cc'ing domes

[ritchieb]

Hi BHQ members,

Can anyone offer some advice on CC'ing pro circuit domes? I have 20cc stamped domes (1 needs replacing) and I want to confirm the actual volume before I buy a new one. I've tried to cc the dome with the plug in and the volume is 23cc. Can someone tell me what's going on and what I need to do?

Thanks

[dirtydownunder]

clean the dome and insert s/plug.

without the oring, lay a piece of transparent plastic ( thick enough for it not to bend or twist) with a lil hole in it, over the dome.

use a syringe and squirt into the hole counting the cc's untill the fluid is up to the fill up point.

 

you can use a tiny bit of grease around the edge of the dome to help seal the fluid against the plastic.

if you cant find some decent plastic you can use the first cd from a pack of cd's (the first one is clear) but cd's bend if you put pressure on them.

[Valin]

clean the dome and insert s/plug.

without the oring, lay a piece of transparent plastic ( thick enough for it not to bend or twist) with a lil hole in it, over the dome.

use a syringe and squirt into the hole counting the cc's untill the fluid is up to the fill up point.

 

you can use a tiny bit of grease around the edge of the dome to help seal the fluid against the plastic.

if you cant find some decent plastic you can use the first cd from a pack of cd's (the first one is clear) but cd's bend if you put pressure on them.

 

I am unsure of how they actually measure the volume of the domes they sell. Maybe when they sell 22cc domes, that really means that the volume of those domes, when referenced on a flat surface is 22cc. I would find it hard to believe that is actually how they do it, because that would not be your actual head volume. Correct me if i'm wrong.

 

Unless you are running perfectly flat pistons, with a 0 deck height, that is not going to measure your head volume for your motor. You need to do it with the head installed in the motor.

 

With the motor sitting so that the cylinder heads are perfectly flat (you might have to jack the motor/bike in order to do so), bring your piston to TDC, and then, using a syringe (I use 10ml syringes), fill the head up with a liquid (I use oil or water) until you reach the bottom of the threads in the spark plug hole, keeping track of the amount of liquid you use. 1 ml = 1 cc. Do it twice to verify your measurement. When you have filled up the cylinder, simply lower the piston slowly to allow the liquid to run out of the exhaust port (do it with no exhaust on it).

Edited May 10, 2010 by Valin

[dirtydownunder]

i think your getting "calculating compression ratio" and "measuring combustion chamber" mixed up.

[Valin]

i think your getting "calculating compression ratio" and "measuring combustion chamber" mixed up.

 

The combustion chamber is the entire area above the piston in which air and fuel gets compressed. It is the head volume in an engine. You need to do it in the motor for this to be accurate to take all things into consideration.....ie. piston dome, deck height, etc.

[dirtydownunder]

yes you are correct, the whole part above the piston and under the dome to the combustion chamber. If your working out unswept volume then yes it has to be done with the engine together. But in measuring domes, dome or head manufacturers dont say this dome/head chamber is 22cc's with a 7cc dome piston, they say the actual dome is 22cc. and its up to you to work out compression ratio.

there is so many piston, gasket thickness's and deck hight configurations that it make more sense for them to say what cc the dome is on its own.