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How to remove the pistons - Honda CX500, GL500, CX400, GL400

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Here's how I removed the pistons from a CX500 motor - the process is functionally the same for the Honda CX400, GL400, GL500, GL650, GL700 or CX650 engines.

Gearbox and oil pump removed:

The two access holes for the underside of the pistons:

Before you remove the pistons, it's a good idea to scrape any debris from the top of the cylinders so that you aren't scraping your pistons up past corrosion and causing damage. If there's a bunch of rust and corrosion in the cylinder it's also fairly likely that the piston will stick on the way out and possibly cause damage to it, so try to clean the bore up as well as you can. Now, turn the engine using the front crankshaft bolt until the piston you want to remove is at bottom dead center, and you will see the bearing cap close to the access hole:

At BDC (bottom dead center), the piston should be this far into the bore:

Using a 12mm socket (I favour the Zeal sockets for fitting well in a confined space, see eBay US, eBay AU, eBay UK, eBay CA and Amazon), undo the two nuts on the bearing cap:

Put the nuts aside in a labelled bag:

Also - when you remove the bearing cap, make a note of which side it came from and it's orientation. Photos help here. You want to be able to put everything back exactly how you found it in terms of which bearing sits where, on what side and which way they are oriented. You can see the bearing cap loosened through the other access hole:

Removed:

Note the coloured marking on one side to indicate which size of bearing it is:

The other side doesn't have the marking:

Note the oil hole in the bearing - this is relevant for reassembly:

Looking closely, you can see some pitting in the bearing:

There's also a locating tab:

Now, turn the motor so that the piston to be removed is at top dead center (TDC) - here you can see the other piston at BDC at the same time:

The piston should be at the top:

At this point the piston should be able to be pushed up out of the cylinder by hand. If the cylinder walls are scored or rusted it may stick - which is why you should clean them out prior to this point. The piston should be visibly emerging from the bore:

Piston out:

The procedure is the same for the other side. I plan on doing a "closer look" at the pistons and associated parts, and will update this page with a link once that's completed.