How a Triumph Bonneville engine works


Jan 15, 2008
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Someone asked me on another forum, so I wrote this up-

Internal (mechanical) operation of a Triumph twin (Bonneville) engine


Basic description

Air-cooled, carbureted, pushrod-operated overhead valve, vertical twin, 4-stroke configuration, dry sump oiling system, with chain-driven primary system, multi-plate, multi-spring clutch, manual foot-shift transmission, and contact breaker point ignition system.

Power Unit (Bottom end)

Utilizing the basic 4-stroke principal design, the pistons are connected in parallel at the crankshaft via plain-bushed connecting rods; the crankshaft is supported at either end on ball bearings. This sub-assembly rotates clockwise when viewed from the “Timing side” (Right side) of the engine.

On the timing side, the crankshaft pinion gear drives the intake (rear) and exhaust (front) camshafts via an intermediate idler gear, timed such that all 4-stroke combustion cycle events occur at the appropriate time. These gears must be synchronized in order for the engine to perform as it should, a simple misalignment of one tooth on one pinion is sufficient to keep the engine from starting, and could result in damage to the valves, pistons, or both.

Power Unit (Top end)

As the camshafts rotate, the raised lobes force the cam followers (tappets) to raise up in their guide blocks, lifting the associated pushrods; in turn, the pushrods force the rocker arms to pivot on their shafts, which depresses the associated valve off of it’s seat, allowing either the fuel/air mixture to enter the cylinder from the carburetor, or the expended combustion gasses to escape through the exhaust.

Primary Drive (clutch)

On the Primary side, as the crankshaft turns, the primary sprocket turns the clutch basket (with its inner edge grooved to accept the spline teeth of the friction plates) via a multi-row chain. Within the clutch basket is a multi-faceted assembly composed of the clutch plate “stack” & pressure plate, the pressure spring array, the cush hub with it’s outer edge grooved to accept the spline teeth of the plain plates, and the center hub which drives the transmission mainshaft.

In operation, the clutch basket, being turned by the primary chain from the crankshaft, turns the friction plates, which have a friction substance affixed to metallic plates with outer spline teeth, meshed to the clutch basket’s inner grooves. Pressure from the spring array at it’s static state, forces the plain steel plates with their inner teeth meshed to the cush hub’s outer grooves, to be virtually bound to the friction plates stacked between one another. As the cush hub rotates, it drives the clutch center hub which is mated to the transmission mainshaft.

When the rider pulls on the clutch lever, a ball & ramp mechanism at the opposite end of the clutch cable forces a shaft to release the spring pressure at the clutch pressure plate, freeing the connection between the drive (friction) and driven (plain) clutch plates; this disengages the connection between the engine’s “power unit” and transmission. As the clutch lever is released, the pushrod retracts, allowing the spring pressure to be fed through the pressure plate into the clutch plate stack, resulting in the power unit once again driving the transmission mainshaft.

Note that since the kickstarter engagement is through the transmission mainshaft, the clutch must be ENGAGED (static, lever out) for the kickstarter to rotate the engine. If the clutch is actuated, this disengages the link between the power unit and transmission; thus, the bike cannot be kickstarted with the clutch disengaged (lever pulled in).

Oiling system

Driven by a pin that is integral to the intake cam pinion nut, the dual plunger oil pump first draws oil (via the main plunger) from the oil tank through its filter screen, to the crankshaft at the timing (right) side end which is drilled though to both connecting rod journals; it is pumped into the mating surface between the crank’s journals and plain bearings of the connecting rod big ends, where it simply squirts out, splashing all of the rotating and reciprocating parts including the underside of the pistons, to keep the cylinder bores lubricated. Oil control rings in the pistons keep excess oil from entering the combustion chamber; thus, if the rings are excessively worn, it becomes apparent from oil-fouled smoky exhaust and excessively oil-fouled spark plugs.

The scavenge (secondary) plunger then draws the oil out of the crankcase bottom through a sump screen, and back to the oil tank; this same circuit feeds a smaller capillary tube that runs up to the head where it feeds the two hollow rocker arm pivot shafts. This oil lubricates the rockers at their pivot points, then drains down through the pushrod tubes, lubricating the cam followers (“lifters” or “tappets”) and the cams. An often eliminated companion feed in this circuit is a small “drip feed” to the final drive chain, which is adjusted by a screw visible in the open oil tank cap; there is also a “telltale” scavenge return orifice which allows inspection and verification that oil is indeed returning to the tank.

