Why do I have to set my spark plug gaps?

There are several gaps that you need to ensure are correct on your YB.  These are the gaps for the

• Spark plugs
• CPS (Crank Position Sensor)
• Distributor Phase Sensor

Having the correct caps for these three items (or six items if you count the spark plugs individually), helps ensure your engine will run reliably each time you start it - and it'll help ensure you spank the foolish M3 driver sat next to you at the lights

Spark Plugs

First off are the most well known ones: the spark plugs.  The YB has four of them.  The spark plugs ignite the fuel/air mixture inside the combustion chamber.  What alot of people don't realise is that the spark they have to generate is just as important as putting petrol in the tank.  If the spark is too small, or if the spark is too weak, the fuel/air will not ignite properly (if at all!), and the engine will run poorly.  This results in poor fuel economy, poor engine performance, and potentially, an embarassing visit to the garage for the sake of a 10 minute job.  Click on the image on the right to see the major components of a spark plug.

Too narrow At the top of the spark plug is the cap, which takes the high voltage from the coil and transfers it to the centre electrode at the other end of the plug.  If the coil is in good working order, the high voltage is usually enough to jump the gap between the inner electrode and the outer electrode.   If the gap is too narrow, the electrons passing from the inner electrode (the cathode) won't get excited enough on their way to the side electrode (the anode).  As a result, the spark won't be hot enough to ignite the air/fuel mixture for optimum power.  The air/fuel mixture will still ignite, but theres room for improvement.
Too wide The opposite condition produces a similar effect - if the gap is too wide, the electrons will get lost on their way to the other side and only a weak spark will be made, if any spark is made at all.  If the gap is too wide on a turbocharged engine, such as the YB, the tiny spark can be blown out by the huge volume of compressed air entering the cylinder.  This often results in a misfire 'on boost'.
Spot on When the gap is set correctly, the spark is strong and hot enough to not get blown out, and to ignite the air/fuel mixture in a way that pushes the piston back down the cylinder with maximum force, generating the maximum torque for whatever speed the engine is rotating at.

What to do

The spark plug packaging will probably say they're 'pre-gapped'.  They are - but not necessarily for your engine.  Spark plugs are made to fit all sorts of engines, and ones for the YB also fit in other engines.  Therefore, you must ensure that the gap between the two electrodes is set correctly for your YB.

Normally, the gap should be set to 0.7mm on a standard or Stage 1 engine.  When running higher boost, as in Stage 2, 3 or above, you may consider reducing the gap to 0.6mm to ensure the spark's not blown out by the higher boost pressures these engines use.

To set the gap, I use a set of feeler blades - there are other tools that can be used.  My feeler blades don't have a 0.7mm blade, so I have to make it up by putting a 0.2mm blade and a 0.5mm blade together.  Slide the 0.7mm blade between the electrodes - you should feel a light 'drag'.

If th gap is too narrow, gently bend the side electrode away from the centre electrode, and try sliding the blade between them again.

If the gap is too wide, gently bend the side electrode towards the centre electrode, then try sliding the blade between them again.  I find that holding the blade in the gap while doing this helps.

When you've got the gap set correctly, smear a small amount of Copaslip (copper grease) around the thread so you can remove the plug when its time to replace them, then screw the spark plug into the cylinder head.

Job done

Now you've got properly gapped plugs, you can check the other two gaps which are worth investigating: the phase sensor and the crank position sensor.