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Thoughts and solutions on the subject of automatic timing chain tensioners

3118 Views 16 Replies 6 Participants Last post by  Beestoys
Yes, I know, a lot has been written on the subject of "automatic timing chain tensioner", but no one has yet described the cause of the problem or offered a solution.

Well, it’s not easy for me to explain it in English, but I try to do my very best. ;)

First, let's take a closer look at the problem:

It is known that the timing chain in the automatic tensioner has a service life of about 50.000 km/31.000 mi.

Many forget to replace the chain or postpone it because they have not noticed anything unusual by then.

Often used CX 500 / GL 500 or CX650 motorcycles from 2nd or 3rd hand are offered with a mileage of about 70.000 km/45.000 mi, where the first chain is still installed.

The problem with the automatic chain tensioner usually arises when the service life of the chain has been exceeded and the chain hits the fully extended tensioner-pin.

Then the locking ball works its way into the tensioner housing and forms a hutch trough there.

This hollow can be seen very well on this broken tensioner.

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What happens if the ball has hit a hutch in the housing?

The path of the ball along the ramp at the tensioner pin is limited by the locking pin, the ball can no longer follow the tensioner pin sliding under it and cannot lock it.
The chain elongates and keeps hitting the tension pin, driving it like a wedge under the ball, which works its way further and further into the housing.

The consequence of this is, that the tensioner is no longer locked in the extended position.

At around ~45.000 mi/70.000 km such tensioners usually have around 3-4 mm of play at the end of their tensioning path.

What does this mean for the tensioner and the new chain?

The effective tensioning path of an ACT (automatic chain tensioner) is 7.95 mm.

If the tensioner is on a new chain, we measure 17.26 mm between the push button and the housing (left)

When the clamp is fully extended, the distance between the push button and the housing is 25.21 mm.

The difference between the two dimensions results in the effective working stroke of 7.95 mm.

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These 7.95 mm are now distributed over around 50.000 km, which corresponds to a tensioner pin range of 1.59 mm for 10.000 km/~6.200 mi.

What does this mean for us?

If the ball has already worked so far into the housing that our tensioner, when we have removed it and press against the fully extended roll pin, has a play of 3.5 mm, for example, then we have to subtract this 3.5 mm from the effective working stroke.

And that means: 3.5:1.59 = 2.27 x 10.000 km = 22.900 km/~14.200 mi, which we have to subtract from the 50.000 km/31.000 mi!

And that means nothing else than that our chain is no longer tensioned after 27.000 km/16.800 mi and then starts to beat again, long before it’s used 50thousand kilometers or 31.000 mi!

If we now install such a tensioner with a new chain and weigh ourselves in the certainty that the chain is good for 50.000 km/31.000 mi, we will drive almost 14.300 mi through “world historie” with a beating chain by then.

The ball works its way deeper and deeper into the housing and this can ultimately lead to the housing breaking.

The fatal thing about the construction is that the rear "oil hole" is exactly where the ball hits the housing!!!

The same thing happens to us when we buy a used CX or GL with ACT that has 80,000 km/50.000 mi on the counter and the seller proudly announces that he has just replaced the timing chain and mechanical waterpump seal. I would deduct just $ 350 from the desired price for this.

But what can we do with the tensioner?

First: There are different versions of the housing.

Some are cast round, the others are flat milled. Some have the oil hole at the back, where it is of no use and also forms a predetermined breaking point (see above), and the newer tensioners, like those of the turbo models and the 650, have the oil hole further up, from where the oil comes from can run nicely over the tensioner pin.

If someone has a tensioner that already has the oil hole at the top and has no play when extended, he can widen the oil hole with a countersink and thus prepare the oil a better way. This modification was mentioned elsewhere and is well known.

My additional modifications I do with the automatic chain tensioner:

Older tensioners, which still have the hole at the back, get an additional oil hole with a countersink on top.

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WARNING: This only works on a milling machine with the appropriate device!

Anyone who tries to work with a normal hand drill or a drill stand will destroy the housing !!!

In addition, instead of the hammered-in roll pin for the working-stroke limiter, my tensioners get an M4 grub screw with which the distance to the tensioner-pin can be set precisely so that the roll pin (which may have been hammered too far) does not scratch the tensioner-pin and then possibly get stuck.

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Tensioners, where the ball has already worked its way into the housing, tension again when the rear locking pin is removed. This allows the ball to move a little further back and locks the roll pin again. This is also a well-known measure that is often practiced.

However, it has the disadvantage that the ball can jump out to the rear if the housing is further worn.

So, my solution is to take the dowel pin and thus also the ball away from this area! o_O

But this is only possible if the zero position of the tensioner-pin on a new chain is shifted by a longer gear cluster.

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The gear-clusters are CNC turned, drilled and then hardened.

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Another advantage of the longer gear clusters with an intact housing (!) would be an extension of the service life of the chain, since the effective working stroke changes with the length of the gear cluster. +3,5 mm corresponds to almost ~14.200 mi longer service life, which means that the ACT comes close to a regularly tensioned and well-maintained manual tensioner.

In the case of a housing in which the ball has already formed a hutch, the advantage of a longer service life does not apply, since the ball then comes back into the area of the trough. But you can drive again about 50.000 km/31.000 mi again without a beating chain. :D

But how do we know when the tensioner is extended and when we have to replace the chain? :unsure:

...Very easily:

We build a modified oil sight glass in the engine cover through which we can see the ball and the tensioner-pin.

We recognize when the ball is at the stop and the tensioner is extended, but we also see when the tensioner is no longer locked and it moves, because the ball has reached the area of the hutch. :D

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(The last picture shows a prototype with a glued-in oil eye. The upper photo shows the final version with a screwed-in oil sight glass.
The plastic insert behind the pane has been replaced by a glued-in aluminum ring, which is more attractive and presses the pane better against the seal)
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Great work as always......we all apreciate the engineering standard.
Even Honda 4s of the 70s and 80s had weak manual tensioners....
In response to #4 if we consider cost and time to put into older machines it mighten seem rational.....BUT many use their older bikes sporadically and once these mods are done theyll see many years of ownership....thats the way I justify it anyways..
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