CX500 / GL 500 / CX650 / GL650

Tappets (Valve Clearances) and Cam Chain Adjustment; clean / reset Sparking Plugs

Skill levels explained.

Skill Level : 2. Dirty Level : 1. Mess Level : 1. Tools level : 1 : 10mm, 12mm ring and socket spanners; ratchet and extension bar; torque wrench; feeler gauges; spark plug removal wrench; slim nosed pliers; wire brush; a hand lamp may also help. Space required : 1

Time : expert 15 minutes, average 20 minutes, "first-time" 40 minutes.

All the hands, tools, and bike in the photos are the author's.

All nut and bolt sizes are given as the spanner size required to fit them.

I am very receptive to comments and suggestions, but you use these pages entirely at your own risk.

This page will show you how to check and adjust the valve clearances (tappets) and the manual cam chain adjustment on your Honda CX500, and variants, engine. If you own a "Eurosport" CX500E-C, GL500, or any of the 650cc or Turbo models, your cam chain tensioner is fully automatic, and you don't have to routinely do anything to it.

The CX engine's cylinder heads project well clear of the petrol tank and main spar and consequently there is nothing to dampen or mask the sound of clicking or clacking tappets. This makes the CX rider extremely aware of when things are in need of adjustment, especially when filtering past stationary traffic and the engine sound is reflected back from the sides of adjacent vehicles.

Tappet and cam chain adjustment is simple and can be done whenever desired; Honda recommend that the procedure is carried out as a matter of routine every 7,200 miles (12,000 km) for UK 500s, UK 650s, and all US models up to 1981; and every 8,000 miles (12,800 km) for other models. In practice I have found it necessary to adjust the tappets very much more frequently than that, even as often as every 2,000 miles, or the rattle becomes annoying, especially as the bike inexorably ages. However the "normal" tappet rattle doesn't do any harm, but a pronounced clattering should be dealt with immediately. You can even do this by the side of the road, if your travelling toolkit is up to it, and you allow your engine to cool.


Note that the engine should be absolutely stone cold; best left overnight in fact. Some Honda engines have the cam chains adjusted whilst the engine is running, but the CX does not. Place the bike on its centre stand on a firm surface and turn off the ignition. As you will be dealing with petrol, don't smoke or use any naked flame during this procedure.

Turn off the fuel supply and disconnect the fuel pipe from the petrol tank to the LH carburettor. My bike has a manual fuel tap with only one pipe, but variants with a vacuum operated tap will have two pipes. Remove the saddle. My bike has two spring loaded catches at each underside of the rearmost part of the saddle; other models may be different, and require the saddle to be unbolted.

Remove the 12mm head bolt securing the petrol tank, and lift off the tank. Some variants have the tank bolted at two points right at the front. Place the tank outside and away from any potential source of heat, flame or electrical spark. Particularly, if you are working in a shed or garage, do not place it at your only means of exit. If there is a fire, you won't be able to escape. I was once trapped in this way and ended up with very nasty burns to one leg.


Pull off both long sparking plug caps from the plugs, then remove both plugs, putting them safely aside if you plan to reuse them. Note that the plugs are unusually deeply recessed and a long reach plug spanner will be needed to get to them.

The chromed curvy bar at the bottom left of the picture is part of the engine protectors. You can also see the chrome water transfer pipe which feeds water between the water pump at the rear top of the engine, and the radiator right at the front. The aluminium L-shaped section at 11 o'clock in the picture is the water transfer pipe between the left cylinder jacketing and the thermostat. It's a good idea to give these parts a good visual check for leaks, whilst you are there, and to brush out any dust or dirt from the crankcase area underneath the petrol tank.



Remove the two 10mm chromed securing bolts and then the RH cylinder head rocker box cover. My bike has had these covers sprayed black. The rocker box covers may need a firm tap with a rubber hammer to dislodge them. Repeat with LH cover. Notice the exposed mechanism; rocker arms and valve springs.

Don't drop anything down any of the exposed works. If you have small children, keep them away. If you are disturbed during the procedure, spread a rag or old towel over the exposed engine to stop foreign object damage - maybe bird droppings - I've seen this happen.



Cylinder head bolts - check

Whilst the cylinder heads are exposed, you should check that the big 14mm head black bolts are correctly tightened. Here are the settings. Check these before you adjust the tappets as if the head bolts need tightening, this affects the valve clearances.

All 500cc except GL inc Turbo
36 - 40 lb / ft
5 - 5.6 kg / m
All 650cc and GL500
36 - 43 lb / ft
5 - 6 kg / m

UNDER NO CIRCUMSTANCES DO THIS "BY FEEL" as a stripped thread will mean removing the cylinder head and having expensive and time consuming repairs done. Always use your torque wrench.