The last circuit in the oiling system is the oil pressure relief, which is in common with the crankshaft main end feed passageway, and incorporates a spring-loaded plunger, which allows excess oil pressure buildup to be “dumped” directly into the crankcase sump to avoid over-oiling of the rod bearings. On newer models, an oil pressure sensor switch is mounted at the front of the timing chest to indicate low or no oil pressure.

Crankcase Breather system

The pressure buildup within the crankcase as the pistons travel downwards would force oil to leak from every possible point on the engine’s many mating surfaces and assembly hardware unless some form of relief was present.

In the early model Triumph twins, the method employed is a tiny disc keyed to the primary side (left) end of the intake camshaft. The two tiny openings in the disc align at the proper time with companion openings cut into a chamber in the left side crankcase half which has it’s exit through a downward pointing pipe just above and ahead of the final drive sprocket (waaaaaaaaay up in there where you need to lie on your back with a flashlight to see it). A tube is connected to that pipe and runs to a “T” fitting which then runs out the back end of the bike, underneath the rear fender. The third connection on the “T” fitting connects to the breather pipe at the top of the oil tank.

Later models eliminated the crankcase seal at the primary side of the crankshaft, and three tiny holes drilled in the case just aft of the bearing aperture, allowing engine oil to lubricate the primary chain, and crankcase pressure to vent through the primary case, out a manifold at the top rear of the case. The hose connected to that manifold connects in the same manner as the earlier models.

Transmission (4-speed)

Power from the engine is fed through the clutch center hub to the transmission mainshaft which has one gear press-fit to it, and several others spline-driven that can be shifted to various positions resulting in different ratios between the input (mainshaft) and output (layshaft) shafts. Power is transmitted from the mainshaft, through whichever gear pair is selected, to the layshaft, which turns the final drive sprocket, which drives the final drive chain, which drives the rear sprocket, turning the rear wheel.

The gear selector apparatus is a somewhat intricate assembly of pins, springs, pawls, cam and forks that operate thusly:

When the gearshift lever is depressed, a toothed plunger clears a retaining brace, allowing the tooth to engage a roughly ellipse-shaped gear which rotates slightly on it’s pivot, it’s opposite end causing a camplate gear to rotate on it’s shaft. The front side of each of the two shifter forks has a pin with a roller slipped over it; these pins with their rollers wend their way through two curvaceous grooves cut in the face of the camplate as it rotates.

The two gear selector forks slide left-to-right on a shaft, with the tines of one fork riding captive in a groove of one gear on the mainshaft, and the tines of the other fork riding captive in a groove of another gear on the layshaft. So, as the gearshift camplate rotates, it moves the position of the appropriate gears one way or another, via the shifting forks, to align each matched pair in succession as the gears are selected. Regardless of the gear selected, power comes in through the mainshaft, feeds the selected drive gear, which feeds it’s paired driven gear, which feeds the layshaft, which feeds the drive sprocket.

The kickstarter engages a ratchet gear on the mainshaft, which turns the clutch, which turns the crankshaft via the primary chain. As the pedal is released, or as the engine spins, the ratchet's teeth release, allowing the pedal to remain free from counter-rotation except as allowed by the return spring, to the point where a peg limits it's travel just beyond the point where the first tooth in it's gear disengages the ratchet gear.


Driven by the timing end of the exhaust camshaft, the ignition system is timed to provide spark to the combustion chamber at the exact point in the 4-stroke sequence for proper combustion and operation of the engine.

In a contact breaker points based system, the exhaust camshaft turns an automatic advancer plate upon which pivots a points cam; this points cam is advanced as engine speed increases, by means of centrifugal flyweights which vary the points cam’s relative position to the exhaust cam by a pair of springs. The faster the exhaust cam spins, the more centrifugal force is exerted on the flyweights, which rotate the points cam via a tooth-in-groove arrangement, effectively advancing the timing to provide the spark earlier in the compression stroke, allowing the mixture in the combustion chamber to ignite and burn appropriately. 12 volts are fed (through the system voltage sources, more later) to the contact breaker points, which when actuated, send a primary voltage pulse to on of the ignition coils, inducing a high secondary voltage pulse through the ignition lead to the spark plug associated with that coil & points set; the same thing happens again 180 degrees of CAMSHAFT rotation (360 degrees crankshaft rotation) later, and the other set of points fire for the other cylinder.

In a magneto ignition system, the exhaust cam drives the magneto’s armature and breaker points cam simultaneously; the magneto’s armature produces the primary voltage for the points, which, when triggered, induce a high voltage in an encapsulated coil which feeds both of the ignition leads to the spark plugs. This results in one cylinder firing, and a “wasted spark” once every cycle of 360 degrees of crankshaft (180 degrees of camshaft) rotation.

