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Technically you need to match the Piston to the squish band the angle of the squish band is what prevents detonation and keeps the explosion over the top of the piston preventing the edges of the Piston from melting.
 

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I strongly recommend you read this article completely before you modify the squish band shape of the hemispherical head that you have.


my advice is for you to match the Piston to the dome and not try to match the dome to the piston.
 

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It is better to modify the head to match the piston than other way around, because usually the piston does not have that much material on it. Also cutting the piston crown can alter port timings.
 

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Discussion Starter #105
I've measured the port timings with the flathead piston and 2.85mm spacer and the timings is exhaust 158 and transfer 118.
I'm aiming for 172° exhaust duration for a blowdown time duration of 27°
i can't sand the spacer down more than maybe 0.2mm, if going further the exhaust port will not open completely.
squish clearance is 2.7mm so I'm thinking of grinding away 2.2mm, or 2mm if sanding the spacer down.

To get the squish clearance to 0.5 without gaskets.
squish band will be 50% area.
Might get it with a 2° taper to get the squish velocity lower.

still waiting for an exhaust to try my porting on the test cylinder.
I could try it on the other pocket bike but that exhaust is so bad that i don't think it's any idea to do so.

havent got any further with the exhaust yet
 

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It is better to modify the head to match the piston than other way around, because usually the piston does not have that much material on it. Also cutting the piston crown can alter port timings.
The hemispherical dome has been carefully calculated to prevent detonation the squish band width and angle are calculated for the inner dome shape to match the shape of the stock piston.

Back in the day they used to sell two different domes with different compression ratios one is for pump fuel the other one is for alcohol basically the higher the compression The wider the squish band I currently own both they were put out by Dave's motors discontinued now only the pump fuel one was copied by the Chinese.

I guess the only way to know is to spend a few minutes and take the time to take a look.

My opinion right now is try to put a crown on the Piston the width of the squish band trying to match the angle of the dome the best you can.

Then get the rest by matching the dome to the modified piston.

Then set the squish using various thicknesses of base gasket material head gasket material and lapping the top of the cylinder.

Last should be to set up all of your ports using the degree wheel

advanced the timing past what a rocket key can give you by JB welding the flywheel to the crank at the desired degree offset

again I would like to put the disclaimer out there that I have not personally tested it but have a similar engine build in the works

You can see I very non-scientifically transferred the width of the squish band from the dome to the top of my piston for reference how I'm going to grind the the band angle into the Piston is up in the air Ill probably have to make some kind of wooden jig or something
 

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I guess it is fine to grind the piston to match the squish band, if it is only a small change. But for matching a flat head piston and a combustion chamber with an angled squish band, I think it is better to machine the combustion chamber. As far as I know, the squish band angle/shape does not matter if it is totally flat or slightly dome shaped(angled). As long as the gap between squish band and piston is constant and not tapered.
 

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Discussion Starter #108
Will see if I can get the parts machined soon.
Going to talk with my friend tomorrow, i had a piston that would work but it never arrives it seems and getting a refund tomorrow.
So i think i stick with the flattop piston and machine the combustion chamber to either completely flat or at + 2-5 degree divergent.
Most likely a bit divergent to get the squish velocity down a bit.


"Influence of Squish Angle
The table below shows the calculated squish velocities for a 125cc engine at 0, +5 and -5 degrees squish angle.

It is evident that even a modest squish angle of +5 degrees (divergent) lowers the MSV by a significant amount. For this engine the MSV with a 5 degree squish angle has a value of 25.8 m/s which is only 66% of the value of 39.14 m/s for a 0 degree squish angle.

A squish angle of -5 degrees (convergent) gives a MSV of 30.63 m/s which is 78% of the MSV for 0 degrees.

Squish AngleMSV (m/s)
-5 30.63
0 39.14
+5 25.8"
 

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Discussion Starter #109
I have started the engine with the test ported cylinder.
Seems to have some power and revs quite a bit.
I put the engine in the hobbit sport pocket bike with the stock boat shaped expansion chamber.
If there's time tomorrow i will take a proper test run and see how it goes.
But seems like the porting of the transfer made a big difference.
 

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Discussion Starter #110
I have not been able to do a proper test yet.
I think it's something with the carburetor.
No fuel reach the sparkplug.
I did get it to run a short time and it did rev to 10 000 according to my cheap tachometer.

Now waiting for a pumper carb instead of the stock carb.
 

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Discussion Starter #111
I have now marked the transfer, will soon be ported.
Earlier this week I've got some new carbide cutters that's gonna make it a lot easier and faster to do the porting.
The transfer will be a lot bigger than stock.
Got some jb weld if I get through the walls.

Piston is marked where there are sharp edges or casting marks that's need to be grind off.

I also got some nice 5mm thick steel plates that will be used as exhaust flange for the exhausts I'm building.
And also a exhaust and intake plate for leak down tests.

I have not decided what exhaust duration and width are going to be yet.
Need to get the head and cylinder machined first then i will see what durations i will use.


I'm thinking of getting the cylinderhead gasket surface machined with not a 90 degree angle but a little less (89-89.5°) 0.5-1 degree angle so when tightening the head to the cylinder it will act like a spring sort of.
So the 2 surfaces meets at the inside and on the outside it's a couple of hundreds mm gap on the outside 0.02mm so when tightening it will spring a little and make a good seal and no need for a head gasket

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Discussion Starter #112 (Edited)
Got some porting done. I did slip pretty bad on the bore though. But I think it will work.
I also gone too far and get through the walls on 3 places.
I think I got the transfer angles pretty good

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Edit

Polished piston
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Cases needs to get a bit smoother
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