MOPED ARMY

Exactly. If the pipe achieves 76% or better trapping efficiency, it'll knock with premium gas. I'm totally making up theories here, but I figure it needs to be expanded by at least 1.5cc to be streetable, but by no more than 2.5cc.

The head is already 8cc, I subtracted 3cc to account for the piston crown in my compression calculations. So I was thinking 9.5-10.5cc, inclusive of bowl & squish, exclusive of piston crown. Do you mean that just the bowl should be 9cc? I imagine it might have problems idling at that point, but you're the guru.

I still need to measure the old piston, or better yet actually bore the case and assemble it before I can give concrete measurements.

Yes, it's going on the A55 with stock retarding ignition & external trigger. I've been looking into other ignition options, but I don't want to give up wattage. I've even considered putting in a 6-pole scooter stator to get more wattage, if I could make that work...

Speaking of ignition, I was working on this guy's Honda Cub the other day and saw that it had retarding points... I thought that was awesome! y'know, if you wanted a performance bike that can survive an EMP. Never know when you might need that kind of thing.

the C70 are actually advancing points, because on a 4 stroke you want them to advance, not retard.

you can use the JOG CDI box, leaving the A55 ignition the same. You'll definitely want to use some kind of auto-retard ignition, plus the stock A55 box cuts out at 9000-ish rpm.

Yes, your math is correct, i read that quickly and mis-interpreted what you were saying. At least 9-10 total CC volume in the head.

you have to remember the pipe supercharging is the issue here. you're dealing with a supercharged engine so you want the compression to be closer to recommendations for supercharged not naturally aspirated. 10:1 nominal is a good starting point for 'max' but you're probably going to want to be closer to 9, 9.5:1

You figure displacement is going to be pretty much the same for kits @ around 70cc, other factors like pipe and porting have way more impact than +/- 5cc of displacement, so i go more off of 'rule of thumb' and also what kind of chamber you can get within the exisiting casting.

if the casting is too shallow to get the desired volume, i'll massage the bowl and squish area to get more volume. If it is deep enough i'll go for a tighter squish and take the compression lower with a bigger dome because ultimately that will be a better performing head.

all the books and stuff are a good starting place but there are so many variables present that the guys designing race bikes or designing from scratch don't have to deal with, that it ends up being mostly guess-and-check more than science.

It would probably be quicker to just carve the head out some, put it on the bike and compression test it. Keep doing that until you get the number you want. 125psi is what most geared 2 strokes come out to. At that pressure (approx 8.5:1) you can get the bike started reliably, less pressure it gets more hard to start. 140 psi is a good number. I myself shoot for 185 psi and can run with large hard hitting pipes but 92 octane is available here. Alcohol in gas can be a good thing. It suppresses detonation, is an oxygenator, and cools better by means of latent heat of evaporation

I find most heads I machine to basically comewading deep in the detonatin zone. Like they were cast as cruel jokes. The amount of fucks not given about these is impressive.

eh yeah 1.8 is pretty wide, i guess 1.5 is probably more accurate. depends on the setup and geometry. You also have to remember that its only measured at the edge before the sqush band begins to diverge, so its really not that important of a number.

you figure by the time the piston expands, rod expands and all the bearing tolerances pull out, they add .2-.3 (when you measure with solder you are compressing all those clearances in the other direction) then you add however much inaccuracy you have from gasket compression, etc. also factor that the piston can rock in the bore and give you .2 mm side-to-side difference.

even with a perfectly setup, 100% dialed build where i've had control over all the variables, i won't go any closer than 1.0, i think given your goals for reliability and longevity over top performance you should stick to that 1.5 mm mark.

the difference between 0.8 mm and 1.5 mm is an incremental decrease in heat transfer, perhaps an incremental increase in power or ability to tune for higher temps... the risk on the other hand is very high for increased chance of contact or something shifting, even a slight tap on the edge of the piston can crimp the ring land shut and pinch a ring which is almost certain failure. IMO the risk isn't worth the incremental benefit.