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A Guide To Valve adjustment
By Zebee Johnstone <zebee@zip.com.au> with additions by
Murray Dawson <dawsonm@faceng.anu.edu.au> and
Alan Pennykid <pennykid@zip.com.au>

Theory - why do the tappets?

The fuel/air mixture comes in via an open inlet valve, which then closes. The piston comes up, compresses the mixture, the spark sets fire to it, the piston is blown down, then comes back up to push the exhaust gases out of the opening exhaust port.

The valves have to be able to open and close properly at the proper time. To do this, they have to have the correct clearances between them and the bits that open/close them.

A too-tight exhaust valve can't fully close and properly seal the combustion chamber. Escaping gases will eventually burn the valve.

Clearances are adjusted by either the adjuster held in the right place on a screwthread by a locknut - screw and locknut system - or by shims placed between the valve and the adjuster - shim and bucket system.

Screw and locknut is easy for the home mechanic, shim and bucket is harder - it's way more fiddly and you either need a stock of shims or a friendly bike shop that you can reach when your bike is not going because you have the top end in bits. Plus on overhead cam bikes it can mean removing the cams and camchain.

This article will be on screw and locknut.

Info you will need:

• The correct valve clearance for inlet and exhaust valves.
• Spark plug gap
• Where the valve clearance should be measured.
  It's almost always when the piston is at Top Dead Centre (that is when it's at the very top of its travel) on the compression stroke, but I have come across bikes where it's "when the cam lobe is flat pointing forwards" and other oddities.
• How to turn the motor.
  You can set the bike so the back wheel is off the ground and the bike in top gear and turn the motor with the back wheel, but I find that a pain when you are trying to get the piston just so, and far prefer to turn it with a spanner or socket or allen key via the cranknut. If you want to do that, you need to know where the cranknut is - which engine cover it hides behind.
• Where to look to find the timing markings.
  You don't need to know these, but if you do it saves having to poke a screwdriver down the plughole to find TDC.

• Rocker Cover Gasket
  not all rocker covers have gaskets, and you don't need to replace it every single time. But you can bet that if you don't have a new one on hand, the one on the bike will tear.

You need to look at your bike to determine what tools will do the various jobs. There is no standard.....

• Feeler gauge
  - a set of blades of varying thicknesses which you can stack to make the required clearance. eg the Guzzi requires .22mm so I use .10mm, .08mm and .04mm blades stacked. Be sure you have the correct sizes to make up the clearance for your valves, and that you are using the right ones - feelers come in Imperial and metric sizes.
• Sparkplug spanner
  - make sure it will reach the plugs and is the right size. My GT550 Kwak needed a weird size spanner.
• Tools to remove rocker cover (cover over the valve gear, on 4cyl bikes this is usually the cover over the cam)
  - usually a 10mm socket or a 5mm allen key, but check your bike to be sure.
• Tools to get to and turn the cranknut
• Spanner to turn the adjuster locknut
  - can be 8mm or 10mm or 13mm or anything really. Rings are better than open enders if you can fit one in.
• Tool to turn the adjuster itself.
  - The "Screw" in "screw and locknut" is a threaded rod, not a screwhead, the bit you use to turn it is usually a square or reactangular tab 3-4mm across. Some bikes have an adjusting spanner in the toolkit, but usually you use a pair of pliers.
  You can make up your own adjuster adjuster by cutting a slot in an 8mm diameter length of round mild steel, and welding a T-piece handle to the top. If you're careful with cutting the slot, you can make it an interferance fit so that it will hold itself on the adjuster when you're working with the feeler gauge and the spanner.
• Container to keep bolts in
  - if you remove anything from the bike, put it in this. Otherwise it will scurry off and hide.
• Plug gapper
  - while you can gap plugs with feeler gauges, a proper plug gapper is so nice and simple to use. But hey, I'm a tool junkie!
• Bit of rag to mop up stray oil and to wipe tools

• - remove sparkplugs and place in container.
• - remove rocker cover
  Be gentle with the bolts. If you need a lot of torque on your undoer, then pause a moment. Steel bolts into alloy like corroding themselves to the alloy. A bit of WD40 down the bolthole and a cup of tea while it works its way in can save a lot of heartache. Make sure you have undone all the bolts before trying to remove the cover.

  Covers can stick on too, especially if someone has used gasket goo. Try not to rip the gasket. If you do, then it must be replaced, or the join will leak. Gaskets are cheap.... A cover that's been gooed down and not removed for a bit may need a gentle thump with your fist or a rubber mallet.

  You may need your rag at this point to deal with the odd oildrip.

  Don't forget to put the bolts somewhere safe.