Onwards ...

Photo shows the rear right crankcase, the timing inspection port with the 17mm round cover, and the manually adjusted cam chain tensioner locknut.

Thanks to Neil Babcock for the very clear photo.







If you have a manual cam chain tensioner, you will see a 10mm head bolt protruding from the rear of thecrankcase, and just below this, a 17mm head round inspection port cover.

Newer models have a rubber tube from the port cover leading to the air cleaner box directly behind; if so, pull the tube off first. You will probably need an adjustable wrench to remove the port cover, as the nut size is about 23mm I think. The older models have the plain 17mm port like the photo opposite.

Remove the round cover, but do not do anything yet with the tensioning bolt. If you can't see a tensioning bolt, you have an automatic cam chain tensioner, and your task will be somewhat easier.



Wiggle the rear wheel and put the gearbox into top gear (left).





Sit down close to the inspection port and peer into the crankcase aperture. If you can't see in, a small lamp will help. You will see a circular flywheel with a smaller dull metal circumference and a larger polished steel circumference. Now reach back to the top of the rear tyre and firmly rotate the wheel clockwise (or forward from the top). You will see the engine flywheel being turned "upwards". Viewed from the front, this would be turning clockwise.






Your engine completes four strokes for each complete cycle; intake with the piston descending and sucking in the fuel / air mixture, compression with the piston ascending and squeezing the mixture, combustion with the piston being driven down by the force of the explosion; and exhaust with the piston ascending again and driving out the burned mixture. Thus, at two stages in its cycle, the piston will be at its uppermost limit of travel. This is called top dead centre, popularly called TDC. The four stroke cycle is often called suck - squeeze - bang - blow.








Inlet valves
Exhaust Valves
Piston is

Valve clearances for each cylinder are done with the engine set to TDC on the compression stroke, and the inlet valves closed. The trouble is, the part of the crankshaft you can see will be in exactly the same position at TDC for both the compression and the exhaust stroke, and you can't tell by just looking at it which stroke it is on.

As you nudge the roadwheel clockwise, watch the four exposed rocker arms on the RH cylinder head, right in front of your nose. There are two pairs - inlet, nearest you; and exhaust, furthest away from you. The nearest pair are the ones to watch, as when they have just finished closing (rising up as you look at them) the engine is at TDC on the compression stroke.

To get this exactly correct, gradually nudge the road wheel clockwise and peer at the flywheel. Scored into it are deep marks which tell you when the piston is at TDC. You will see first of all a pair of unmarked lines come into view; then a mark with FR, then a mark with TR.

I did photograph this view of the engine but it didn't come out, so here is a drawing of what it looks like. Note that the calibration mark is the grey-shaded pointer on the left of the aperture, and the FL/FR and TL/TR marks are on the dull part of the flywheel.

Ignore the FL and TL marks for the moment.

Nudge the roadwheel forwards and watch the inlet valves nearest you. When you see them rising, the piston is coming up on the compression stroke; continue to nudge the roadwheel gently until you see the TR mark align precisely with the calibration pointer. If you are on the compression stroke, you will be able to rattle both the inlet and exhaust rockers on the RH cylinder head.



There should be a small but feelable up-and-down movement, and an audible clack. If you have the piston on TDC of the exhaust stroke, there is no free movement of the inlet valves, they are locked up solid, and you need to turn the roadwheel enough to nudge the engine round another complete turn to the TR mark again.





One good dodge here is to take a T-handled screwdriver and slide it into the hole where the spark plug was. Whatever you do, don't use anything that could disappear completely down the hole or you will have very major problems. A T-driver can't fall right in. As you turn the engine, the T-driver is moved up and down by the piston as it rises and falls, and it's consequently dead easy to see where it is in its cycle of travel, as you can see it and feel it.





Ok, you've got the RH cylinder at TDC on the compression stroke. Setting the tappets or valve clearances means checking and possibly adjusting the tiny amount of free space between the top of the valves themselves, and the rocker arms (the mechanism which actuates them). As the engine gets very hot and its metal expands, this clearance ensures that the whole thing works correctly. It is measured in thousands of an inch ("thou") or tenths of a millimetre, and varies according to which model of bike you have, as per the following chart.

Valve clearances (engine cold)

CX500Z, A, B, C, all CX650 variants inc Turbo
0.1 mm / 0.004" / 4 thou
0.12 mm / 0.005" / 5 thou
All other 500cc variants (Eurosport, GL500) inc Turbo
0.08 mm / 0.003" / 3 thou
0.1 mm / 0.004" / 4 thou

Your feeler gauges are a set of slim steel tongues which are precisely machined to a certain thickness, which is etched or stamped on them. For my bike's inlet clearances, I will be using the gauge which has the thickness of 4/1000ths of an inch, or "4 thou" or 0.004". If you work in millimetres, the gauge is marked 0.1mm, i.e. 1/10th of a millimetre. For the exhaust clearances, I will be using the gauge of 5/1000ths, "5 thou", 0.005", or 0.12mm. Your bike may be different - check with the above chart. Even if the clearances are different, the technique of checking and adjusting is exactly the same.




