Mk II Oliver Tiger Cub
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Here we see an Oliver Tiger Cub Mk II, circa 1976. Serial number, as stamped on the far lug is in the mid 900's and stamped on the visible lug are the letters "MOD", indicating this engine was factory re-worked for extra RPM. Definitely not a factory mod is the compression screw, and I mean screw! Why do people do these things? The thread for the real Cub screw is 1/4-32 - nominally a glow plus thread, but I'd suspect the factory used the "Model Engineer" series which have a 55 degree included angle thread, while glow plugs are 60 degrees. The monstrosity in the head is neither 32TPI, nor 1/4".
Fortunately, I had what I mistakenly thought was a Mk I Cub to copy the replacement comp screw from. Stan Pilgrim put me right here, pointing out what I actually had is an earlt Mk II (more on that later). Running a 1/4-32 (ME) tap thru the butchered head thread did an acceptable job of cleaning it up. Quite aside from its size, the screw design is interesting. It is hollowed out by drilling 1/8" and the bearing face of the contrapiston is cone shaped so thrust is taken on the inner rim of the hole! The thread was screw cut to a firm, shake-free fit in the restored head thread. The Ball on the top was produced using a form tool which was ground using a cone shaped wheel in the trusty Dremal tool.
Early MK II BEFOREAnd for comparison, here is my earlier Tiger Cub Mk II showing signs of having been owned by a young teenager (me!) with no regard for anything but use. There are cosmetic differences in the shape of the cooling fins and prop driver "spool", plus some crankcase differences. I had the distinct impression that the MK II case was sand cast, while the MK I appears to be a bead blasted die casting. Like the Mk II above, this is a factory "MOD" engine whose serial number in in the mid 300's.
MK I Cub (Quite rare)Will the real MkI Cub please stand up? Here is the genuine article, circa 1954 from Stan Pilgrims massive 1.5cc diesel colelction. The case is actually a 2.5 casting. The engine has the same stroke as the larger engine, but a reduced bore.
Oliver Cubs - variousTo finish off, here is the pride of Cubs under Stan's protection. Note the rear exhaust on the engine at the far right and the cast exhaust collector ring (available as an option) on the one with the black head.