Piston trim on exh. side crown

breno · Feb 1, 2012

Hi all,

I've heard that trimming a small 45degree angle on the exhaust side of the piston crown, in line with the exhaust port but not down too far down to the top ring may add some degree of performance.

Whats the verdict on this one?
Where does it add the power (if at all)?
Is this a hit and miss kind of mod?

With the pipe I am currently running power band doesnt come into effect at its best until 30mph with my 36 tooth sprocket even with the long header pipe and long stinger as its a relatively largeish (making up words) pipe. .

While I'm at it whats the best way to mod the fuel cap in order to gain more airflow without making it more likely to leak fuel when I've got a full tank?

Thanks for your thoughts guys.

Cheers
Breno

maintenancenazi · Feb 2, 2012

breno said:
Hi all,

I've heard that trimming a small 45degree angle on the exhaust side of the piston crown, in line with the exhaust port but not down too far down to the top ring may add some degree of performance.

Whats the verdict on this one?
Where does it add the power (if at all)?
Is this a hit and miss kind of mod?

With the pipe I am currently running power band doesnt come into effect at its best until 30mph with my 36 tooth sprocket even with the long header pipe and long stinger as its a relatively largeish (making up words) pipe. .

While I'm at it whats the best way to mod the fuel cap in order to gain more airflow without making it more likely to leak fuel when I've got a full tank?

Thanks for your thoughts guys.

Cheers
Breno

Thats what a lot of the 2 stroke tuner books tell you to do. I haven't tried it myself yet, but just did that very same thing to my second HT. Once I try it out I will let everyone know how it worked out.

breno · Feb 2, 2012

Gr8 stuff look forward to hearing the results

linnix13 · Feb 2, 2012

yeah keep us posted, i might try this for the heck of it, i already trimmed a semi circle into the exhaust side skirt and it responded well

breno · Feb 9, 2012

maintenancenazi said:
Thats what a lot of the 2 stroke tuner books tell you to do. I haven't tried it myself yet, but just did that very same thing to my second HT. Once I try it out I will let everyone know how it worked out.

Hey bud just wondering if you got round to testing it out.

maintenancenazi · Feb 10, 2012

breno said:
Hey bud just wondering if you got round to testing it out.

Hey breno, as of yet no. The engine I did this to happens to be my spare one. I am still tuning and tweaking my first china girl engine that is actually on my bike. It is also the one I had to rebuild, the crank main bearings self destructed after only 5 minutes of running. Currantly, I am just happy to have this engine running again, and running stronger and stronger every time I ride.

I guess I am in no real hurry to test my other HT, the one I did the piston mods on, as it was a proven good running engine when I reinstalled my original ( the one that blew ) china girl. I'm sure I'll pull it out and test it out one of these days!

Peace, James

Lazy Dog · Mar 6, 2012

The exhaust timing On these motors is pretty good as it sits, you would be better off doing the same bevel to the intake runners on the piston sides. I have tried both separately and a simultaneously. The best gains have defiantly come from the angling the sides of the piston. It will lower the compression slightly but it is very well worth it combined with a high comp head you'll never know the difference.

This is with no jug porting, only a little port clean up (deburred only) Drop the piston to BTDC and mark the intake runner opening ports on the piston. Do about an 8-12 degree pitch on the top side of the piston. You should see/feel an immediate difference in low end grunt. I have been running this for well over 500 hard miles with no probs. So much that this part of the standard install routine now.

I did kinda notice an increase beveling more heavily towards the intake side of the piston (so to try and keep more of the new mixture inside the chamber) maybe 6-7 on the exhaust side, and 10-12 on the intake side.

biknut · Mar 7, 2012

I trimmed the intake side of my piston, and I like. I don't notice any downside.

zwebx · Mar 9, 2012

im about to do this, but when i trim it do i trim it at a sharp 90 degrease?
or do i make it slightly round?

breno · Mar 9, 2012

Lazy Dog said:
The exhaust timing On these motors is pretty good as it sits, you would be better off doing the same bevel to the intake runners on the piston sides. I have tried both separately and a simultaneously. The best gains have defiantly come from the angling the sides of the piston. It will lower the compression slightly but it is very well worth it combined with a high comp head you'll never know the difference.

This is with no jug porting, only a little port clean up (deburred only) Drop the piston to BTDC and mark the intake runner opening ports on the piston. Do about an 8-12 degree pitch on the top side of the piston. You should see/feel an immediate difference in low end grunt. I have been running this for well over 500 hard miles with no probs. So much that this part of the standard install routine now.

I did kinda notice an increase beveling more heavily towards the intake side of the piston (so to try and keep more of the new mixture inside the chamber) maybe 6-7 on the exhaust side, and 10-12 on the intake side.

I need to take my head off and check out how things look at different parts of the stroke.
With what I think your saying how much meat did you remove from the edges and was the new angle given just enough to allow the bottom edge of the new angle on the piston to be inline with the bottom of the transfer port window?
Or is the 10-12 a ref. to the degrees you took off on the piston sides? (probly a dumb question..)

