If all this technical mumbo...
If all this technical mumbo jumbo has your head spinning don't get frustrated. Revisit our guide periodically and it will all start to make sense.
If you've ever spent any time checking out the tech specs on a new bike you might have found a few terms that left you scratching your head. And to be honest, we sometimes use jargon without explaining ourselves too well either. So put an end to perplexing terms with our handy guide to the top end of your engine.
Top end:
First and foremost, what do we mean by 'top end' anyways? Traditionally, it means the parts of the engine above the pistons. That's to say the cylinder head and its constituent parts: valves, combustion chambers and valve operating gear. A slightly wider definition includes the pistons and bores as well. Conversely, the 'bottom end' is everything below the pistons: crankshaft, conrods, primary drive, clutch and transmission.
Compression ratio
This is another key determiner of an engine's performance, and it refers to the ratio between the volume of each cylinder when the piston is at the bottom of its stroke, and the volume when the piston's at the top.
As an example, if the total volume of a cylinder and the combustion chamber above is 100cc with the piston at the bottom, and 10cc with the piston at the top, then the compression ratio is 10:1. When the engine is running, it sucks in (around) 100cc of the fuel/air mixture, and then compresses it into a 10cc space before igniting it with a spark. A higher compression ratio gives better efficiency and more power from an engine, but there are limits. Road bikes don't go much over 13:1, although race bikes with race fuel and careful development can go to 14:1. Anything higher than this leads to knocking and pre-ignition (which wrecks engines).
Cam chain tensioner
Most engines use a chain to drive the overhead camshafts, and this chain needs to be kept tight. Tensioners used to be simple mechanical threaded adjusters that were manually checked and adjusted at each service. Now they're almost all automatic devices that use springs, wedges, ratchets or hydraulic pressure to keep the chain correctly tensioned.
OHV
Stands for Over Head Valve, and is one of the most basic valve layouts, now only seen on Harleys and some Japanese cruisers. As the term suggests, the valves live in the top of the cylinder head, and it was coined when many engines had 'sidevalve' layouts (with valves located alongside the cylinder, pointing upwards, and opening into a combustion chamber that extended out to the side of the piston).
Most OHV engines have their valves operated by pushrods that run up the side of the barrels, transferring the opening motion from a camshaft that runs alongside the crankshaft in the bottom of the engine.
OHC
Stands for Over Head Cam, and is a more advanced layout than OHV. This design has the camshaft physically located in the top of the cylinder head, so the cams can push on the valves more directly. The camshaft can be driven by belts, chains, gears or shafts.
Bore x stroke
Simply a measure of the cylinders in the engine, usually in millimeters. A CBR1000RR has a bore of 76mm and a stroke of 55.1mm. So the bore is 76mm wide, and the piston moves 55.1mm from the top of the bore to the bottom.
SOHC/DOHC
Single or Double Over Head Cam tells us how many camshafts are in the cylinder head. A SOHC design is cheaper and easier to make, but is harder to incorporate into a high-performance, high-revving design. DOHC designs have two camshafts, one for the inlet valves and one for the exhaust valves (usually). Positioning each cam right above its respective valves makes for a shorter, lighter, stiffer valve operating mechanism. This allows for higher revs and more performance.
Two/three/four/five valve heads
The number of valves in each combustion chamber is fundamental to an engine's performance. Most older designs use one inlet and one exhaust valve per cylinder, and this gives pretty good performance at low and mid-range engine speeds. But more valves can give better performance at high revs, so four valves per cylinder are now essential on performance bikes: two inlets and two exhausts. Yamaha used five valves on early R1s and the old FZR1000R. Three inlet and two exhaust valves gave even better breathing at high rpm on those bikes. Ducati's ST3 used three valves per cylinder, as have some Hondas (like the Goldwing).
Top dead center/bottom dead center
TDC and BDC are the points when the piston is at its absolute top or bottom position in the bore. If you were to look at the engine side-on, then the crankshaft would be positioned with the big-end of the conrod at either the 12 o'clock (TDC) or 6 o'clock (BDC).
Small end
The end of the conrod that the piston is attached to. So-called because it's smaller than the other end which attaches to the crankshaft (and is called the 'big end').