Primary chain tensioner

The workshop manual is a little vague on this one. Can anyone help me with advice on the primary chain tensioner, does it need any maintenance, I take ie that it is tensioned with oil pressure?

I assume you are referring to the MkIII hydraulic tensioner? This should need no more maintenance than an occasional dissassembly for cleaning. It has springs and a check valve to automatically adjust chain tension. The polymeric feet that contact the chain do eventually wear and require replacement and if the chaincase oil becomes very dirty, the check balls can become obstructed and allow the oil to flow back out under chain pressure. These are usually rare occurences.

Vince,

The tensioners are similar in operation to a shock absorber; there is no oil pressure per se. As Ron mentioned, the two primary chain tensioners require little maintenance and never need adjusting. I’ve never seen the balls that Ron refers to so did panic if you can’t find these in your tensioner either.

Jason

OK Jason, now you've got me doubting myself. It's been long enough since I had my own MkIII primary apart that I'm not sure, but I seem to recall that at the bottom of the "trough" formed by the sheet metal plate designed to funnel the primary oil into the adjuster is a ball bearing check valve to allow oil into the chamber, but not out. Can somebody confirm or deny this??

Ron L said:

but I seem to recall that at the bottom of the "trough" formed by the sheet metal plate designed to funnel the primary oil into the adjuster is a ball bearing check valve to allow oil into the chamber, but not out. Can somebody confirm or deny this??

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I can indeed confirm there is a single ball bearing that acts as a one way valve that feeds the tensioner gallery.

Vince,

The 850 MkIII tensioner uses the primary oil that is picked up by the lower run of the chain when the engine is running, the oil then drips from the upper run into the 'trough'. When the bike is left standing the oil drains from the tensioner mechanism, and takes a while to re-prime itself after the engine is restarted.

I

Thanks for all the help guys, your responses and this wesite are fabulous! So, following your advice am I right in thinking that the tensioner springs take up the slack in the primary chain and when running the oil doesn't add to the tension it simply holds the plungers in place by virtue of the non-return ball??

vince said:

am I right in thinking that the tensioner springs take up the slack in the primary chain and when running the oil doesn't add to the tension it simply holds the plungers in place by virtue of the non-return ball??

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Well you are on the right track but it is slightly more complicated than that.

The oil acts as a damping medium for the plungers (like a shock absorber as Jason mentioned) but imagine that the two tensioner plunger rods are pistons, one at each end of an oil filled tube, if you push one piston inwards this will have an effect of moving the other piston outwards (like a hydraulic brake system).
As the upper and lower chain runs constantly alternate between slack and tight when the engine is running as the crankshaft doesn't really rotate at a constant speed (which is more pronounced at lower revs). The tight run of the chain will try and push the tensioner plunger back into its housing (against the spring pressure), this 'action' will then cause a 'reaction' pushing the opposite plunger outwards increasing the amount of tension on the run of chain that is loose.
The one way ball valve closes as this happens otherwise the pressured oil between the plungers would simply escape back to the feed trough.

Excellent, got it! thanks L.A.B., Ron L, Jason

Wow, I'm intrigued by this ball check!

I looked through my parts manual for this ball but it's not shown. Perhaps it's an integral part of the aluminum tensioner housing and trapped in place and hidden from sight?

Jeez I must be getting old. As few as five years ago I would've never forgotten or overlooked something as fundamentally obvious as a ball check.

Pouting,

Jason

Jason Curtiss said:

I looked through my parts manual for this ball but it's not shown. Perhaps it's an integral part of the aluminum tensioner housing and trapped in place and hidden from sight?

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I think you are correct that the ball and the screw plug that locates it are not available separately from the tensioner housing.

The head of the screw plug can be seen drawn in the parts book in the Group 10 illustration. It's the screw head shown immediately to the right of the stud labelled '33' (part 06-3189).