Squish

**barty5** · 26th June 2009, 21:57

Originally Posted by cheese

Re the holes in teh piston, I got a OEM one for my old RM and then I got an Athena one and it had the holes in it. I found no real difference in power between them. I assumed that it was to spread oil better into the crank etc. I found be well wrong there.

I could be wrong also thought i read some place it was just to cut weight??? as they dont lead to any place and anything that went in would have to come back out same place???

**cheese** · 26th June 2009, 22:10

Hmm maybe. 2 holes would be 5/8ths difference though. Mine you every little bit I guess....

**F5 Dave** · 26th June 2009, 23:49

Firing solder down the hole to the edge of bore turning it over & retrieving it is easier & then repeat for 4 corners (well yeah a circle but. . .).

The site is good info I didn't bother to read it all again, but typically once you get the squish down to ideal (0.7 would be min for a 125, bit more for bigger) the compression tend to get a bit high. On several bikes (my GG included) I had to take some metal out to get com back to where it would be fine for the gas I ran. I don't take compression gauges as being accurate for ratio measuring, only for gauging change once ratio determined.

**camchain** · 27th June 2009, 10:09

Pic worth 1k words alright. Hard to properly measure that any other way.
(Thanks IIIRI, 'plastigauge' back into part of memory where it used to be. Solder looks good as well though!)

Noobi, keep in mind more gaskets=more gasket crush to take into account, Reckless got about 0.05 squash with 0.75 gasket?. Ask that guy about the piston dent/hole. Seems unlikely, but maybe to keep a hot part of the bore away from piston? I know katooms sometimes have an issue where nip-ups happen at the mini power-valve exhaust ports.

**Reckless** · 27th June 2009, 13:42

Originally Posted by barty5

I could be wrong also thought i read some place it was just to cut weight??? as they dont lead to any place and anything that went in would have to come back out same place???

Originally Posted by cheese

Hmm maybe. 2 holes would be 5/8ths difference though. Mine you every little bit I guess....

The piston windows do heaps!
As I understand it the piston windows are always in the back of the piston lining up with the inlet port. They let more fuel in as the piston travels. I probably put you guys wrong with calling them the wrong term boost ports?? which are in the barrell. My Ktm has two really trick ones that revolve as the power valve activates.
Back to Piston windows: As you can imagine as the piston travels up letting in fuel, if there is a window in the rear, the fuel starts entering as the window passes the inlet and doesn't have to wait for the bottom of the skirt to pass the inlet. This enables a larger volumn of fuel to enter and be pushed up the transfer ports as the piston fires and goes down again. I would imagine this is far better than shortening the skirt and making the piston less stable in the bore. Some pistons have them some don't. My Ktm has a section of the skirt cut higher but similar priciple. But you all know how a 2smoker works so I'll leave it there.

**barty5** · 27th June 2009, 14:02

Originally Posted by Reckless

The piston windows do heaps!
As I understand it the piston windows are always in the back of the piston lining up with the inlet port. They let more fuel in as the piston travels. I probably put you guys wrong with calling them the wrong term boost ports?? which are in the barrell. My Ktm has two really trick ones that revolve as the power valve activates.
Back to Piston windows: As you can imagine as the piston travels up letting in fuel, if there is a window in the rear, the fuel starts entering as the window passes the inlet and doesn't have to wait for the bottom of the skirt to pass the inlet. This enables a larger volumn of fuel to enter and be pushed up the transfer ports as the piston fires and goes down again. I would imagine this is far better than shortening the skirt and making the piston less stable in the bore. Some pistons have them some don't. My Ktm has a section of the skirt cut higher but similar priciple. But you all know how a 2smoker works so I'll leave it there.

good point makes sence guess wiseco figure its not needed ans shorter skirts are ok im sure he'll get it sorted.

**noobi** · 27th June 2009, 14:14

Originally Posted by Reckless

Back to Piston windows: As you can imagine as the piston travels up letting in fuel, if there is a window in the rear, the fuel starts entering as the window passes the inlet and doesn't have to wait for the bottom of the skirt to pass the inlet. This enables a larger volumn of fuel to enter and be pushed up the transfer ports as the piston fires and goes down again. I would imagine this is far better than shortening the skirt and making the piston less stable in the bore. Some pistons have them some don't. My Ktm has a section of the skirt cut higher but similar priciple. But you all know how a 2smoker works so I'll leave it there.

how does a blind hole increase fuel volume?
isnt a piston widow in the skirt? like this http://www.sip-scootershop.com/wwima...8c42bd504b.jpg
i couldnt see how the blind hole in my piston could increase this fuel flow, this blind hole is above where the gudgeon pin goes ?

**Reckless** · 27th June 2009, 16:26

Originally Posted by noobi

how does a blind hole increase fuel volume?
isnt a piston widow in the skirt? like this http://www.sip-scootershop.com/wwima...8c42bd504b.jpg
i couldnt see how the blind hole in my piston could increase this fuel flow, this blind hole is above where the gudgeon pin goes ?

Yes Noobi you are correct I assumed they where talking about windows in the skirt I prob didn't read it properly!