CMS-Japanese Motorcycle Supply Parts

Head and Valve performance improvements - Honda CX500, GL500, CX650, GL650

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Need to upgrade your Honda CX500, GL500, CX650 or GL650's valves or heads? Considering porting to improve performance? Get the down low here...

Here is some information on the head and valve modifications you can make for performance gains on a Honda CX500, GL500, CX650 or GL650 motorcycle.


Larger stainless steel valves can be acquired courtesy of Honda's CB750 SOHC - you have to use the springs, retainers and keepers from the CB750 SOHC to get it to work. Oh, and get the seats re-cut and the guides reamed or replaced. Apparently they are bigger than either the 500 or 650 stock valves, so it's going to be easier to put them in heads from a 650. Not sure if they'd fit in a 500 head - anyone ever tried?

You can also have the profile altered at the valve T-join to improve flow by ~20%.

The 650 heads will accept KPI oversize valves with larger seats. Jim L found that while the 500 heads can be CAREFULLY carved out (seats and ports) to accept 650 valves, the heads would not be long-lived and there was a real risk of a seat drop out. Jim L did make one set of 500 heads with valves bigger than stock 650 size, and 5.5mm valve stems for a 500cc engine going into a streamliner.

Jim L says that if you really go to town on the heads to maximise HP it gets expensive... figure close to $1000USD a pair. He also says that with high compression the bikes really shake, so add another pile of money for metric nylock nuts all over the bike!


The heads flow more than the stock motor needs, so gains here are not low hanging fruit. With perfect porting and bigger valves Murray estimates you might get 10% more available power out of the motor from these mods, but you'll likely have to make other changes to the motor to access those gains.. So... if you were considering only doing one mod to try and get more power out of the motor, this wouldn't be it.

If you are porting don't polish up the intakes - it seems 80 grit is a good choice for those - while the exhaust can be polished. The reason here is that the slightly rough surface will help mix the air and fuel, where the polished surface will not. The factory cast texture is actually not the worst thing for this for the stock size/angle/etc.

Mark's porting for his build:

Here is some information on the head and valve modifications you can make for performance gains on a Honda CX500, GL500, CX650 or GL650 motorcycle.

Port matching can be helpful - Murray's custom intakes are a pretty good match for most heads out there but you may find that they need a very slight adjustment to perfectly match the inlet shape. Murray recommends to over-size the head side by about .75mm compared to the inlet, to allow for slight shifts in the intake position when installing or slight intake runner movement on the O-ring due to heat cycles.

For his 555cc builds Murray from Murray's Carbs uses CB750SOHC stainless valves in 650 heads on the 500 block. How's that for mixing and matching?

Jim says that the porting and head work is a difficult area to deal with. Any local head-work shop can handle these small heads. The oversize valves he uses are the stock CX650 parts, installed into new brass seats. The heads are cut (remove old seats and make larger diameter hole), brass seats machined and dropped in liquid nitrogen, then dropped into the heads. The brass seats are then cut for 3-angle. This is how we did the engines for Shaun Austin and Dan Wagner. If you want to spend even more, you can build heads like mine. These heads have bigger valves (from KPI ...Kibblewhite in San Bruno, CA). The valves have tiny 5.5mm stems, to promote intake flow, which require custome bronze valve guides. These also require customer retainers and cotters, along with stem caps to match the contact area for the rocker arms. Expect to spend $700-$1200 for a pair of head mods.

As far as skimming the heads goes, to maintain stock dimensions and clearances you can get away with 20-30 thousandths of an inch max. 10 is usually all that is required to take out warpage.

CX500 heads on CX650

So... running 500 heads on a 650 increases compression and thus performance, but both Murray and Ray have apparently found that when drag racing the valves were hitting the piston on deceleration, resulting in bent valves... which had rather the opposite effect on performance. A thicker head gasket may work, though I don't believe either has tried that yet. You probably wouldn't want to do this with 500 heads that had been skimmed at any point in their life, either, as that will just make life harder... If you're going to go ahead with this you should probably look into a company who will make you thicker head gaskets (Cometic etc) and then plasticine the motor to make sure that it's safe.

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