Quote:
Originally Posted by AlCan
Oh, and, I've watched too many Alan Howatt videos...!
|
Like these:
https://www.youtube.com/watch?v=UBs1FLVyxdk
https://www.youtube.com/watch?v=aVFa8H1RpSY
https://www.youtube.com/watch?v=-jLzRoDQdyo
https://www.youtube.com/watch?v=08VYLAQ5xr0
It looks like the MD has many more cam chain issues than its predecessors.
The cam chain is a redesign, which on the face of it, looks like it should be good. It saves space by turning the tensioner the other way up, allowing an oil filler / handy cam chain inspection hole to be placed in the rocker cover, but this may be also be why it seems to fail so often.
In the earlier models, the cam chain tensioner presses down on the upper pass of the chain. This causes the chain to wrap further around the sprockets as it wears. However, the new design pushes UP on the upper pass of the chain, which has the opposite effect. I can't see that that's a big deal because either way, it's on the slack pass of the chain, and because the presser foot is facing upward, you'd imagine, if it's still pushing oil out through a hole in the foot, that this would lead to improved oiling of the chain, but they seem to snap more often. Maybe they reduced the oil supply at the same time, making it more prone to blockage?
However, when you look at the way the chain sits when the tensioner is not tensioning, such as at 7:48 in the
https://www.youtube.com/watch?v=aVFa8H1RpSY video, you start to get an idea of what might be happening.
With the tensioner pressing on top, it's working in the same way as gravity. The chain will naturally hang down even if there's no tensioner, and the tensioner plunger itself will come to rest after a stop, hanging down on the chain after the oil has drained away. The spring inside it will ensure that too.
However, in the case of the tensioner pushing upwards, once the internal spring breaks, the tensioner will drop over time causing the chain to take on an M shape as per 7:48. It's easy to imagine that when the engine starts from cold with no oil pressure, the slack chain coming off the driving sprocket (LHS, [rear] exhaust cam) is going to crash into the side of the tensioner foot, and if it's a bit worn and stretched, it's likely to jam and be dragged between the tips of the sprocket teeth and the side of the tensioner. It can only break...
I would almost guarantee that's what's causing these chains to snap. I haven't surveyed it, but I'm willing to wager that snapped MD chains always have broken tensioner springs (first). And that they always break on startup, after a longer period of not running.
Howzat, Alan Howatt?