a given engine capacity, the longer the stroke, the smaller the bore tube (and hence combustion chamber), and vice versa. Short-stroke engines have larger combustion chambers, making possible much larger valves for a given engine capacity. The Blackburne engine has cheated a bit by fudging the edges for larger valves - clever. As the spigot itself is almost completely chamfered away under the valves, I wonder if they have problems with compression leaks? I would assume the head is lapped onto the barrel with no head gasket, but a little gasket cement for luck.
If you have an interest in Rex-Acme motorcycles, and are looking for other Rex articles, try Classic Bike Mar. '85, and Classic Motorcycle, Aug. '90. There are more recent CB articles on Rick Parkington's Rex (he writes for the magazine, and is a ray of hope there), and Rick can be found hosting the Classic Bike Forum. It's time somebody wrote the book.
[A note on the video; it's a repeat of the video from my earlier post on the Rex-Acme, but I'm fairly sure this is the very same machine as above; the youtube caption says the Rex is in Australia.]