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• What do the marks on the flywheel mean?

What do the marks on the flywheel mean?

When adjusting your valve clearances or camchain tension, you'll find that you need to open up the inspection port so that you can set the engine's position to a specific point. What do the markings actually refer to?

Here are the markings - first FR/TR:

..and quite close to these, FL and TL:

As you rotate the engine using either the crank bolt at the front or the rear wheel, you will see these markings spin by repeatedly. The one you'll probably look for most is TL - which is TDC left, or top dead center left. If you're not familiar with the term dead center, it refers to the position where the piston is either farthest from or closest to the crankshaft, and you can have top dead center and bottom dead center. TDC is top dead center. TL is TDC for the left side, and TR is TDC for the right side.

These markings will go by twice for each 4-stroke cycle (intake, compression, combustion, exhaust) as the piston will be at top dead center twice in that cycle - once between compression and combustion, and again between exhaust and intake.

FR and FL refer to the point where the ignition fires - you'll notice that it is just before top dead center for each cylinder. This is because fuel takes time (a very short amount of time, but time!) to burn and expand and if you were to ignite it at the exact point that the piston reached TDC, the expansion would reach the piston after it had already started moving down the cylinder with negative consequences (lost power, unburned fuel etc.).

The FR/FL marks represent the minimum amount of advance; there are an unmarked pair of lines further up which show the full advance points.

You could in theory use a timing light to check that time engine's ignition matches the FR/FL marks but with the amount of oil splashing around that area of the engine while it is running, it would probably be quite a messy business.

Don't adjust anything (valve clearances etc.) based on the FR/FL lines - you want the TR/TL lines for that.