6-7 degrees on the exhaust side doesnt seem like it would be alot..I'm not saying thats wrong or anything as I have never tried it but Is there any benefit in going to say 10-15 degrees (bit more angle)?
I'm thinking if you go too far possibly it may hurt power due to the loss of comp. due to being a bigger volume also not giving as much firing comp. time on the top of the piston due the exhaust port opening too early..hence maybe why you say 6-7 degrees.

Do you have any pics of the dremel bit you used to carve off the piston egde?

I would love to see a pic of your piston.. I'm thinking theres meat removed from 4 sections... right?
My tachometer should be turning up soon so I plan on timing the idle to max rpm before and after this mod along the same stretch of road to see how the mods turned out.

Thanks for your comments

ivan H · Apr 6, 2012

I have ramped the top of the piston adjacent to the exhaust port & also the top of the exhaust port, it changes port timing & exhaust gas flows into the port with less turbulence, but u only do in small amounts, nowhere near down towards the top ring. If u take 1/2mm of top of piston & port its like raising the port 1mm. I have also done the bottom of inlet port & piston skirt. Dont do top of piston for inlet, the intake inlets to the crankcase, nt the cylinder. Radius the bottom of the skirt a little. Its an upper rpm thing. Cheers

fredgold52 · Apr 6, 2012

One of the great techniques for getting more performance out of Vespa 2 stroke engines is to install a 2 to 4 mm spacer between the crankcase and the cylinder base. Mill an equal amount off of the top of the cylinder. Doing this changes the port timing to give greater torque and horsepower. If you have the tools, you can grind the bottoms of the ports to match up with the top of the piston at bottom dead center.

You can also drill holes in the sides of the piston, below the wrist pin, that line up with the transfer ports. This gives the engine more opportunity to transfer gasses from the crankcase up into the cylinder.

With a good pipe, these mods will get you bunches more power.

multipaul · Apr 14, 2012

There are engines, where porting is very difficult to do. Or because of the ports are perfectly manufactured in the factory.
We say 1mm at the piston is equal 1mm rising the port.

This is from a Sachs 30cc Motor. The piston costs 50 dollars, a new cylinder about 700 bucks. The loss of compression isn't remarkeble. But the gain of power is.

Any bevel on the end of the top of a piston from factory follows this idea.
It costs nothing and they don't have to work at their tools. The first piston, they built in, was flat, I'm sure.

breno · Apr 16, 2012

With the piston to the left side how come the there are 3 grind marks in that config.?

I get there is the exhaust, and 2 transfers... but why are they like that?

Would have thought there would be meat removed from inline with the zorst port ( the way the arrow points) and then either side of that to the 2 transfers if anything.

Also when you say there is more power do you mean at the point where the pipe kicks in or down low, maybe even a few more rpm's?

Personally I'm after tuning the power band...not related but I love my turbos

Cheers
Breno

multipaul · Apr 16, 2012

breno said:
With the piston to the left side how come the there are 3 grind marks in that config.?

I get there is the exhaust, and 2 transfers... but why are they like that?

Hi Breno,

The ports are simply rotated counterclockwise to save width. But a picture tells more than thousand words...

There is a lot of gain on the upper end and only few loss at the down end.
The transfer ports are only 84° open from factory.

Cheers - Multipaul

rustycase · Apr 16, 2012

A GREAT picture, MP!
Explains a lot!
Best
rc

ivan H · May 13, 2012

Hi MP, thats a nice crankcase design. What sort of crankcase compression ratio it has would be interesting. On the china transfers I cleaned up the ridge on the roofs but didnt reshape to alter flow direction or timing, or do any ramping. In the future I might try ramping the piston top at the transfers as well 2 c how it goes. Cheers

multipaul · May 15, 2012

Hi Ivan.

Sorry for my late answer.

I have never measured that. It's not such a good idea to fill the crankcase with oil. In the right half of the crankcase there is the clutch! You see, that part is wider.
The compression ratio must be rather low and I can hardly change it.

Is there an instruction in this forum how to measure the pre-compression ratio?

Multipaul

ivan H · May 15, 2012

Hi MultiPaul, dont worry bout the late answer, no prob. If u've got an old piston & rings, the easy way is dril the crown & put a fuel tap in & with it at TDC measure the kero 2 fill it, then with the tap open & a hose on, push piston 2 BDC & measure wot come out & subtract from wot went in. Otherwise its a lot more math than I care to imagine calculating volumes, of everything internal etc. Cheers

ivan H · May 15, 2012

Hi again MultiPaul, I forgot 2 add (& I'm not sure of this) the TDC figure divided by the BDC crancase figure (wot came out subtracted from TDC's amount gives this figure) give u the compression ratio when compared to the TDC figure as 1. Thats a lot of figures, I dont do well at math. Cheers

Piston trim on exh. side crown

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