Use pliers to release the spring clip securing the fat rubber engine breather hose and pull the hose away from the cover.

Use your 10mm socket to undo the two bolts securing the rear cylinder's rocker box cover and remove the cover. In the photo the right hand bolt has already been removed.

You can get the rear rocker box cover off without removing the air filter box, but not the front cover.

The front right hand plug is much easier to deal with.

As is the rear cylinder left hand spark plug.

But the rear cylinder's right hand plug is another of the awkward angle ones.

After removing the spark plugs, stuff a rag or a ball of kitchen roll into the plug holes, to stop anything falling in.

General view inside the rear cylinder head after removing the rocker box cover.

The valve stems and rockers upper centre are the two inlet valves which draw their fuel/air mixture from the right-hand carburettor, the intake of which is just visible at the top right of the picture.

The single exhaust valve and its rocker arm at at the bottom left of the head, and the curvy chrome pipe carries coolant.

Colin Miller comments - with regard to accessing the front cylinder head - "When I did the tappets, to access the front cylinder cover I removed the thermostat housing as I simply couldn't get access with my hands; but now that I have done it once I probably could work around that."

Use your pliers to unclip the front cylinder's engine breather pipe, and pull the pipe away from the rocker box cover.

Remove the front cylinder's two rocker box bolts. These are harder to get at and I found that a 10mm ring spanner was the only way to reach and loosen the left hand front bolt.

Once it's loose, reach in with your fingertips and unscrew it all the way out.

With both the rocker box cover bolts removed, wriggle off the front cover, forwards and to the right hand side. It's quite an art to get this cover right off, but be patient and ease it away.

If the thick rubber gasket should be dislodged from the bottom edge of either of the covers, take care not to tear it, and then replace it once the cover is right off.

With both covers removed you can get at the valve gear for clearance checking and making any necessary adjustments.

The procedure is very similar for both cylinders. The front cylinder's two inlet valves face backwards, towards the rear cylinder's two inlet valves which face forwards. The single valve for each cylinder is that for the exhaust.

The graphic here demonstrates the 4 stroke cycle; intake, compression, combustion, exhaust (popularly called suck, squeeze, bang, blow). At the start of the induction stroke, the inlet valve(s) open to allow fuel/air mixture to be drawn into into the combustion chamber as the descending piston creates a vacuum above it. At the bottom of the induction stroke the chamber is full of fuel/air and the inlet valve(s) close.

The rising piston compresses the fuel mixture as a compressed gas will explode with greater power than uncompressed. At the top of the compression stroke, the sparking plug ignites the mixture and the piston is driven back down its bore, rotating the crankshaft and providing power to the engine.

At the bottom of the ignition stroke, the exhaust valve opens and the rising piston pushes out the burned mixture. Then it all happens again. If you are doing 80 mph on your Deau this cycle is happening 1,500 times a minute per cylinder. Scary isn't it.

We will work on one cylinder at a time, first the front one, To check the clearances it is first necessary to ensure that the valves are completely closed. This can't be done by a simple visual inspection, so Honda have made it easy for us to tell by casting a special mark on the camshaft to show when the piston inside the cylinder is at Top Dead Centre, popularly called TDC. But there is a snag. Only when the piston is as TDC on the compression stroke are the valves all shut - at TDC on the exhaust stroke, the exhaust valve is not yet closed and the inlet valves are just thinking about opening.

How do we tell the correct stroke? Go round to the left hand side of the Deau and locate the big circular port shown in this photo. Use your 10mm Allen key to unscrew and remove the inspection port. This exposes the left hand end of the engine crankshaft, which has a handy 17mm nut on the end.

This large port cover is quite soft metal and may become stuck, or even damaged, especially around the hex hole where the Allen key fits in. If you need to replace it, the part number is "90087KT7000, Cap 30mm". If you can't remove it because it's stuck or the hole is chewed up, first soak it in ACF50 overnight. Then you'll need a large screwdriver and a rubber hammer. Place the tip of the blade on the perimeter of the cap with the blade facing towards the centre, and tap in a small chink so that the screwdriver's edge can get a good grip on the soft alloy of the cap.

Now give the screwdriver's handle some smart blows with the rubber hammer and try to knock the soft cover around in the anticlockwise (unscrew) direction. As soon as the cap starts to move and unscrew, you should be able to get it out with more conventional methods!

Ensure that the gearbox is in neutral, and the ignition is off.

Use your 17mm socket and ratchet handle to locate on the crankshaft end and get ready to wind the crankshaft slowly round anticlockwise.

As you tickle the crankshaft round anticlockwise, look closely at the exposed cam chain and valve gear, you'll see them start to move. If this is your first excursion into engine spannering, it's a profound step forward for you as you can now see how things happen as the engine is running.

What you are looking for is a tiny arrow or chevron cast into the steel sprocket which carries the cam chain. When this is at the vertical in relation to the line of the cylinder - remembering that the engine cylinder is angled forward - the piston is at TDC.

This photo shows you the chevron just past the vertical - I've done this so you can see what you are looking for.

The aluminium pillar is where the rocker box bolt screws in and is the reference point for the vertical line.

Remember that THIS photo just shows you what you are looking for, not where to correctly place it.

