CX500Z/A/B/C/D Cam Chain and Manually Adjusted Tensioner

changing the cam chain and adjuster

500 Eurosports and all 650s have automatic cam chain tensioners which are different to these parts, although the camshaft, sprocket and actual chain are the same, or very similar. Auto tensioners are at the end of the page.

You can't see inside the rear engine casing - so what do all these bits look like?

The mounting plate shown here is the extended plate fitted from the A model onwards, and cures the early CX500 cam chain failures.

I didn't have all the bits to show you, I'm afraid - missing are the adjusting quadrant which is pulled down by the spring when the locknut is loosened.

As the quadrant is pulled down by the spring, the tensioner blade is bowed inwards against any slack which has developed as the cam chain wears and stretches. The tensioner blade is anchored at the bottom, and the top is attached to the quadrant.

The guide blade does not move. It is anchored at the top by one of the guide plate bolts, with its bottom part resting in a slot cast into the crankcase.

Both blades are made from a very tough black nylon-like material which itself wears very little.

Cam tensioner failure occurs when the tensioner blade snaps at the upper or lower "neck". This leaves the cam chain free to rattle around, and it then chews it way through various portions of the crankcase. This is very noisy and if the chain breaks, the engine will probably be wrecked when the pistons hit the valves, bending the con rods and possibly even twisting the crankshaft.





Camshaft, with sprocket fitted and chain looped over. Farmost end drives the rev counter (tachometer) and on 500s, the cooling fan.

If you had SuperVision, this is what you could see from kneeling close to the rear left hand side of the engine!








Tensioner apparatus mounting plate. The ruler is in inches. This is the extended plate, which corrects the early trait of cam tensioner blade failure. The plate here is shown reversed - when bolted into place, its other face is towards you.

This plate has suffered abnormal wear, visible where the cam chain has rubbed a shiny line down the plate on the guide blade side. This was because the locknut thread was stripped when I bought the bike in April 2002, and the chain was thrashing about a little. The tensioner blades were not damaged. I removed the engine and had the thread retapped, fitting all new cam chain and tensioner parts.

If you have a 1978 or 1979 original model CX500Z (barrel shaped brake master cylinder, black tabs over Comstar spokes where they join the wheel rim, black radiator shroud) and your plate lacks the upper extension, you should immediately fit the modified parts, which are available as a kit from Honda. The original modification was done free of charge but I don't know if Honda will still honour this!


Here are the punch marks adjacent to the engine number on the lower left crankcase, to show that the cam chain tensioner modifications were carried out. Even better, inspect inside the rear casing with a dental mirror and a lamp, to make sure that the long plate has been fitted.

(Thanks to John for the clear photo.)

CX500As onwards are ok.



This photo shows the parts, as much as they lie inside the engine, shown against the inch calibrated ruler.

The "SuperVision" view would be from by the clutch cover.

Note that in this picture the spring is upside down - whoops, sorry!









Thanks to Maurice McAllen for the following photo set, showing the cam chain assembly before removing the guide plate (right) and afterwards (below) with the various components listed and identified.






PARTS PICTURE When following the instructions on changing the cam chain, the numbers on the picture below refer to the items in the parts list.

I've ringed in yellow (right) the most likely places to find where a slack cam chain has chewed away portions of the aluminium engine.

Look inside the oil filter element for the tell-tale silver or copper flakes or debris. This is bad news, and indicates major problems, especially if the flakes recur after flushing the engine and replacing the oil filter.

Coppery flakes indicate terminal damage to the shell (big end) bearings. This usually means a new engine. Replacing the shells is simple enough once the engine is out, but removing every trace of metal contamination in the oilways is a most daunting and difficult task.








A "fine example" of what damage a slack cam chain can do (below). All the metal ground off by the slack chain has to go somewhere ... into the lubrication system ... and the crankshaft is directly below the wear line. See also the Disaster Zone page for more dreadful things to worry about.

















The cause of the problem - a snapped cam chain tensioner blade (right), with the classic "back of the neck" fracture.









The slack chain caused abrasive damage also to the mounting plate (left).









John Clarkson sent this very clear pic of a cam chain which wore out very suddenly, making the machine-gun noise. Here his screwdriver shows how much slack was thrashing about on the left hand run of the chain. This demonstrates how and why the metal parts get chewed away.












ALTERNATIVE PARTS PICTURE : Another view of the components (below).

Judging the wear on the cam chain and manual tensioner system

You judge by how much the cam chain is worn out by how far the manually adjusted tensioner arm has been pulled down by its spring. The more worn the chain is, more of the slot is visible BELOW the shoulder of the tensioner locknut once it's tightened down. In these very clear pictures (thanks to Joe Robinson) the system is about half worn out,as you can see that the slot is about half way down its travel.

In the photo, once the locknut is tightened, none or very little of the slot was visible. So in this example the cam chain is fine. Only replace any damaged parts.

The yellow line shows the tensioner arm's direction of travel and the red line shows where the wear is best measured, below the locknut's shoulder.

If the "red line" or "below" clearance is clearly more than the corresponding clearance above the locknut shoulder (when it's tightened down) or exceeds 1/10 of an inch, replace the chain, spinrg, o-ring on the locknut, tensioner and guide blades.




Automatic Tensioner

The automatic camchain tensioner is apparently rather less reliable and long-lasting than the manual type. I've heard that it can fail as early as 20,000 miles, compared to double that for the manual type.

The auto tensioner can crack its casing, as the photo (right) shows. Thanks to Sture Lindberg for the photo.






This automatic camchain tensioner has broken in a similar place.











The photo (right) shows an automatic tensioner unit which had done 34,000 miles and was completely worn out with a dreadful engine rattle noise. Well over 1 inch of the spring and plunger are visible, and we reckon we caught the engine just in time before serious damage occurred.