Some feeler gauges do not have a single tongue for the required thickness. It's fine to slide two or more tongues together to build up to the necessary thickness. For example, if you don't have a 5 thou gauge, you can use a 3 thou and a 2 thou slid together to add up to 5 thou. This is quite ok, but be careful that no other gauges have accidentally slipped in between the ones you want.


I've now bought the special Honda tool for adjusting CX tappet screws - it's pictured here and is part no 07708-0030400. If your dealer can't find this part, try the alternative part number 07908-KE90200.

It's a handy mushroom-shaped tool with a knurled handle, 1½" tall and with the exact size square hole for getting those clearances spot on. Recommended, and only about £3.




Dave Harwood tells me of this alternative tool. He says "I think it's overpriced at US$20 but it is longer and gives fine adjustments."






Slide the gauge between the top of the valve stem and the bottom of the adjuster on the rocker arm. It should be a close, snug fit, but should just be able to slide in and out. If it won't go in at all, or rattles about too freely, the clearance needs adjusting. Leave the feeler gauge in place, and use a 10mm ring spanner to slacken the locking nut and a pair of slim nosed pliers to grip and turn the adjusting screw gently.

Useful Tip! After much experience with this class of engine, I have found that the tappets will start to rattle somewhere about the 1,000 mile mark after adjustment. This is an annoying thing to happen, and now I set the clearances to very tight - so I can just withdraw (drag!) the feeler gauge out afterwards.


Tweak the adjusting screw whilst you slide the feeler gauge in the free space, until the gauge can only just move, but is not trapped so tight that you can't extract it. Now whilst you use pliers to hold the adjuster still, tighten the locknut a little more than finger tight. Remove the ring spanner.

Now check the clearance again. If it's still ok, use your torque wrench to tighten the locking nut to the correct setting of 11 - 13 ft lbs (500cc engines) or 14 - 18 ft lbs (650cc engines). Check the clearance again, and readjust if necessary. Don't forget to use the torque wrench to finally tighten the locknut.






(A rather unusual thing happened when I did the tappets on 15-Apr-03. Two of the adjusting screws, on the same cylinder head, stripped, despite using the correct torque. Luckily I had spares. I now recommend not tightening them more than just above the lowest setting.)

This is all a little fiddly, and sometimes you will wish that you had three hands - one to hold the feeler gauges, one to hold the pliers and one to hold the spanner! But it ensures that your valve clearances are spot on, and this staves off the dreaded tappet rattle for longer than usual. It's all mostly a question of practice.

Repeat, checking the clearance on the other inlet valve.

Now select the other feeler gauge and check / adjust the slightly wider exhaust valves. These have different settings because they have very hot exhaust gases rushing past them, whereas the inlet valves are cooled by incoming unburned fuel / air mixture, and consequently they don't get so hot.

That's dealt with the RH cylinder and the LH one now needs the same procedure. Go back to the inspection port, and this time nudge the rear wheel backwards, from the top of the tyre (as if you were reversing the bike) as you peer in through the hole. When the TL line (not the FL line) is against the calibration mark, the left piston is as TDC. Check that the LH tappet arms are free and "clackable". If you find they are locked up, nudge the engine round 360 degrees until the TL mark comes up again, and recheck for free movement of the valve mechanism. Check and adjust the clearances in just the same way as for the RH cylinder head, remembering that the inlet and exhaust clearances are different.

When adjusting the left hand valve clearances, the action of tightening the locknuts does tend to loosen the adjusting screws very slightly. To avoid this, screw in the adjusters so that the feeler gauges are just about slideable through the gap. Then check them after torqueing the locknuts.

Don't turn the rear wheel or engine just yet.

If you have a manual cam chain adjuster, use a 10mm ring spanner to undo the locknut 2 turns. Just in case the adjusting arm has stuck, give the tensioning bolt a couple of firm taps (not a hefty great clout) and then set your torque wrench to 6½ ft lbs and tighten the bolt. NEVER do this "by hand" as if you strip the thread on the tensioning bolt, this is an engine-out job to repair.

Replace the inspection port cover, finger tight plus 1/16 of a turn.





Insert one of the sparking plugs into the special spanner you used to extract it, and examine it. The colour of the plug gives a good indication of whether or not the fuel / air mixture is correct. If the plug is a light or golden brown colour, this is fine. A sooty, oily or obviously burned plug indicates a problem.