Turn the engine using your ratchet handle until the arrow timing mark on the cam sprocket is at the maximum upper part of its travel. It should then be concealed behind the aluminium pillar, when viewed square on from alongside. To further help you, there are two scribe lines on the sprocket, which lie parallel to the cylinder head's lip once the piston is at TDC. You can see these scribe lines on the above photo.

This photo makes it look as if the timing mark is past the vertical, but it's an optical illusion due to the camera angle being from the front. In fact the mark is immediately behind the pillar.

One quite good way to find out if a cylinder's piston is at top dead centre is to slide a wooden or metal rod through the spark plug hole and rest it on top of the piston. Holding the rod in one hand as the engine is gently rotated, you will be able to feel the rod move with the piston. When the rod has reached its highest point, that is top dead centre, although you won't know whether it's the compression (correct) or exhaust (wrong) stroke.

Just be careful not to trap the rod anywhere as the engine turns, and if you use a wooden one, not to break off the end inside the engine!

So now you have the front cylinder at TDC - but which stroke - compression (correct) or exhaust - (wrong). How do you tell? This is easy and is called a clack test.

For each valve in turn on the cylinder you are working on, grip the end of the valve rocker in your fingers, directly above the valve, as you see in the photo. Now give each one a good rattle up and down. If all three valves clack, they are not under tension, and the valves are closed at TDC on the compression stroke. This is correct.

If they don't clack - and you should be able to feel this movement as well as hear it - the valves are under compression at TDC on the exhaust stroke. This is incorrect, so use your ratchet handle to rotate the crankshaft a full circle, and check the timing mark - or rod, if you used that method - is again at the top. Repeat the clack test.

the Red Fox says : "For first-timers, I recommend, purely as a test, setting the valves to the incorrect position (ie top of the exhaust stroke) and feeling what it's like when it is wrong, so you get to know what the difference feels like."

If your feeler gauge set doesn't have a single blade of the required 5/000ths thickess, it's fine to slide a 2/000 and a 3/000 together to make up the required 5/000. Just add up the numbers carefully, and ensure that no unwanted feeler blades are stuck between the ones you actually want.

Once all the valves are clackificatious, take the 5/000ths of an inch feeler gauge (aka five thou or 0.005") - or 0.15mm in funny foreign thingies - and choosing one of the two inlet valves, slide the feeler gauge between the top of the valve stem and the underside of the rocker arm.

The gauge should be a close sliding fit, not quite bending up as you slide it into the gap. If it won't go in at all, the gap is too narrow. If it goes in and slides about very freely, the gap is too wide.

To adjust the valve clearance, leave the feeler gauge in place and use a 10mm ring spanner to loosen the locknut and your pliers to very gently turn in or out, as necessary, the square-headed adjusting screw. It's important not to push down on the mechanism as you do this. As you adjust the clearance, slide the feeler gauges in and out until they are trapped and won't move. Then unscrew the adjuster just a tiny amount until the feeler gauge can just be withdrawn.

Whilst holding the adjuster screw firmly in position with the pliers, tighten the locknut to a good firm finger tight and then use your torque wrench - and if you don't have one, or don't use it, you must be completely mad - to tighten the locknut to 16 ft/lbs. Repeat the feeler gauge test again and re-adjust if necessary, as fully tightening the locknut can slightly reduce the valve clearance.

Repeat the procedure for the other intake valve, using the same feeler gauge.

Choose the 7/000ths of an inch feeler (aka seven thou or 0.007") - or 0.2mm - and check/adjust the exhaust valve clearance.

Here, the single front exhaust valve clearance is being checked. Notice the 0.2mm feeler gauge.

For the rear cylinder, rotate the crankshaft again to get the cylinder to TDC, this time watching for the arrow timing mark or horizontal scribe lines on the rear cam sprocket. These marks are on the left hand side of the cam so you will be able to see them as you nudge the crankshaft round. The technique is the same as for the front cylinder.

Don't forget the clack test.

Here, the inlet valve clearances are being adjusted on the rear cylinder.

the Red Fox says : "If you are doing this for the first time, it's a good idea to have a more experienced friend give you a hand and check your work. If you get this wrong - the classic mistake is setting the clearances on the exhaust rather than the compression stroke - your engine will sound like a tinker's barrow, and you'll have to remove all the bits again to redo the work.

FIRST TIMERS - go slow and easy, don't rush!"

If you forget which rocker box cover goes on which cylinder head, this is the correct orientation. The rear cover is the one with the thick breather pipe connection to the air cleaner box.

In fact the covers are cast identically - but the front one has the breather pipe junction blanked off!

Honda claim that the bolts should be torqued to 7 ft/lbs. I disagree and recommend 6 ft/lbs, for the simple reason that years of working on CX500 engines taught me that the bolts are undone frequently enough to warrant caution regarding wear on the threads into which they engage. These are soft aluminium alloy and whilst a Helicoil repair is easy enough, it's a right pain if you strip out a thread because of over-tightening.

In fact there is no oil pressure inside the rocker box covers and no reason for the two bolts to be much more than a really good finger-tight. If you are paranoid about tired and worn threads, set the bolts to 5 ft/lbs torque. If there should be an oil weep, the bolts can be easily reached and tightened a little more.

Don't forget to re-attach the breather pipes on the covers, and secure then with their spring clips.

Replace the spark plugs with either NGK DPR8EA-9 or their expensive but longer-lasting NGK Iridium equivalents, DPR8EIX-9.