• - Rotate motor until first valve is ready to adjust (I am assuming normal TDC Compression adjustment here, if it's a weirdo, you are on your own...)
  • - place finger on plughole of cylinder.
  • - rotate motor in normal direction. If via rear wheel then as if the bike is going forward, if via cranknut then usually clockwise.
  • - as motor rotates you will feel pressure against your finger. This is compression. As the pressure increases, you can stick a screwdriver down the hole to feel the piston rising and work out when it is at the top of the travel. Be careful not to get the driver jammed or scraping your cylinder! Slow and careful is always the rule.
  • - When the motor is at TDC compression, check the flywheel/alernator for timing marks. Note which mark is TDC compression on that cylinder for future reference.
  • - grasp the rocker arm and wiggle it up and down. The rocker arm is the arm that curves over with its end sitting on top of the valve stem
  It should be able to move. If it doesn't, are you sure you are on TDC Compression? Rotate the motor again and watch the valves, you should see them moving. Get to TDC Compression again and woggle the bits you saw moving.
• - Insert feeler gauge into slot between valve and adjuster (the slot that opened up when you wiggled the valve). Check clearance.

  Correct clearance is when the gauge blade(s) go in but it's a tight fit. They should feel like someone is holding them firmly, but not so tight you get a metal-to-metal graunching feeling.

  Try the next largest size of gauge blade. Can it get in?

  Remember - too loose is better than too tight!

• - To adjust,
  • - loosen the locknut
    be careful, don't round it and don't jam your spanner against the rocker gear.
  • - holding the adjuster still with your tool, (pair of pliers), screw the locknut back a half turn or so to give yourself moving room
  • - turn the adjuster until the clearance is correct
  • - holding the adjuster still, tighten the locknut down, carefully and not overtight.
  • - recheck clearance. Until you get the knack, the damn thing *always* tightens up no matter how tight you hold the adjuster.
  • - redo until correct.
• - repeat check/adjustment for other valves which can be checked at the same time. On some bikes that's the inlet and exhaust for that cylinder, on other bikes, it's another inlet valve on a different cylinder. You need to know for your bike. If it's a mild old thing without wild timing, it's probably OK to check inlet and exhaust on the same cylinder when that cylinder is at TDC on compression.

  Allan Pennykid says:

  "What my father taught me here has always worked for me, for a four-cylinder engine it would be "rule of 5".

  When the tappets are 'rocking' (Exhaust closing & Inlet opening on overlap) on No 1 for instance, you would adjust the tappets on No 4, derived by subtracting 1 from 5. eg Inlet/Exhaust rocking on 3, adjust no. 2. It works for any 4stroke piston engine I've found yet(except a single, for the pedants amongst us)".

• - Now repeat the TDC thing for the next cylinder. Some multis have more than one piston at TDC at a time, which can be a real timesaver.

  If you don't know which ones are so on your bike, play about until you do and make notes.

• - replace rocker cover
  • - you can seal the gasket by smearing it with a tiny bit of grease. Tiny. Smear. Not a great dollop plastered all over it. You seldom if ever need gasket goo on rocker covers if the mating surfaces are clean and you tighten it properly.
  • - make sure the gasket and mating surfaces are clean. If there are bits of gasket or goop, then the join will leak. Remove carefully without gouging the alloy. Sometimes a bit of kero will help.
  • - tighten the bolts in a crisscross pattern so the cover is tightened evenly. Snug them down lightly, then go around again in the same crisscross, tightening them, then again. Do not overtighten, if you strip the threads then that's a $60+labour helicoil job. No more than a solid-but-not-massive tug on a normal sized spanner or 3/8" socket handle.

    Overtightning will not stop leaks, clean mating surfaces, new gaskets, clean light application of grease or proper sealant to absolutely flat surfaces stop leaks...

  • - it can be a good idea to dab a bit of grease on the bolt thread before putting the bolts in. This can help with the steel/alloy corrosion problem. Bolts stay tight because the threads are tensioned against each other. Greased threads do not cause the bolt to slip but can stop it corroding.
• - check and fix spark plug gap. Clean or replace plugs if needed.
• - replace spark plugs like the rocker cover bolts, don't over tighten. They are not going anywhere, and helicoiling sparkplug holes is a right pain.

  If you can beg, borrow, or liberate some Never Seize or Copaslip, then smear a bit on the plug threads before putting the plugs back in. I don't think normal grease will be any good considering the temperatures, but high temp moly grease should be OK.

• - replace engine cover if you removed it.

  Again, tighten bolts in a crisscross pattern and don't overtighten. Grease them before replacing.

All done... Now wipe your tools and put them away in the toolbox.

Start engine and wonder what that 'orrible clunking noise is. Stop engine, place gearbox in neutral. Kick self. Restart engine. :)