If you are not fitting new plugs, give each old one a good going over with your wire brush and use your feeler gauges to set the plug gap (between the central core and the little arm that projects over it) as per the following chart. In fact always check the gap on new plugs - they probably aren't correct. If the gap needs adjusting, gently bend the outer curved-over electrode.

Now clean and gap-check the old plugs, and put them in the boxes your new ones came in. Then put the boxes in your travelling toolkit. Who knows when you may need a spare plug?

Sparking Plug Gap / Type

Spark Plug Type and Gap
UK all 500cc variants except Turbo
NGK DR8ES-L or ND X24ESR-U set to 0.6-0.7mm / 0.024 - 0.028" / 24 - 28 thou
US all 500cc variants (carbs with accelerator pump)
NGK D8EA or ND X24ES-U set to 0.6-0.7mm / 0.024 - 0.028" / 24 - 28 thou
All 650cc variants except Turbo
NGK DPR8EA-9 or ND X24EPR-U9 set to 0.8 - 0.9 mm / 0.032 - 0.035" / 32 - 35 thou

Check that the drain holes at the bottom of the spark plug recesses are not blocked, then replace the spark plugs in the cylinder heads; finger tight plus 1/8 of a turn with the spanner. To eliminate the threat of cross threading a spark plug, push a 6" length of flexible garden hose or aquarium pipe over the top of the spark plug ceramic body and use that to twiddle the plug into place. If the thread tries to go in cross threaded, the garden hose will slip free before any damage occurs. This is especially useful on cylinder heads where the angle of the plug is such that you can't see what you are doing, or where your fingers or plug box spanner can't get square-on to the head.

Check that the two (inner and outer) cylinder head cover black rubber gaskets are not damaged; ensure no debris or tools are left in the head spaces, clean the mating surfaces and refit the covers, tightening the chromed bolts to no more than 6½ ft / lbs. If the threads strip out - a common wear problem as the bike ages - see this page for how to repair them.

UNDER NO CIRCUMSTANCES TIGHTEN THE 10mm CHROMED HOLDING BOLTS "BY FEEL" as the threads are very easily stripped. Always use your torque wrench. In fact, these bolts don't need to be very tight. Little more than finger-tight is quite enough.

By "finger tight" I mean placing a 10mm socket on the top of the chrome nuts and turning the socket by hand as far as you can comfortably do so. If afterwards you get an oil weep, you can tighten them down a touch more than finger-tight. But these important threads get a lot of wear with the necessity to check the tappets so often on the CX engine and anything which reduces the danger to them is a good idea. At least repairing them is a simple task provided you have the correct threading kit, and isn't an engine-out job.

The left and right head covers are interchangeable, but they only fit one way up, as there is a moulded valley for the spark plug leads, which you now snap back on.





Before you replace the fuel tank, remove the radiator cap and if necessary top-up the coolant level with antifreeze or deionised water. When refilling or topping-up the radiator, use a 50/50 mixture of distilled or deionised water (battery water) and silicate-free antifreeze, with its distinctive orange or pink colour. Silicate-free antifreeze is much better for the ceramic seal inside the water pump, and cooling system generally.

UK readers note that if you buy antifreeze from Halfords, it should be the more expensive "Advanced" formula, which does actually say silicate-free on the rear of the bottle.

Refit the cap and replace the fuel tank; connect the fuel pipe(s). Turn on the fuel, checking for leaks.

Put the gearbox back into neutral and start the engine in the usual way. A good method of identifying the source of any noises is to place a small socket, on an extension bar, against various places on the engine casings, and rest the other end of the bar just inside your ear. You'll hear every sound. Experience is the best guide, but regular whirrings are ok, anything heavily scraping, knocking or hammering needs investigation. A child's play-doctor stethoscope is equally as good for listening to engine noises.

Short road test and leaks check, and you're done. To check the oil level, remove the dipstick, wipe it clean and reinsert it without screwing it in. Remove it again and the level should be between the lower and higher marks, anywhere in the criss-cross hatched area of the dipstick is fine. Top-up oil if necessary but do not exceed the higher of the two levels.

If you get an oil weep or minor oil leak from where the rocker box cover meets the cylinder head, or from the water drain hole half way down the outer face of the cylinder head, it is most likely to be a misplaced rubber gasket, probably the centre "ring" one. Stop the engine and remove the cylinder head cover. You can of course replace both the black rubber gaskets, but a temporary fix is to use non-permanent gasket sealing compound such as Hylomar, smeared on the mating surfaces. A leak here will send oil all over your upper boot and shins, as well as dropping hot oil on the exhaust collector box.

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