Just cc'd my 066BB (and I apologise if we are veering off the Makita BB topic). I did it three times. 11.5, 12.0, and 12.5 cc. That's with 0.020" squish.
The hard part was locating TDC precisely. I used a chopstick through the plug hole as an indicator, which was not super precise. In hindsight, I should have pulled the muffler and determined TDC by observing piston movement through the exhaust port.
The BB's 0.020" squish takes up 1.25 cc. Subtracting the squish leaves 10.75 cc for the combustion chamber alone.
Then I cc'd an OEM 066 P&C. This was off the saw, so I could push the piston all the way up until it touched the squish band. 9.5, 10, 10cc. That particular jug had only blown 135psi with new rings, so I'd say its 10cc chamber is larger than ideal.
The 066BB's static compression ratio is 8.2. The
best it has ever blown was 130 psi, and last time I checked it was only 115 or so.
The 066 OEM's static compression ratio would be 8.14 if set up for the same 0.020" squish. As mentioned before, it blew 135 psi when it had a new piston and rings. Makes sense.
I'd like to have 160 psi at my altitude. A little math says that would require a 10.3 static compression ratio. Gotta do a little more math to calculate how much pop-up is needed.
Sorry about hijacking the thread, but I hope the data helps someone. I know it helped me.
Update: Did the pop-up calcs for the BB kit. A 10.3 static compression ratio requires a 9.85 cc chamber, a 2.15cc reduction (BTW the 2nd generation BB kit is supposed to be 1cc smalller).
Let's assume that the maximum practical pop-up is 0.050" high and 35.2mm diameter. That buys 1.24cc, not nearly enough, darn it.
The math is similar for the OEM jug. An 0.050" pop up would only raise the cr to 9.23, yielding perhaps 145 psi at my altitude. Darned ! ! !
My best hope at this point is to wait for the 1cc smaller big bore kit, then install it with minimum squish, ending up with perhaps 150 psi.