Honda C90 - cam chain tensioner

26th Feb 2022
Tags: Engine

Operation

As shown in the diagram below the cam chain tensioner is made up of two parts, the tensioner push rod (8) and the chain tensioner (5).  The rod pushes against a tab on the tensioner rotating it in the direction of the arrow so the roller mounted on the end of the tensioner takes the slack out of the chain.

C90 cam chain tensioner (1970s)

Honda don't seem to have had much luck on this front and the cam chain tensioner department were kept busy for a couple of decades trying to get a satisfactory design.

v1 automatic (A)

Honda started of with all best intentions having introduced their new overhead-cam engine in the C65 (1964),  saying that they had invented a self adjusting system that would put the correct tension on to the cam chain without the need for periodic adjustments.

The solution is rather exotic. The chain tensioner spring compartment is flooded with oil that enters through an oil strainer mounted in the left hand crankcase at the base of the compartment:

The shop manual says the oil strainer - which incorporates a mesh screen - acts in conjunction the centrifugal filter and the filter screen under the oil pump to  purifies the oil. It's hard to believe this tiny device made much of a difference to oil purification but I suppose it at least prevented any bits in the oil jamming up the tensioner.  Presumably the strainer also plays a role in once the oil fills the guide compartment it stays there, but it is not obvious how it works from the pictures.

This same system was used in the CM91 (the first cub to use the new 89cc OHC engine) and the early C90s:

After the engine starts, the reciprocating motion of the pushrod forces the air above the oil level in the tensioner spring compartment out of the hole in the pushrod (see "A" in the diagram below) and into the crankcase through a hole (B).

Eventually the level of the oil rises sufficiently to submerge hole B so that the the tension spring compartment is constantly flooded with oil, making it perform as a damper.   This is the theory anyway, the design was quickly modified so presumably did not work as well as expected.

v2 automatic (B)

Attempt #2 still aimed to use a combination of oil pressure and spring power to maintain tension automatically, but the vented rod was replaced by a shorter unvented version and a substantially larger spring, perhaps indicating the original version was not able to exert sufficient tension to prevent chain noise.

v3 mechanical (A)

These modifications appear not to have worked either and at some point in the early 1970s Honda replaced the sealing plug with this contraption:

The spring applies the correct tension and then the rubber tipped rod shown above is screwed up through the spring to the base of the pushrod preventing it from moving downwards.  Presumably this solved the noisy chain problems, but the mechanism could no longer be properly be said to be automatic.

v4 mechanical (B)

At some point in the 1970s Honda got rid of this idea too and came up with a more elegant design (this is the version used on the C90z2 and C90zz cubs made between 1977 and 1982).  

The two small collars (shown 2nd from the right in the picture below) slide past each other and lock the guide rod (second left) in position when pushed by the adjusting screw that fits in the sealing plug, and this holds the pushrod tight against the tensioner.   Simply  loosen the adjusting screw so it releases the guide rod and the spring will apply the correct tension, tighten the screw and the plunger is gripped in the correct place.  A lovely simple solution!

Three of the four version used on the UK c90s are shown in the 1976 parts list:

The modified oil damped version in the center, the mechanical gizmo that was added to lock the same in place and finally the version used on the Z2/ZZ models 

v5 deluxe

Honda treated those countries that got the "deluxe" C90K series (e.g Australia) with yet another version.  This one - based on a design original introduced with the C70 in the early 1970s - is arguably the most byzantine of the lot.

The pushrod is still spring loaded but locked in place by a screw accessed from the side of the crankcase.  The mechanism also allowed for secondary adjustment that could be used to increase the preload spring tension.    

v6 automatic revisited

in the early 1980s the Honda cam chain tensioner department had another brainwave and reverted back to their automatic adjustment/oil damped approach, but with a twist.  This is the version used in the 12v cubs.

Apparently this design hit the spot as it has not been altered since it was introduced.

The tensioner pushrod position is maintained by the presence of a constant amount of oil in the chamber.   Oil splashed from the cam chain enters the ports in the side of the pushrod and fills the void on the underside of the rod.  

When the tensioner spring pushes the rod up to remove the chain slack the one-way ball valve opens and allows oil collected in the push rod to flow into the chamber below.  When excess tension starts to move the rod downwards, pressure from the oil in the chamber forces the valve closed to the position of the rod is maintained.

Adjustment

The CT90 shop manual and late 70s C90s owners manuals say that the mechanical tensioners should be adjusted while the engine idling, which I suppose is logical since  when the engine is running the tension will be on the underside of the chain and the slack is on the trailing side that the tensioner pushes against.

Some people prefer to put the engine at TDC on the compression stroke - this works because you are rotating the engine to a point where the camshaft is not trying to rotate the engine so the adjuster spring can take up all the slack.

standard adjustment (Z2/ZZ models)
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