When I acquired my c1908 Lewis in the 1990s (as one of a number of machines and parts that came in exchange for a never-missed 1936 SS80 Brough Superior), the motor had been "done up". I guess most of us have heard this before, so I didn't pay too much attention: I was more interested in acquiring a complete, restorable machine than worrying over the wear inside of the motor. After I acquired the bike I had a look inside, and made a mental note that there was still work to be done.
It's now time to try to get the Lewis back on the road. One thing worried me about the motor. The cylinder that came with the motor is not the original: it carries number 1280 (one of the last of this type) compared with the engine number of 997. These later barrels have slightly larger ports, and a slightly more horizontal exit for the exhaust pipe. Of course "slightly" is just that - in real life is is almost impossible to tell the two cylinder types apart, but that didn't stop me wanting the original style part. I managed to locate three barrels of the correct type, but all three had major problems of one sort or another that rendered them close to unusable. Since 1280 is in very nice order, I'll use it until someone comes forward with the original - does anyone have Lewis cylinder 997 lying about the workshop? Lewises of this period were hand made. Fitting the cylinder to the cases meant opening out the mouth of the crankcase by 20 thou to accept the spigot on the barrel. Thanks Chris! Other than the mis-matched barrel, the motor was not too bad. A bit of patient work with a stone and emery tape and the mainshafts were dressed up to be quite useable. The big end had been done, but the flywheel assembly was about 0.005" out of true. Easily fixed. The cast iron piston (the original running in the standard 3.000" bore, worn only 0.005" at the top) had a good-sized piece broken out of the bottom of the skirt, but was otherwise in good condition. Since the bike will be no GP racer, I simply filed out a smooth profile around the missing piece. I replaced the top ring - originally 1/4" wide - with two 1/8" rings in the original groove. New main bushes were put in, and reamed to fit the now-round mainshafts. While the engine number is 997, all of the major component parts are stamped with the fitting number 17: crankcase halves, timing cover, both cams and both cam followers. It was only late in the rebuild that I noticed that the leading flank of the original #17 exhaust cam - although not noticeably worn - was no longer hard. Luckily I had a spare cam, albeit with a broken tooth on the gearwheel and rather tired shaft. Since the missing tooth was not on a high-load part of the gear wheel (I suspect it had been dropped rather than damaged while running in an engine), it was built up with weld and reprofiled. The shaft was hard chromed and reground. Restoring this cam was the only real expense associated with the rebuild. Fitting new bushes for the exhaust cam highlighted an inadequacy of the original design. The timing cover is bronze, and originally the exhaust cam shaft ran directly directly in it. Not a problem - it could be bored out and re-bushed with bronze - but the lack of dowel pins to locate the timing cover on the crankcase is a bit poor. I knew it before I started the job, but with 7 screws to locate it, how bad could it be? Pretty bad as it turned out. I struggled through the job (thanks to Rod and Philip for helping with their machines), only to relent at the final assembly stage when I added two 5/32" pins, took a final lick out of the bushes with an adjustable reamer (thanks Terry), and now the timing cover can't move anywhere it shouldn't.
The rest of the assembly was straight forward, provided you ignore my mistake that meant I had to move the crank assembly across 20 thou to stop it kissing the wall of the crankcase when the motor was leaned over on one side. I'll do better next time. So other than cosmetics, the motor is looking pretty good. The next job on the list is one that I haven't encountered so far in a rebuild: to fix the leaks in the radiator... Copyright © Leon Mitchell 2007
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