Rock 'n' Roll design for XT660
Yamaha's XT660X has been fitted with the first roller type rocker arm valve lift mechanism ever on a Yamaha motorcycle engine.This system uses a lever to operate the valves, which enables a high lift and shortens the overlap between the intake and exhaust valves. The result is plenty of urge in the low to middle speed range and excellent exhaust performance.
Using a unique Yamaha design, a needle roller bearing is fitted to create a continuous revolving motion at the slipper that significantly reduces reciprocating resistance and thus improves reliability.
Although this type of rocker arm has been used in some automobile engines, applying it to a motorcycle engine required a more compact version to be developed.
The more common valve operation method is direct-lift, where a cam pushes directly on the valve to lift it. The direct-lift cam is limited in terms of the lifting height that can be achieved but has the advantage of compactness. It also has fewer parts, retains its rigidity well and involves little friction loss at high rpm, which is why direct-lift cams are used on many DOHC engines.
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