Medium speed four stroke engines and small two stroke engines use a simple method of checking and adjusting chain tension. The tension is checked by turning the engine so that the longest side of the chain is the slack side. (this is usually when the engine is running astern). After stopping the engine, the chain is then pulled back and forward by hand, and the transverse movement should be between ½ and 1 link. Adjustment is carried out by tightening or slackening the adjustment nuts on the tensioner; see diagram.

On larger engines, the method described above becomes increasingly difficult to carry out, so the tension is adjusted automatically by using the compression of a spring to give the correct chain tension. This negates the need for measuring the play in the longest side of the chain when slack (½ - 1 link). The engine is continuously turned in the ahead direction (so that the longest side of the chain is tight). The nuts are slackened off at top and bottom, and the spring is compressed by the set amount by adjusting the lower nut. This will give the correct tension in the chain. The top nut is then screwed down carefully until it just locates on the fixed bar. The lower nut is now tightened further until the spacer piece is up against the collar. This further compression of the spring will not affect the chain tension. The lock nuts are then tightened and locking washers bent into place.

Later engines rely on a hydraulically damped tensioning arm, the weight of which supplies the correct chain tension. In this case the adjustment is to the damper as the chain elongates.


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