Notched Piston Skirt

[Biker Mike]

Hi everyone, I ran across a post by Egor, and also had a friend mention this as a great mod. I am seeing and hearing that you can cut a little off the bottom of the piston skirt and it is supposed to change the timing and increase HP. Could someone explain how to do this and post pic's. Does the notched side go to the intake side or exhaust side? How far can you notch it? Thank for any info you can provide.

 

[eDJ]

You may want to read this page and understand the message presented. Hirth has been around awhile and are definately doing something right.

Hirth Text.html

Probably the simplest, least expensive mod one can make on a 2 cycle is the addition of an expansion chamber. It's a straight up bolt on that will provide impressive results no matter how sloppy the ports are in a production cylinder.

Unless it's time to tear down and re ring concider a simple bolt on for now.

In expansion chamber designs there is what's called a "torque tube". Among the truncated cones of the chamber there will be a parallel band somewhere about the middle. This chamber will increase torque all thru the rpm spectrum and provide the feeling of added power.

I've seen guys go thru trick pistons and fancy porting in their cylinders only to
wind up with less than what they had to start with, at great cost of time and money when they could have been riding and enjoying.

Many owners will take their piston and insert it into the cylinder using a scribe
and the top edge of the piston as a straight edge to mark the ports tops and bottom edges so they are all equal. Thus each will flow about the same opening and closing all at the same time. I've seen pistons with holes in the skirts to allow transfer. There many things that can be done but bear in mind that on the evolutionary chain these little "China Wonders" are primitave at best.......cheap.....and just fun to bolt on and go play with. Unless engine building is more fun than riding then you can make your choice and I'll post what I can to support it.

 

[misteright1_99]

I would suggest lowering the intake port on the cylinder rather than raising the skirt on the piston. Most pistons are wider at the bottom than at the top, and cutting the bottom might produce more noise and bore wear.

 

[Biker Mike]

Here is a pic of what I was referring to. It was posted by Egor in another thread. Looks fairly simple. I got mine put back together a while ago and will begin break in tomorrow. Cant wait.

 

[mralaska]

Egor says he uses that mod "in a pinch". I would be curious what he considers a pinch and how the engines hold up for the long term after that mod. Any mod that increases hp introduces the possibility of an increased wear, of course, but removing part of the skirt is also removing part of the piston support, especially from the bottom point adjacent to the pin where the greatest force is.

 

[Biker Mike]

I was told by a friend that he use to do this to the old 2 stroke Harley's back in the day. Said it made a big difference and held up fine. I went ahead and took the plunge and notched mine yesterday and put it back together. I'll keep you posted on how it effects the engine. I could not resist trying this. Wish me luck.

 

[mralaska]

Good luck! Let us know how it performs both short & long term affects.

 

[Biker Mike]

I fired it up! It sounds great so far. I am going to run 10 min and shut down and let it cool several times before riding. Just to seat the new rings. It seems to run much smoother, not as much jerking at low speed coming back up the drive. I also drilled the hole inside the carb as suggested. More to come later.

 

[hill climber]

let us know how this goes after the rings seat. ive go a powerking "80cc" just did the porting with measurments from a post by dave. 1/4" lower on the intake and brought the exhaust up to 1.1" from the top and cut .045 from the head on my mill. just reassembled the motor last night, ill put it back on the bike today. i was thinking of doing the carb mod, drilling the hole,"idle circut", and slotting the slide. is there any mods i missed. intake port matched, exhaust expansion chamber and port matched. HEY!! DO A WHEELIE!! hill climber

 

[Biker Mike]

Just got back from a 10 mile ride. The engine defiantly runs better. Much better throttle response at low speeds. Also a much smoother operation. WOT is sweet and seems to have more RPM.s. I do not really feel there is a gain in HP however the quality of the way the engine runs is MUCH improved. I would do this MOD again.

 

[Andyinchville1]

Hi Biker Mike,

Which side of the piston did you notch (intake or exhaust)....How wide is the bottom of the notch and how high did you go up?

I may try some tinkering too! ;-)

Thanks!

Andrew

 

[Biker Mike]

Hi Biker Mike,

Which side of the piston did you notch (intake or exhaust)....How wide is the bottom of the notch and how high did you go up?

I may try some tinkering too! ;-)

Thanks!

Andrew

Hi Andrew, I notched the intake side of the piston skirt about 1/4" deep and maybe 1/2 to 3/4" wide tapering it out. These are just estimates. I did not actually measure. I just took the pic that Egor posted and eyed it and took my best guess. From what others have told me, It does not have to be precise. Hope this helps. Let us know how yours comes out.