CMS-Japanese Motorcycle Supply Parts

Changing the spark plugs - Honda CX500, GL500, CX650, GL650

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Transverse V-twin engines are some of the most straight-forward to change spark plugs on - most other engines don't have the cylinder heads just hanging out in the wind on either side of the bike! First, we need some tools. Assuming you are running one of the standard sized plugs for these bikes (DR8ES-L or D8EA etc.) you need an 11/16" or 18mm deep socket and ratchet wrench:


Oh, and an extender:


Two fresh spark plugs:


That's it! Now, pick a side to start on - we are working on the right side to start with:


Lift the plug cap away from the head slightly - it should pop off the top of the spark plug without too much force:


There should be enough slack in the high tension lead to lift the plug all the way out of the well; if not, you may need to undo the bolt at the rear of the tank and lift it slightly to pull the cap out. You're likely better off doing that than unscrewing the leads from the cap, as the copper wiring inside the leads may deteroirate with too many insertions. Here we can see the plug lifted out all of the way:


Looking into the well we can see the top of the spark plug:


Attach socket to extension, attach extension to ratchet, place socket onto the spark plug and unscrew (counter-clockwise) until you feel that the plug has come away from the threads and is sitting in the well.


If you haven't done it before and aren't sure what you are feeling for, just stop to check every couple of turns and eventually you will see the plug sitting against the well like so:


If you have skinny fingers you should be able to grab it; if not, improvise! You can look through the hole where it was and see the crown of the piston beneath, if you're curious. Spark plug lifted out:


Examine the spark plug; it's one of the few ways of getting an idea of what's actually happening inside your engine's combustion chamber. You can read up a little on how to interpret the appearance here:

These plugs are being swapped out for fresh ones after changing carburetor settings; the settings used for these plugs were too rich, leaving the plugs black:


Out with the old, in with the new:


Carefully insert the new spark plug into the well - don't drop it in. Now, take the ratchet wrench off the end of the extension. Why? You shouldn't require more than light finger strength to screw it in almost all of the way, and if you find that there's a lot of resistance you should stop and start over as you may have cross-threaded it. If you've done this often you probably won't bother to take the socket off, but if you're reading this guide to learn how to do it for the first time play it safe! Attach socket to plug, carefully:


Screw in with your fingers until you feel the resistance build up near the end. This is because you have started to crush the crush washer, which looks like this:

ngk-dr8-esl-spark-plug-honda-cx500-7 This washer is designed to crush and give a good seal when you screw the plug in. Once you feel this, add the ratchet to the end of the extension and turn it a further quarter of a turn.



If you are re-using a spark plug the crush washer will have already been crushed and you will likely just feel the plug come to a hard stop; if so, don't try to push a quarter turn past this point as you may cause damage.


Lift the socket off, re-attach the plug cap and you're done!

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