Retensioning

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.

Renewing the chain

The camshaft chains will have to be replaced when their elongation reaches 1% or after 15 years. The reason for placing a limit on it’s elongation is that as the chain gets longer the pitch increases and is not matched to the wheel teeth, and continued use would result in excessive wear.

The chains are replaced one at a time. This is so that the connected chain can be used to turn the engine whilst the new chain is pulled round.

To change a chain on a large 2 stroke engine, the chain is slackened off at the tensioner. The top chain case cover is removed, and one of the chains is split at the point indicated. see here on how to split the chain.

The new chain is attached to the old chain with a temporary link, and as the engine is turned, the old chain is pulled out of the engine, the new chain is fed in. Once the new chain has been joined, the process is repeated for the second chain and the chain retensioned. The camshaft may now have to be retimed as described here, as it may have been adjusted to compensate for elongation in the old chain.

chain elongation

Students sometimes have difficulty in visualising the effects of the elongation of a timing chain on the timing of the camshaft. This model may help in understanding what happens.

It should be made clear from the start that timing chains do NOT stretch, The factor of safety is too high. However, they wear on the bushings and rollers, and this causes elongation and thus a change in the pitch of the chain.

For the model, I used bicycle parts, two large wheels of 42 teeth and a smaller 21 tooth wheel for the jockey on the tensioner. Two chains were used; a brand new chain with a pitch of 0.5 inch (12.7mm) and an old worn chain. The percentage elongation between old and new chains is about 2.5% which is way above the max allowable on an actual engine of 1%, but because my model is not to scale, will show the effects more clearly.

It will be noticed that a series of 29 links have been painted red and white on both chains. This is to highlight the difference in length and also to highlight why the camshaft timing alters as the chain gets longer. The 29 links spans the distance between the chain leaving the camshaft drive wheel and engaging with the crankshaft drive wheel. These points on the wheel teeth are also highlighted.

For our model the crankshaft and camshaft are turning clockwise when running in the ahead direction as indicated by the arrows. TDC of the crankshaft and camshaft for No 1 unit are indicated by the marks on the crankshaft and camshaft wheels. It can be seen that with the new chain the timing marks are correctly aligned.

As the chain gets longer so the pitch of the chain increases. BUT there is still the same number of links between the camshaft wheel and the crankshaft wheel. Because of this the camshaft chain wheel is effectively retarded when turning the crankshaft wheel in an ahead direction. This can be clearly seen from the timing marks. Because of this, chain elongation results in alteration of valve, fuel pump and air start distributor timing.