mtngun
Addicted to ArboristSite
Since the pitted, scratched, OEM jug didn't seal up well, and since I have a nearly new BB kit laying around, and since I need more practice at woods porting, why not woods port the BB ?
Well, because a pop-up will make the exhaust free port even worse, that's why not. :rant:
On the other hand, perhaps the woods port will do more good than harm, and if it doesn't work satisfactorily, I'm not out anything except my time.
So here we go......
I bead blasted the carbon off the BB chamber and piston top so I could get a clean chamber volume measurement. The piston was pushed all the way against the squish band while a 10cc syringe dribbled water into the spark plug hole. The process was repeated just to be sure. 10.3 cc and then 10.4 cc, we'll call it 10.35 cc for the "zero squish" chamber volume.
How much can be safely shaved off the BB piston ? Not a heck of a lot. The top land is only 0.110" (2.79 mm) thick, grrrrr :bang:
Grande Dog, can you please change all your 066 pistons to the more conventional 0.140" (3.6 mm) top land thickness, so we'll have a meaningful option to cut a pop-up ?
I felt like throwing the piston across the shop, but decided to shave the top land down to a risky 0.085" thick, for a 0.025" pop-up.
I used electrical solder to measure the profile of the chamber, and then wrote a CNC program to closely match the chamber profile.
Last time I used a 4-jaw chuck and it took me over an hour to zero the piston decently. This time I remembered that I had a 6 jaw chuck that might be well suited for holding a piston. Sure enough, the 6 jaw held the piston just fine. 0.001" run-out on the face, without even trying.
Cutting the pop-up.
The finished product, complete with wimpy top land.
Once again the 10 cc syringe was used to measure the "zero squish" chamber volume. 9.5 cc, 9.2 cc, 9.3 cc, let's take an average and call it 9.3 cc, for a 1 cc reduction. That's pretty good for a 0.025" pop-up, because this time I did a better job of matching the chamber profile.
1cc is about 10% of the chamber volume, so it should produce a 10% increase in compression. The original BB only blew 123 - 130 psi, depending on its mood, so maybe now it will produce a whopping 143 psi. Woooo - hoooo !
Grande Dog, if you are reading this, the 1 cc smaller chamber on the 2nd generation BB is nice, but it needs to be more like 1.5 - 2 cc smaller, IMHO. Plus, the compression and mid-range power are hampered by excessive exhaust duration, so the roof of the exhaust port should be lowered, not raised, p*l*e*a*s*e.
And that's as far as I got tonight.
Well, because a pop-up will make the exhaust free port even worse, that's why not. :rant:
On the other hand, perhaps the woods port will do more good than harm, and if it doesn't work satisfactorily, I'm not out anything except my time.
So here we go......
I bead blasted the carbon off the BB chamber and piston top so I could get a clean chamber volume measurement. The piston was pushed all the way against the squish band while a 10cc syringe dribbled water into the spark plug hole. The process was repeated just to be sure. 10.3 cc and then 10.4 cc, we'll call it 10.35 cc for the "zero squish" chamber volume.
How much can be safely shaved off the BB piston ? Not a heck of a lot. The top land is only 0.110" (2.79 mm) thick, grrrrr :bang:
Grande Dog, can you please change all your 066 pistons to the more conventional 0.140" (3.6 mm) top land thickness, so we'll have a meaningful option to cut a pop-up ?
I felt like throwing the piston across the shop, but decided to shave the top land down to a risky 0.085" thick, for a 0.025" pop-up.
I used electrical solder to measure the profile of the chamber, and then wrote a CNC program to closely match the chamber profile.
Last time I used a 4-jaw chuck and it took me over an hour to zero the piston decently. This time I remembered that I had a 6 jaw chuck that might be well suited for holding a piston. Sure enough, the 6 jaw held the piston just fine. 0.001" run-out on the face, without even trying.
Cutting the pop-up.
The finished product, complete with wimpy top land.
Once again the 10 cc syringe was used to measure the "zero squish" chamber volume. 9.5 cc, 9.2 cc, 9.3 cc, let's take an average and call it 9.3 cc, for a 1 cc reduction. That's pretty good for a 0.025" pop-up, because this time I did a better job of matching the chamber profile.
1cc is about 10% of the chamber volume, so it should produce a 10% increase in compression. The original BB only blew 123 - 130 psi, depending on its mood, so maybe now it will produce a whopping 143 psi. Woooo - hoooo !
Grande Dog, if you are reading this, the 1 cc smaller chamber on the 2nd generation BB is nice, but it needs to be more like 1.5 - 2 cc smaller, IMHO. Plus, the compression and mid-range power are hampered by excessive exhaust duration, so the roof of the exhaust port should be lowered, not raised, p*l*e*a*s*e.
And that's as far as I got tonight.
