The engine is a liquid cooled four-stroke type, of three cylinders (75 models) or four cylinders (100 models). The cylinders are arranged inline but the crankshaft is disposed longitudinally, parallel to the machine's center line and the cylinders are laid flat so that the cylinder head (or 'top' end) is on the machine's left and the crankshaft (or 'bottom' end) is on its right. All castings are of aluminum alloy, the main crankcase being made as light and compact as possible by the use of plated cylinder bores instead of separate (usually cast iron) liners. The pistons run in bores which are accurately machined in the crankcase and given a hard bearing surface by having a thin layer of nickel/silicon carbide ('Scanimet') deposited electrically and ground to the required tolerances. Passages for coolant are included in the cylinder head and block castings. The forged steel crankshaft incorporates four (75 models) or five(100 models) plain main bearing journals which rotate in split shell bearings and are secured to the crankcase by large bolted-on caps. There rearmost crankshaft web is fully circular with gear teeth machined in its periphery, and a small sprocket and rotor flange are attached to the crankshaft front end to drive respectively the camshaft and ignition trigger assembly. The connecting rods have detachable bolted on big-end caps; split shell bearings are fitted at the big-end bearing and a plain bush at the small-end bearing. The pistons are flat-topped and are fitted with two plain compression rings and one oil scraper ring. The valves are set in deep wells in the cylinder head and are each closed by a single coil spring. An inverted bucket-type cam follower (or tappet) is fitted over each valve spring assembly; these cam followers have a recess machined in their upper ends into which a thick steel shim is placed to permit adjustment of the valve clearances. The shims are hardened to withstand the action of the camshaft lobes which bear directly upon them. The valve opening is controlled by two overhead camshafts which run in bearing surfaces machined directly in the cylinder head casting and are each retained by four (75 models) or five (100 models) separate bearing caps. They are driven from the crankshaft by a single-row roller chain which has plastic-faced guide blades between the camshafts and between the intake camshaft and the crankshaft, and a plastic-faced pivoting tensioner blade which is pressed against the chain 'slack' run (i.e. between the crankshaft and the exhaust camshaft) by a hydraulically-operated chain tensioner assembly. Drive from the crankshaft is transmitted via the large gear on the rear web to a secondary shaft which is disposed parallel to and underneath the crankshaft along the machine's ' center line. The matching gear on this secondary, or engine output, shaft is of the same size as the crankshaft gear to give a 1:1 reduction ratio but incorporates a spring-loaded anti-backlash gear to reduce noise.
The shaft serves not only to transmit drive to the clutch and transmission (see Chapter 2 - Clutch) but also drives the combined oil/water pump assembly from its forward end. On 75 models two balancer weights are incorporated in the shaft to cancel out the rocking couple produced by the motion of the two outer pistons and thus eliminate the only vibration source inherent in any 120o triple; on 100 models drive is actually transmitted via a large housing, with vanes protruding from its inner surface, through rubber blocks to damp out transmission shocks to a vaned shock absorber inner which is splined to the output shaft. The shaft rotates in a needle roller bearing at its forward end and a ball journal bearing at its rear end, both bearings being clamped to the underside of the main crankcase/cylinder block casting by the crankcase lower section, which also acts as the engine oil reservoir.
The fourth major engine casting is the bellhousing which is attached to the rear end of the crankcase and houses the clutch and alternator/starter motor drive components. An auxiliary drive shaft is driven via a 1,5 : 1 reduction ratio from the crankshaft gear, rotates in a needle bearing in the crankcase and a ball journal bearing set in the top of the bellhousing and has the drive flange of the alternator shock absorber bolted to its rear end. The electric starter motor drives via an idler shaft set in the bellhousing through a starter clutch mounted on the auxiliary drive shaft; a total reduction ratio of 27 : 1. Early UK only K100 and Kl00 RS models ware fitted with a clutch containing three rollers looked by spring-loaded plungers, while later models are fitted with a sprag-type clutch containing fourteen locking elements.
Since the output/balancer shaft and the auxiliary drive shaft are gear-driven from the crankshaft they rotate in the opposite direction to it. Their combined mass, with that of the alternator and clutch, cancels out the lateral torque reaction which would otherwise be evident from the crankshaft of an engine of this layout.
Next Section: 2: Compression test