THAT'S SO SIMPLE!" you say. "WHY do you need a Photo-How to for unscrewing a spark plug?!?!" That's what I thought; & even Tef, who will never give the 'short' version if a 10,000 word essay will suffice, didn't think it was THAT worth mentioning.... BUT, after answering the FAQ "Stripped Spark Plug Hole, What's a Helicoil' umpety hundred times...... he thought it MIGHT actually warrant a mention! Prevention is better than a cure, after all! So!
EXCESSIVE FORCE IS NOT REQUIRED
& Socket Sets are your ENEMY! Far to easy to apply far too much force. But, often only tool in the box that will fit a motorbike spark-plug, and buried deep in a casting with double overhead camshafts either side it can sometimes be awkward and the extensions & knuckle joints, can be useful, but be warned, more between your finger tips and spark plug, less 'feel' you have to avoid over tightening or cross threading.
Keep the socket as square to the spark plug as possible. Let the socket 'fall' over the plug, grip the ratchet 'loosely', and apply a gentle even pressure to the end to start it turning, and hold the centre of the socket over the centre of the spark-plug.
As soon as the initial pressure is taken off the plug, STOP using the leverage of the ratchet. Remove it and if you have to use the socket or extension, twist it like a screwdriver, with 'finger force' only, until you feel the plug starting to 'wobble' in its threads.
THEN wind the last way out, wherever possible PURELY with your fingers.
Refitting plugs is pretty much the reverse.
Putting the plug in; START the plug VERY gently. Avoid just 'dropping' the plug down the spark plug rebate. You do NOT want the electrode on the bottom holding the critical 'electrode gap' to get bashed on the cylinder head and closed up. Engine wont run well if it does. So try and lower it in, preferably with your fingers, and get it central in the hole, before you get metal touching metal.
THEN, again, with your fingers, GENTLY get the spark plug square and twisting into the threads, without cross threading. This high up the hole the plug can 'wobble' a bit, so don't force it, tickle it into the threads.
As removal, if you need to use socket and or extension like a screw driver to wind the plug to the bottom of the hole. ONLY when it has bottomed out, do you need to use the ratchet, and THEN only with finger force to just 'nip' the sealing ring between the plug and head.
Again, cradling the ratchet to keep it central over the plug, and using 'just' finger tip pressure on the handle to apply that 'nip'.
This handy guide was offered on the matter of what that 'nip' may be! Discussion was also had on the topic of using a Torque Wrench, but if you know how to use a torque wrench, you probably don't need this How2!
The Tightening torque on a Spark plug is VERY small; maybe 8ft-lb/10nm/1.1kgm, and most Torque Wrenches, sold to do up cylinder head bolts and things, don't go down to torque settings that low. So the Danger is if you were told to use one, you would very possibly pick up a 'High Range' torque wrench, and confused by the scale, pick a number that is 'similar' to the specified setting, but could be 10, 15, 100 or even 150 times what's needed, and actually be MORE likely to strip your cylinder head threads than NOT using a Torque Wrench!
So be careful of your spark-plugs and gentle in their handling, and your cylinder head will thank you by NOT stripping its thread.
If you are clumsy, brutal or lazy, with them? Well, then the likelihood is you will suffer a cross or stripped thread.
If you do, and you are lucky, the thread may merely be cross threaded, and a 'cleaner die' might be used to reform the damaged threads.
If not, then it will probably require helicoiling; which is a process of drilling out the damaged thread, and cutting a new larger one, the same pitch, and winding in a coil of metal wire to sleeve the thread back to the spark plug size.
If you are very lucky, then this might not need the engine removed from the bike, or necessarily the engine stripping so that the cylinder head can be drilled. But don't bank on it!
And once helicoiled; make the same mistake again, and you can strip the insert AND thread in the cylinder head, and do THAT, and chances are there is probably no cheap or easy fix!
So, prevention is better than the cure, and ALL it takes is a little care and attention.