In an typical modern electronic ignition, the exhaust cam drives a rotor assembly with two magnetic triggers affixed to it; as these magnets pass in proximity to two pickup coils mounted in the stator plate, an electromagnetic pulse is triggered in the primary circuitry. This primary pulse travels into the ignition’s “black box” which in turn magically (well, through the wonders of modern electronics), sends a single12 volt primary pulse through a pair of 6 volt coils, inducing a secondary pulse in both coils, causing a spark at both spark plugs simultaneously, resulting in one cylinder firing, and a “wasted spark” as with the magneto ignition.

Alternator (single phase)

On the left end (primary side) of the crankshaft, a magnetically polarized alternator rotor is keyed to the shaft; the rotor spins in close proximity to the fixed alternator stator which has multiple copper-wound coils encapsulated in it. These coils are wired in such a manner that an alternating electromagnetic pulse is triggered as the rotor’s magnets spin past the coils. The stator’s output leads are wired such that two wires carrying alternating current (AC) feed into a diode array called a “rectifier”. The rectifier’s diodes are composed so that the alternating current pulses that are sent through the array, exit in a flow of direct current (DC). This voltage is sent to the storage battery, and all of the electrical systems, as well as a Zener diode which regulates the maximum voltage allowed into the electrical system at just above 12 volts; any additional voltage above that (from the alternator spinning at high speeds), is “bled off” in the form of heat. That is why Zener diodes must always be mounted to some form of heat sink, or they will rapidly fail due to excessive heat buildup.


The various makes and models of carburetors operate in essentially the same manner:

Fuel enters the carb and encounters an orifice that is controlled by a float needle; as the bowl fills with fuel, the float rises to a fixed point at which the paired needle rises in it’s bore until it seats at the orifice, block flow of fuel into that carb so that it doesn’t “flood”.

At idle, a fixed system consisting of a restricted passage or a replaceable “low speed” or “idle” jet allows a tiny amount of fuel to bypass the carb’s main circuits, allowing the engine to idle without intervention; the ratio of air to fuel at idle and low engine speeds is controlled by the low speed mixture screw.

When the throttle twistgrip is actuated, the cables pull on the carb’s slides, lifting them out of the airstream, allowing more air into the carb; as the opening increases, a vacuum is formed by relatively high pressure in the “venturi” of the carb, relative to the low pressure in the float bowl. The greater the air pressure, the more fuel flows though the carb’s main jet; this flow is restricted, moderated and adjusted by the slide needle affixed to the carb’s slide. The general running condition fuel-to-air ratio (mixture) is controlled by an intricate combination of different sized and shaped slide, main jet and needle jet, and different shaped and positioned needles.


The spent products of combustion cannot simply be dumped out of the combustion chamber into open air without drastically negative effects on the engine’s performance. The exhaust headers exit the head and minimally channel the exhaust to the rear of the bike; a secondary effect of the bends in the exhaust pipes causes a pressure buildup that interacts with the flow in such a way that a return pulse moderates the flow to a point at which nominal performance is gained. Customizing the diameter of these pipes, and the complexity or simplicity of the circuit, can result in increased effectiveness of the system, resulting in increased performance of the engine. The mufflers may or may not contribute to (or detract from) performance, depending on the design, but are mandatory for road use.

Any questions?
Feb 10, 2009
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"In a magneto ignition system, the exhaust cam drives the magneto’s armature and breaker points cam"

Is that the American aftermarket magneto, such as Hunt?

Most British mags have stationary cam and rotating points.
Feb 10, 2009
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grandpaul said:
The last circuit in the oiling system is the oil pressure relief, which is in common with the crankshaft main end feed passageway, and incorporates a spring-loaded plunger, which allows excess oil pressure buildup to be “dumped” directly into the crankcase sump to avoid over-oiling of the rod bearings.

OK- a bit trivial but the valve dumps directly into the timing gear chest, whence it oozes into the crankcase.

I'd say over-oiling the the big ends, or rather passing so much oil through them that you over-oil the rest of the bottom end, is minor. Once the big ends are worn it happens anyway! Excess pressure is a bad thing because it would strain the pump and its drive, find leaks between the pump and the crank, invert the crank end seal, and waste power.


Jan 15, 2008
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Yep, hunt magneto. Technically correct on the oil pressure relief, too. I should eliminate the word "directly".

There's also an inaccuracy in the transmission, regarding the output sleeve shaft beign driven by the mainshaft in top gear.

"close enough", ah, it'll never be perfect...