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How to change the spark plug on a Honda CB125E or GLH125SH

Here's how to replace the spark plug on a Honda CB125E or GLH125SH.

To do this you will require:

• Spark plug wrench from toolkit, 16mm deep socket or 5/8" deep socket
• Spark plug gap tool

The spark plug - just one on this motor - sits on the right hand side of the engine amidst the engine fins:

It's hidden underneath the spark plug cap:

You can lift the cap off with minimal pressure:

Place the cap and lead somewhere where they won't be bent or crushed. Now you can see the top of the spark plug:

If you don't have the spark plug wrench that comes as part of the bike's toolkit, a 5/8 deep socket or 16mm deep socket work to remove it, though given it's easy access you can probably manage it with a wrench as well.

It's easily removed with a ratchet:

Mostly undone:

You can assess the condition of the fuel/air mixture by examining the end of the spark plug (an article for another day):

Also examine the condition of the center and side electrodes. If there is obvious damage or the edges are eroded, replace it - they're not particularly expensive items and there's no good reason to use a damaged plug. This is also a good time to check the spark plug gap (another article).

Grab the plug that's going back in - new or old and OK. If re-using the old plug, make sure that the crush washer is present and undamaged. When reinstalling a spark plug, thread it in by hand to begin with to ensure you don't cross-thread it. If you meet resistance, back it out and start over.

Once it bottoms out by hand, you will have to turn a new spark plug more than an old one to get it to seat - effectively crushing the crush washer. For a new plug, ighten the plug 1/2 a turn after it seats, then back it off slightly so it's loose then tighten 1/8 of a turn.

For an old plug, simply tighten 1/8 of a turn after it seats.

Reinstall the spark plug cap:

...and you're good to go!