Couple of points:
Yes, absolutely, given enough time, a cam chain will wear and that wear will be seen as "stretch", which we have already established is not really stretching per se, but an accumulation of wear that results in an elongation of the chain. This elongation is the main reason that a chain driven cam system has an automated tensioner. Otherwise we could just take up the slack in the new chain with a manual tensioner and never look at it again.
There are a multitude of reasons that a cam chain and sprockets wear so much more slowly than a drive chain. Obviously, the cam chain lives an ideal life, immersed in a continual bath of lubricating oil, protected from the elements in the confines of the engine, etc. Another is that the load on the camchain is much smaller, (even in proportion to it's smaller size) only driving the cams. But another important difference is that a drive chain must have slack in it to accommodate the articulation of the rear suspension. This slack allows the chain to climb the teeth of the sprockets under load and results in uneven wear to the sprockets and chain. Because the cam and crank sprockets do not change distance from each other, the camchain can (and should) have no slack. That is the function of the CCT.
The chain will never become shorter, so the chain tensioner only needs to take up additional slack throughout the life of the chain, never give any back (except during a service procedure such as removing the cams). That is the premise behind a ratcheting pawl type of CCT. It takes up any chain slack with a spring loaded plunger (some have the spring assisted with engine oil pressure), and as the plunger extends the ratchet ensures that the plunger won't be inadvertently driven back into the tensioner body allowing the chain to go slack with the catastrophic consequences.
Here's a picture of a CCT from a Kawi ZX-11 with a ratcheting pawl:
This is a pretty standard way to design a CCT.
The method that Yamaha engineers chose to design the tensioner is completely different. It is a torsionally loaded (twisted) spring that drives the plunger out via a spiral cut groove in tensioner body. There is no anti-reverse feature in the Yamaha tensioner other than having to overcome the bevel of the spiral drive. Apparently, as ionbeam, Howie and now Patriot can testify, this isn't enough to prevent the chain tensioner from retracting and the chain going slack.
Another missing feature of the Yamaha tensioner design is that there is no way to gauge chain wear (stretch). On a ratcheting CCT, the SOP is to remove the end bolt and withdraw the spring before then removing the tensioner body from the engine. The ratchet will remain in position and you can measure how far extended the plunger is, and compare that to a spec. The Yamaha design it is impossible to remove and retain the plunger in it's present extension.
If we knew all of the physical measurements of the Yamaha tensioner and the required plunger range it is entirely possible that there would be a ratcheting tensioner already available from some other bike application that we could transplant into our FJRs.
Otherwise, IMO, a manually adjusted tensioner would be a better long term bet than continuing to use the poor Yamaha design and run the risk of expensive head damage.
The
web page that I glommed the picture of the ratcheting cam chain tensioner from has an interesting dialog about CCTs on Kawis. It seems they have had the same kind of complaints historically and the ratcheting CCT in the picture is their fix.
