Unit single engine design (motorcycle)

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The design of the "unit single" sits squarely in the tradition of British motorcycle engine design.

The design is derived from Edward Turner's original Triumph Tiger Cub design. The gearbox and crankcase is cast as a single moulding "Unit Construction". This simplifies manufacture and in theory reduces maintenance albeit at a loss of some flexibility. The engine is of dry-sump design - oil is scavenged from the bottom of the casing and pumped to the parts to be lubricated. This requires a separate oil tank which is often incorporated as part of the frame.

The timing-side main bearing on most earlier models is a plain shell bearing. As most models did not have any meaningful oil-filtration system, and many unit-singles were owned by young men with a cavalier attitude to regular oil-changes, this single design point has become a major Achilles heel of the unit single engine. Some engines (notably, competition engines and War department models) came with a more robust roller bearing. For everyone else, after-market conversions to roller and taper bearings are a popular modification. However, Users who have fitted oil filters and made regular oil changes have had many trouble-free miles from the standard bearing.

The crankshaft is of pressed construction, and the plain big end bearing has suffered some of the same problems as the timing side main bearing (although to a somewhat lesser extent). The primary side ball-bearing causes few issues.

Primary drive is via a duplex chain, with a somewhat weedy chain tensioner on most models. The clutch is wet multi-plate. On some unit-singles a mechanical clatter can be heard at idle from the plates flapping around in the worn grooves in the plates. This is no great cause for concern and can be cured with a cash injection.

The crankcases are vertically split. The mating faces throughout the engine are disappointingly narrow, making most joints prone to oil "misting" at the very least. The majority of unit-single owners were inclined to home-maintenance, and often used the barbaric practice of using a screwdriver to pry mating faces apart. It is no surprise that there is often a spotting of oil beneath a parked unit single.

As would be expected from a British engine of this era, it has an overhead valve design. The timing gear and cylinder head are well-designed and rarely give significant trouble. There is considerable scope to improve the performance of the typical unit single by judiciously re-working the inlet and exhaust ports. Generally these engines tune very nicely and pleasing gains in performance can be gained with no more than a higher-compression piston, bumpier cam and gas-flowed head. This has been another factor in making these engines popular with competition users.

If well-lubricated, the gearbox tends to be robust. However, the method for checking the oil level on earlier models is whimsically awful, meaning that on many bikes, the gearbox has at some time been run with low oil. This leads to wear on the mainshaft, leading to wear on all dogs, leading to the gearbox eventually being scrap. Fortunately, most gearbox parts are still readily available. The gear-change mechanism underwent some improvement for later models. If well maintained and carefully shimmed, all unit single gearboxes can be a delight to use, but neglect them at your peril.

From 1959 to 1964, the distributor was a "mushroom" design, driven by a worm gear. A worn distributor drive (perhaps combined with weak timing advance springs and a nicely sloppy AMAL slide) is a well-known cause of lumpy running. This was addressed with the 1965 "F" engine, when the points were moved to the end of the camshaft. This change was formalised with the 1965 "G" engine, in which the hole for the distributor was completely removed (it was merely blanked off on the "F" engine) and some long overdue improvements were made to the gearchange and kickstart mechanisms.

The B25 engines of 1966 onwards have largely identical bottom-ends, but square alloy barrel and cylinder head. As more performance was required, the capacity was stretched to 440cc (the B44), then to 500cc for the B50. Although the B50 has a loyal following, many feel that the optimum capacity for this design was a little lower, where vibration could be contained and the bottom end kept together for a longer period of time.

"Soft" unit singles, such as the original C15 can run uncomplainingly for many years with only regular oil-changes. Some of the more highly-tuned models (notably the Starfire) require relatively expert care if they are to be used with any gusto. There is considerable expertise available for making these engines run right, but the most valuable source is "Rupert Ratio" of the BSA owner's club. His "Unit Single Engine Manual" has the quiet authority of a true master, and should be compulsory reading for all owners of Unit single motorcycles.