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This page has some information on upgrading the pistons/over-boring the motor and altering the crankshaft and flywheel on your Honda CX500, GL500, CX650 or GL650.
As a note - if you up the compression via higher dome pistons you will end up having to run higher octane fuel to prevent detonation/pre-ignition.
CP-Carrillo makes pistons. In 2011 Harly Legowski was the person in the know (949-567-9000), and she has specs in the computer for Jim Leininger's 500 pistons and 650 (~680) pistons. You'll need to tell her how much compression you want (Jim says you can't get past about 12.3:1 on the 500 due to short stroke). Lower compression ratios will save the motor if you can't get high octane fuel but you will have reduced power gains accordingly. The 650 pistons must apparently be made 0.5mm overbore (83mm) because there is not supposed to be any available ring packs for 82.5mm. A pair of pistons, with pins and rings, has been stated to be anywhere from $330 to $450. The pistons will arrive with valve reliefs sufficient for oversize valves (think 650 valves in a 500) and suited to the WebCam lift and duration. Jim took some performance built heads to CP-Carrillo so they could model the pistons accurately and they can be acquired in either the standard bore or an oversize figure.
CP-Carrillo can make your pistons with the pin moved up (to allow use of stock conrods). It will add a little cost because the wrist pin will be under the oil rings (requires a bridge part, which they provide with the kit). Jim's destroked cranks required pistons with the pin moved DOWN .060" (half the stroke change is how much you move the pin) - he had to make a bike with less than 650cc capacity to run in a certain racing class.
The CX650 pistons from CP-Carrillo have less dome than the CX500 variant but are a higher compression ratio (over 13:1) due to the longer stroke available in that motor.
Over-boring is not easy/cheap on these motors - the sleeves are cast into the block so it's not like you can just swap out the sleeves for some of a different bore. The pistons have a 22-degree twist to the valve cutout so you won't be able to find pistons from another bike/car model in the size bore you want, you'll have to get them made.
Another option is to do a sleeve swap between a 650 donor and 500 recipient motor - so you can use 650 pistons in a 500, giving a 555cc engine. Murray from Murray's Carbs does this sort of work for some of his CXs.
High compression pistons alone won't make a huge difference to a CX - when combined with other mods (intake/exhaust, cams, ignition), however, you will see the benefit.
Jim had this to say about over-boring or over-stroking:
It looks like I can get to 746cc from the 674cc by using the 83mm bore (stock head gasket) with 69mm stroke. That is 7mm over stock stroke , and will requre a very small amount of shaping in the crankcase to clear the rod nuts (stock rods). It looks like maximum available, without sleeving, would be 800cc with 84mm bore and 72mm stroke. That will be a pretty short lived piston, because the wrist pin will be under #2 ring and #3 oil rings. #2 ring is actually the heat transfer ring for the ring land area of the piston, so it will have to be a little loose around the crown and still prone to trouble (my best guess).
Murray from Murray's Carbs has the boring plates to install over-bore liners, hence his ability to make himself 555cc engines as per above.
Shaving down the flywheel of a Honda CX500, GL500, CX650 or GL650 can certainly make a difference to the bike's responsiveness, particularly at higher revolutions. In theory depending on how much you take off idle smoothness and ease of stalling may suffer, but Ray Platek reports that there's no such issues with the CX that he has encountered.
Image above courtesy of Ray. You need access to a lathe and an experienced operator to achieve this - you don't want to unbalance the flywheel while you're lightening it...
Ray's bike with the above flywheel (GL650, with .020 off the ported heads, 36mm Mikuni Flatslides and a custom exhaust setup) achieved 12.45sec @ 104.75mph (168.58km/hr) in full street trim. My bike weighs 450# (204kg), I weight 218# (98kg) in riding gear. That's only about a half second slower than a late '90s GSX-600F Katana.
If you are going to stroke your crank, CP-Castillo can do them for about $350 to $400 (Jim Leininger had two de-stroked cranks made to build his 648cc CX650 engine).
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