Ok, now to work out the unswept area in the cylinder, and the compression ratio.
First, calculated areas based on the manufacturers specifications.
Yamaha gives the compression as ratio as 12.4:1,
The bore as 65.5mm, stroke as 44.5mm.
It also lists the cylinder head volume as 10.3 - 10.9 cc.
This would mean that the total combustion chamber area would work out to 13.16mm (roughly.)
area of bore = pi x r(sq)
= pi x 3.275cm(sq)
= 33.6955cm2
vol = bore (in cm sq) x stroke (in cm)
therefore vol = 33.6955 x 4.45
= 149.945 cc
which is what you would expect from a 4cyl 600cc.
Compression ratio is given as 12.4:1, and with that in mind you can find out combustion chamber volume. Because compression ration (CR) = (swept vol + unswept vol) / unswept vol,
12.4 = (149.945 + unswept vol) / unswept vol
therefore unswept vol = 13.15cc (close enough to.)
So with that in mind, you could measure the thickness of the headgasket, and times that by the area of the bore, and work out how much of that 13.15cc is made up by the stock headgasket. And them subsitute a different thickness to work out how it would affect the compression ratio. In theory.
I decided to actually measure mine, being as the engine was in peices anyway. It is a good chance to work out the actual compression ratio, just in case yamaha are as liberal with the truth about compression ratio's as they are with claimed power and weight.
To do this, you'll need to measure the volume of the cylinder head, the thickness of the headgasket, the height of the piston above or below the block at top dead center and the volume of the weird cut outs and lumps in the piston.
First thing I started with was the area of the piston head.
The first thing I saw was that although the piston sits just below the block at TDC, the raised portions of the piston would certainly take up some of the area of the cylinder head and head gasket.
Firstly, the piston top needed to be cleaned of carbon, washed clean of grease and fuel residue with a mild detergent, dried, and set at exactly TDC. I used a dial indicator to find TDC bang on, but frankly the timing marks were right on the money anyway.
Then, using an engineers square flat across the machined surface of the block, slide feeler gauges under the gap untill the appropriate one is found.
Once that is done, you need to move the piston down the bore far enough for the piston top to just be under the block height, using the dial indicator for absolute accuracy.
Once it is there, zero it or make a note of the reading, then move the piston a few cm further down the bore. Put a thin smear of grease close to the piston on the wall of the bore, and move the piston back up to the zeroed point.
Now wipe off all the excess grease, leaving only the grease above the rings bewteen the piston diameter and the bore. This will seal the ring gap.
Find a flat peice of perspex, drill a 5mm hole in it. A CD case works well, after you have ground off the raised egdes.
Smear a thin line of grease around the top of the block, and stick the perspex there firmly. This will waterproof the top of the cylinder.
Get an appropriate size syringe and needle. Bend the needle slightly. Mix up some water with a very small ammount of detergent (it will help with any residual fuel repelling the water). Put in an exact known ammount of liquid into the syringe, and inject it slowly into the bore untill all air is expelled, and no water is spilled. (it may take a few goes to get it right.)
Do this a couple of times to confirm an accurate measurement.
*note - I'm sure if you have one, you could use a burette for this too. I don't have one.
Next, the easy one - the head gasket.
I tried measuring it first with a micrometer, however it was too accurate. The headgasket changes size slightly in a few places, so a set of vernier calipers did a better job.
Times this value by the area of the bore to get the cc it gives to the combustion chamber.
Now for the head.
Clean all traces of the old head gasket off. Clean any carbon out. Degrease with a mild detergent as before, dry, and install spark plug.
It's the same trick as with the block, but more simple - a smear or grease, the same water mix, and inject water in untill all the air is expelled. Again, do it a few times to confirm.
Now you will have the three measured volumes! I measured
Cylinder head volume = 10.6cc
Headgasket volume = 2.696cc
Piston volume = took 7.9cc to fill. BUT
for piston volume, I had to move the piston down the bore for clearance, and take into account the block height. So once that is taken from the volume required to fill it, i was left with -0.18692cc from the piston head volume.
So, 10.6 + 2.696 - 0.187 = 13.109
so
(149.945 + 13.109) / 13.109 = CR
which equals 12.44:1. That is really close to the CR yamaha claim - so I am happy!
Now that is all out of the way, I know exactly what changing the head gasket will do to the CR. As you can see, the measured CR is so close to the claimed CR, I could have just used yamaha's one, however, now I know and my mind is at ease!
Doing the calculations from that, if I use a copper gasket of 1.6mm thickness, as opposed to the .8mm stock one, I will get a new CR of 10.5:1. I can get my hands of copper sheet that size fairly easily, so all I have to do is machine it up now and install it!
Whew! That was a lot of writing. Who actually read it all?