Thanks to Neil Goodall who sent these two comparison pictures of a worn cam chain (left) and new cam chain (right). Here you can see the difference in the extension of the spring loaded automatic tensioner. On the new one, the top lip of the tensioner blade is well to towards the tensioner unit and there is a visible lead-in to the blade. On the left, the worn-out blade is almost flat against the chain and the tip of the blade is level with the tensioner's guide.

< Worn chain, old tensioner unit

New chain and new tensioner unit >







Changing the cam chain, tensioners and adjusting apparatus

You will need the following parts. Numbers refer to the big "PARTS PICTURE" higher up the page.

This photo of a new and well worn cam chain shows that a worn one bends far more and is visibly longer than a new one. The more sideway it bends, the more worn out it is.

A recent (September 2016) examination of a worn and thoroughly rattling cam chain, with the spring loaded adjuster at maximum travel, showed a length of 9.6" (24.2cm) and a droop of 1.5" (3.9mm).

Use of a torque wrench is mandatory - serious or terminal engine damage can occur if bolts come undone, or threads strip out.

On CDI ignition variants (Z, A ,B ,C or D), check the stator condition BEFORE dropping the engine.

engine oil and filter, DO NOT buy cheap pattern oil filters

50/50 mix of nonsilicate coolant and battery (distilled) water

rear crankcase gasket (look on Ebay and buy 0.4mm to 0.8mm gasket roll and cut your own casing gaskets - it's easy!)

water pipe transfer o-rings (8) (again, Ebay will get you a box of 400+ assorted o-rings for about £10)

cam chain (3, or 2 on the alternative picture) - it's common to ALL variants including the Turbos. DO NOT buy the cheaper pattern cam chain

CX500 Z, A, B, C, D Models : manual tensioner arm (6, or 5 on the alternative picture); spring (10, or 6 on the alternative picture); tensioner locknut (7) and o-ring; tensioner blade (5, or 4 on the alternative picture); guide blade (4, or 1 on the alternative picture); you can re-use the r-pin and locking pin unless they have been damaged.

all others : automantic tensioner unit (picture immediately above), tensioner blade, guide blade.

Remove the engine and the rear crankcase, water pump etc and alternator rotor. Remember that if you are changing the cam chain it's sensible to change the mechanical seal at the same time.

Remove all traces of the old rear crankcase gasket from both faces. Don't allow fragments to drop into the engine.

Unbolt the tensioner mounting plate - there are 4 x 10mm bolts holding it in place - then it just lifts away.

ZABCD : disengage the tensioner arm spring where it hooks over the mounting plate. Note which way round it goes - long leg downwards. Remove the tensioner locknut, r-pin and pull the tensioner blade away, it's not anchored at the bottom, it fits into a slot. Pull off the tensioner arm.

OTHERS : remove the 14mm bolt holding the auto tensioner unit and pull it away, it can be tight fit on the steel locating pin at the right hand end.

Remove the two short 10mm bolts which hold the camshaft driven sprocket (1 on the main picture) to the rear of the camshaft and slide the sprocket off the cam end. Unloop the old chain from the sprocket and from the bottom of its run where it engages with the crankshaft. Note the steel key embedded in the crankshaft end - this key is called a Woodruff key, and it not only secures the alternator rotor in the correct position on the crank, but also serves as an alignment mark during reassembly.

Loop the new chain over the crankshaft teeth and ensure that the Woodruff key is pointing the alignment mark. If you need to rotate the crankshaft and it's too stiff to move by hand, slip the alternator rotor on it and use that to move it.

Whilst pushing inwards on the right hand run of the chain, to simulate the tensioner pressure, jiggle the chain along the camshaft sprocket teeth until the bolt holes align with the corresponding marks adjacent to the water transfer pipes. This should leave the bolt holes exactly horizontal.










Here is a photo of the correctly aligned Woodruff key and set mark.

The yellow block and join line show what you line up and what it should look like.

In practice I've found that the cam chain timing can appear to be 1/4 of a tooth out of alignment, but this doesn't matter.

Attach the sprocket and fit the bolts finger tight.

NOW STOP WORK and have a coffee break for at least 10 minutes.

On return, check the timing marks again, and have another person check them against the diagram opposite. It DOES NOT MATTER which stroke the engine is on, I know this feels strange but take my word for it. Tighten the cam sprocket bolts to 11-14 ft lbs.

ZABCD : fit the guide blade on the left run (the flat face goes inwards), it's anchored at the bottom in a slot and at the top by one of the mounting plate holding bolts. Attach the tensioner blade (curved face inwards) to the left end of the tensioner arm and anchor it with the steel pin and r-clip (see pictures of how these locate).

Attach the spring, long leg downwards, to the retaining plate, hook it up to the tensioner arm with pliers and fit the tensioner arm onto its pivot pin, engaging the tensioner blade,s bottom hole on the steel pin close to the crankshaft teeth. Now allow the tensioner arm to be pulled down by the spring, pulling the tensioner blade inwards to take up slack in the new chain. Finally fit the locknut and its new o-ring, tighten to 6-9 ft lbs. If this bolt hole strips out, you can easily repair it.

OTHERS : fit the guide blade as above and the tensioner unit which pivots on the steel pin, it can be a tight fit. Anchor it with the 14mm black bolt and tighten that to 13-18 ft lbs. Fit the tensioner blade before pulling out the new auto tensioner's shiny steel securing pin, this allows the unit to extend and take up the chain slack.

ALL : Fit the retaining plate and tighten the holding bolts to 6-9 ft lbs.

Fit a new crankcase rear gasket - I use Hylomar on both faces to make a good join - and use new o-rings on the water transfer pipes.

After refitting the engine, set the valve clearances. Change the oil and filter after 500 miles.

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