+1mm OS Valve Clearance

[Treesus]

Ok. I just installed new guides and valves on my CB360 head and am about to put the upper end all back together.

When stepping up the valve size 1mm, do I need to check piston to valve clearance? My head has not been skimmed at all, and I am using a copper gasket.

I have read about needing to do this when the head has been skimmed, but not heard anything about just changing the valves.

Thanks

 

[Sonreir]

You haven't mentioned it, but I'm assuming you had the head at a machinist for the valve fitment?

Also, yes, you do need to check clearances. If you change ANYTHING to the workings of the engine you need to check. In this case, it's important because the oversized valves may not fit into the flycut of the pistons.

 

[kopcicle]

1mm increase means .5mm closer to the piston . Doesn't sound like much does it ?
Imagine the sound if the valve does contact the piston .

Intake chases the piston down , piston chases the exhaust valve up .
Now think bigger valves and heavier , little cam chain growth , retarded (late) exhaust closing and the piston slows for nothing .

Dial the cam in and a few degrees camshaft advance hurts nothing but actually helps the bottom a bit .

Clay it all is what we are saying , clay it all ....

~kop

 

[Treesus]

Ok I clayed the pistons, put everything together, rotated the engine through 4 times very slowly, and took it apart. Valves barely touched the clay at all.

I was happy and confused until I realized I didn't mess with the valve adjusters at all. Guess I get to do it again...

 

[Treesus]

Did it (hopefully) right the second time. Results are posted in my build thread.

Both exhaust valves left plenty of room at .058" and the Intake valves didn't even touch the clay that was an median of about .08" thick.

Thanks for the help.

 

[kopcicle]

...Any time , that's what we're here for . That and the free beer .
So now that you know that the valves don't touch . ( I knew that the exhaust would be closer and around 60 something) do an investigation into the effect of squish or quench on combustion flame front and over all efficiency . Get back to us when your head is about to burst and we'll take the pain away with a few simple generalizations that would mean nothing if you hadn't done the homework .
The results will be better fuel economy , cleaner combustion chamber , broader torque curve (more area under the curve , the right way) and even if it is a subjective indicator the butt dyno says much improved response to arbitrary dropping of the wrist at near any rpm .

Look you only have so many cc's to play with you might as well make the best of it right ?

~kop

 

[Treesus]

Damn. True.

Ok I've been reading about that most of the day now. Actually I've seen those terms pop up occasionally and have been happily ignoring them because every time I started to try and figure them out I got a headache.

But as I now understand it, squish is basically important for creating combustion chamber turbulence that aids in quicker burning of the fuel/air mixture. Looking at my new versus old pistons the original CB pistons have a flat band around the radius while the GS pistons are more domed. And I can see the flat band around the combustion chamber while looking at the head.

BUT. This all seems rather unchangeable. What can I do to take best advantage of all this?

 

[Sonreir]

Basically, the "best" squish will come from having a very close clearance between piston and deck (< .010" is not unheard of, but anything under .040" will start becoming useful). The angle of expansion formed by the piston and the combustion chamber should be between seven and ten degrees for idea squish.

What this does is to force the air and fuel toward the center of the chamber as the piston ascends toward the head. Increased turbulence allows for faster burning of the mixture as well as a decreased likelihood of forming hot spots. On its own, a good squish band will produce questionable performance benefits, but the real plus is that it allows you to run higher compression ratios on lower octane fuels.

It sounds like you have a bit of clearance to work with, so if you don't mind treading a little bit closer to the edge, you could get away with shaving a bit of metal off of the cylinder jugs. This will provide better compression and better turbulence. Likewise, you could repeat your clearance experiments without the bottom base gasket in place and see how things measure up.