Pocketbike Forum banner
101 - 120 of 157 Posts

·
Super Moderator
Joined
·
3,080 Posts
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.
 

·
Super Moderator
Joined
·
3,080 Posts
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.
 

·
Registered
Joined
·
59 Posts
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.
 

·
Registered
Joined
·
196 Posts
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
 

·
Super Moderator
Joined
·
3,080 Posts
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
 

Attachments

·
Registered
Joined
·
59 Posts
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.
 

·
Registered
Joined
·
196 Posts
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"
 

·
Registered
Joined
·
196 Posts
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.
 

·
Registered
Joined
·
196 Posts
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.
 

·
Registered
Joined
·
196 Posts
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

159179
159180
159181
159182
 

·
Registered
Joined
·
196 Posts
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

159187




159184
159185
159186
159188
159189

Edit

Polished piston
159190

Cases needs to get a bit smoother
159191

159192

159193
 

·
Registered
Joined
·
59 Posts
Looks very good. Are you going to add the JB weld to transfer duct corners later?
Too bad that the exhaust port has that stupid notch. It's going to lose some power because of that. Cannot even weld it because it will ruin the plating.
 

·
Registered
Joined
·
196 Posts
Discussion Starter · #115 ·
Thanks yes the transfer ducts are planned to be done later.
I'm thinking of getting jb weld steelstik, with that i can press the jb weld in with my fingers and smoothing it out. Wearing gloves of course.

I installed some new ball bearings to the cases and waiting for my oil seals to arrive.
And going to purchase some steelstik too so I can get those transfers nice and even.

Edit.
What do you think of the boost ports?
Shall I make them wider? Deeper? Reshape them to get the angle upwards the sparkplug?
Now they are so shallow so i don't think they do much difference
 

·
Registered
Joined
·
59 Posts
I filled them with JB weld. I wouldn't try to use the "steelstik". I imagine it is some kind of mass that you can bend and form to shape, but I would think that it may not stick well particularly on a smooth metal surface. It would be bad to have the epoxy drop when the engine is running... The normal JB weld seems to stick very well when first sanded with rough grit paper and then cleaned with acetone. You just pour a large blob of the epoxy there and grind and sand it smooth.

About the boost ports, yes I think they could be made deeper. And make the angle towards spark plug.
 

·
Registered
Joined
·
196 Posts
Discussion Starter · #118 · (Edited)
I don't know if I should leave the boost ports alone.
If I make the angle towards the sparkplug they will open earlier than transfer ports and i have already a bit low blowdown time so it might be better to leave them as is.
Like they are now I suppose they flow almost nothing through them.

I will try the jb weld on the transfer ducts.

Edit.
Grinded some more, exhaust duration are 175° and transfer 125°.
So blowdown are 25°.
Maybe a bit low but will try that for now.

Jb welding the transfer ducts are next on the list and polish the transfer and exhaust ports.

Got some new bearings in engine cases.
I hope that next week I can get the cylinder and the cylinder head machined.
 

·
Registered
Joined
·
196 Posts
Discussion Starter · #119 ·
I don't like the compression notch in the bore of the 44mm cylinders.

So i thought why not make a 40mm cylinder too.
I've got no lathe or drill press and such things. Even though I would love to have it.
So i just took a saw and saw the stock 40mm head off.
Needs more work of course but it's a starting point.

I will need one more cylinder head also or a 40mm flat head piston.

40mm specs.
182° exhaust duration
120° transfer duration
31° blowdown

159245

159246


159247
 

·
Registered
Joined
·
196 Posts
Discussion Starter · #120 ·
Back to the 44mm bore cylinder.
Which of these two looks best in your opinion?
Used model clay.

One side is just enough clay to get rid of the 90° angle.

The second side is a full reshape, with the transfer ducts angle towards the sparkplug.

(I know there is some bad chipping, i slipped bad with the dremel tool.)

159259

159260
 
101 - 120 of 157 Posts
Top