Shaving Heads on a Honda CX500

[AlphaDogChoppers]

I have a CX500 rebuild underway in my shop. Engine had really low compression, around 70 psi in both cylinders, and left cylinder smoked quite a bit. After checking valve adjustment, decided to tear it down. Cylinders turned out to be perfect, still showing cross-hatching on the walls. Rings were still within spec, too. Still, was VERY lucky and found some NOS standard rings on eBay for $50, so it's getting new rings anyway. Turned out the intake valves are leaking a little.

Anyway, to my question.
I've been told that this bike will perk up quite a bit if you raise the compression some. My question is how much should I have the heads shaved. I seem to remember 50 thou as a number that was thrown out, but it seemed prudent to ask first. This bike is going to be ridden on a cross-country trip this year, not be used as a bar hopper. So, it would be desirable if we can raise the compression a little but still stick with regular gas.

Maybe it's unrealistic to hope for staying with regular gas while raising the compression, but it's a pretty low compression engine to begin with.

This is going to be a sweet bike. Not doing much in the way of modifications. Just paint, polish, and powdercoating.

 

[PancakeShake]

Ever get this beast running right? What PSI did you get after the new rings in the head?

JUST finished new rings, valves (lapped and set), and im only getting 75 PSI...I pretty pissed at the machine right now haha.

Let me know!

 

[AlphaDogChoppers]

I got the machine running like a raped ape with Murray's Mikuni carb kit. Compression was about 150-160 when I first built it. After a couple hundred miles of break-in, it was running 185 psi.

 

[PancakeShake]

***Sighhhhhh*** welp, i guess my cylinder walls are just shot or warped. Bummer.

Thanks though!

 

[mg42gunner]

From my experience I've seen guys running 93/95 (depending on what you have in your area) on 14:1 static compression, but it's all about dynamic. You have to take into account the duration of the intake valve being open. (more time open during the cylinder's intake stroke=more air/fuel to squeeze when it starts it's compression stroke. Granted the exhaust valve would have to have less overlap resulting in generally higher cylinder temps and dirtier chambers) How close the combustion chamber is to the cylinder (deck height/head gasket thickness). The bore of the head gasket comes into play, not that there is a little bit of a ledge or crevice there that would add volume to the cylinder. It's a weird thought I know, but I've seen factory engines where the head gasket's bore is larger than the cylinder's bore and of the combustion chamber by a couple thousandths.

If possible I would send the whole motor with the heads for machining so the machine shop can shave off set amounts at a time, then put the heads on,(don't forget to include the final 'crush' measurement of your head gasket for them to reference) roll it over and see how much clearance there is before you start playing piston/valve tag. This is what we do when we send out the motor and heads for machining on the race car. If you can machine 50 thousandths off I would start with only cutting out 20. With an air cooled motor the more metal you take away the less heat sink that cylinder has. And if there is an issue down the road where the head would warp you have some metal left to deck it.

If you're looking for a quick way to up the compression a little bit (I'm talking in maybe a tenth of a point or so) look for a set of headgaskets with more crush to bring the chamber closer to the cylinder. Buy two sets if you go this way so you can lay some clay on the piston face and roll it over to make sure you're safe.

 

[cxman]

i would not shave the heads any more than a clean up

you have no ignition adjustment on a cdi bike and they are close to the max advance for not pinging

if you have a later ti or an ignitec and know how to adjust the timing then your engine might survive

 

[AlphaDogChoppers]

***Sighhhhhh*** welp, i guess my cylinder walls are just shot or warped. Bummer.

Don't make that assumption. Rent a cylinder leak-down tester and determine where you are losing compression. Dollars to doughnuts it's your valves.

The thing about lapping valves is that there is a limit to how wide the valve to seat contact area is. (The service manual is quite specific on those limits.) If you lap them too much, your valve job will not last. You should only lap just enough to get them to seal. Too many guys lap the valves WAAAY too much.

After doing a little research, I realized that the CX500 already has a pretty high compression ratio, and one should not attempt to raise the compression. The real key to performance on this engine is Murray's carbs. (CXMAN) They are easy to install and adds 20% to the horsepower. The one I did dynoed at 59 hp at the rear wheel. It would easily lift the front wheel in the air in second gear, even if you didn't intend to. Power starts coming on at around 2,500 RPM and pulls like a freight train right on past redline. With the stock carbs, it didn't start making good power until 5,500 RPM.

 

[MR915]

No one mentioned the effect on cam timing when you start milling heads and/or decking cylinders/upper cases.

 

[AlphaDogChoppers]

Enlighten us. How the Hell does decking the cylinders or milling the heads affect cam timing?

 

[cxman]

cuz they think its a over head cam engine

 

[AlphaDogChoppers]

Makes sense.

I'm in the process of moving into a larger shop. Good traffic exposure and in an excellent area only 22 minutes from home.

 

[cxman]

good luck to you Scott

let me know when you land

 

[AlphaDogChoppers]

I'm hoping to be up and running shortly after June 1. I have a LOT of motorcycle work on deck. A CX500 is my first project.

 

[Maritime]

I'm hoping to be up and running shortly after June 1. I have a LOT of motorcycle work on deck. A CX500 is my first project.

Nice so are you going to deck the heads on it LOL ;D Nice space you have there.

 

[MR915]

I apologize for the statement made.....I haven't touched one of those engines since 1995. They were actually banned from the shop after 2 stator replacements as they were very expensive and no joy to the owner. I am sure they were "Silverwings" ? Like the Sabre 45, Magna 65/45....all hated and banned. :
I liked the RC51-1000 Twin....I have the 33rd one off the line in 2000....first year made. That is the Honda twin for all time. They are nice.

 

[Sonreir]

Liquid cooled motor... surely a little extra compression is possible with high octane fuel?

 

[AlphaDogChoppers]

It's already at 10:1 bone stock.

 

[Sonreir]

12:1 it is, then!

 

[AlphaDogChoppers]

LOL!
I does have pretty long duration cams, so even with the 10:1 compression ratio, it will run on regular.

I wouldn't mind setting up one of these bikes as a vintage racer, and exploring performance modifications beyond CXMAN's Mikuni carburetor upgrade. If you shaved the head, you would have to be very careful about piston/valve clearance. The valves already open to below the head gasket. I'm pretty sure you would have to deepen the valve pockets in the pistons. It occurs to me, that the CX500 has a very thick head gasket. One could experiment with making an annealed copper head gasket for purposes of experimentation.

 

[Rich Ard]

Looks like you have about eighty places to plug a mill in