7 mil stroker motor

[waxhead]

OK
you can get port timing right of a 5mm but it gets crazy when you do it off a 6mm and above

 

[#ZERO]

I always use OEM kawasaki pistons in GPR's. I bet every pro in Havasu in the 2000-2004 era used OEM kaw's in the Yamaha cylinder on the GPR1200.
Yes they do wear more over time because the ring ends clip the roof of the rear transfers but it will work. Tons of Lameys are also out there with OEM Kaw's or Wiseco Kaw's

Why use those when Riva & Fennic made some nice looking Wiseco flat top pistons with the ring pins and skirts designed for the Yamaha engine?

It's a lot of trouble to trim the skirts down and relocate the pins for the price difference IMO.

 

[#ZERO]

OK
you can get port timing right of a 5mm but it gets crazy when you do it off a 6mm and above

You can always mill the case top or cylinder bottom down to lower the port timing on the strokers with the stock length rods.

Spacer plates below the cylinder will usually work well with the strokers with the longer rods.

 

[waxhead]

its not the size of the cylinder its the port timing it gets advanced the longer the stroke is
You can only go so far out the top , if you go to far the ring will pop out

 

[#ZERO]

That's one reason why flat top pistons work well on stroker engines, because they have a lower compression height.

 

[waxhead]

yes but you still have to take the piston up the bore
The flat top having a 3.8mm higher pin height doesnt mean your pistons will not come out the top trying to get the porting heights right

It means you can have a shorter engine but because the piston sits lower in the cylinder it opens the port sooner
Your back to the same issue
Its stroke and rod length the effect the overall port timing not the piston ( other then the port themselves)
then you have to jack the cylinder around to get it right
It doesnt matter what the height of the piston is it still moves the same amount per degrees of reveloution

 

[#ZERO]

The flat top having a 3.8mm higher pin height doesnt mean your pistons will not come out the top trying to get the porting heights right

The flat tops don't have a higher wrist pin height; it's a lower piston surface or compression height.

Its stroke and rod length the effect the overall port timing not the piston (other then the port themselves)
It doesnt matter what the height of the piston is it still moves the same amount per degrees of reveloution

All that and more effects the overall port timing even the piston surface; using longer rods & shorter piston compression heights will increase the dwell time and slow down the piston speed.

 

[waxhead]

its nothing to do with the piston height
its about the rod length and stroke
this is why every time you get software to convert degrees into mm of bore travel they only ask for rod length and stroke
http://john.rushworth.com/Pics/Piston/index.html
have a look a this link

The choice of piston only determines the cylinder height you will have to run at
If you have to run the piston out with a dome piston the situation will not change with a flat top

the pistons are the same height
the kawi just has the pin higher
so what you take from the top goes on the bottom
here you can see a picture i just took of a yami dome piston and a kawi flat top

in this picture you can see the higher pin height on the kawi piston
you still have to run the piston out the top on a flat top the same as a dome top if you wish to keep the same timing

 

[#ZERO]

The piston compression height is measured from the wrist pin center line to the edge of the compression surface.

Pistons with a lower compression height rock less in the same bore and don't require longer skirts.

Here's a Riva flat top, Kawi flat top and Wiseco dome piston.

The flat tops are 3.8mm lower in compression height and raise the port timing and the longer rods slow down the piston speed which changes the dwell time.

 

[waxhead]

my digital verniers must be wrong then

i may run different pistons in my ski will save me having to port the cylinder in the future
good information thanks for that

any idea on how to get rid of the 3.8mm of extra squish clearance

 

[#ZERO]

That lower compression height is a benefit if your using a stroker with longer rods and not too good for the port timing for the stock stroke for freestyle.

You have to build the engine around those pistons by changing the height of the sleeves, cylinder, crank stroke, rods etc. to get the port timing to work just right for the application.

 

[waxhead]

you havent explained how your going to get rid of the squish
your talking about running a piston that doesnt go up the bore as far because it runs a higher pin
so therefore it going to have a massive squish clearance
the only way around this is to move the cylinder down so it goes to tdc
by moving the cylinder down 3.8mm your getting the same timing as you had before

This is why i am saying to you the port timing is not effected by the piston
but by the stroke and the con rod length

 

[SUPERTUNE]

you havent explained how your going to get rid of the squish
your talking about running a piston that doesnt go up the bore as far because it runs a higher pin
so therefore it going to have a massive squish clearance
the only way around this is to move the cylinder down so it goes to tdc
by moving the cylinder down 3.8mm your getting the same timing as you had before

This is why i am saying to you the port timing is not effected by the piston
but by the stroke and the con rod length

If you want the same port timing, then machining the base down to lower cylinder would keep the same port timing, if you what it more aggressive then mill some off the top of the cylinder and less off the base.
But, there's always limitations with the intended stroke design of the sleeve, no matter where you set the cylinder height you can't change the floor of the exhaust port to the top of sleeve.
So in some cases you can run the piston with a positive deck height.

 

[SUPERTUNE]

its not the size of the cylinder its the port timing it gets advanced the longer the stroke is
You can only go so far out the top , if you go to far the ring will pop out

Exactly.

 

[SUPERTUNE]

That's one reason why flat top pistons work well on stroker engines, because they have a lower compression height.

Not always true, Like Jared was saying, as the stroke increases the problem (correcting port timing) gets worse with a shorter piston height, now it does help with some of the skirt length issues, but port timing is really critical.

 

[SUPERTUNE]

Up to 6mm is really pushing the limit for any stock 62T or 61x re-sleeved cylinder for freestyle powerband.
5mm's are fairly common for a stock Yamaha cylinder.

 

[Kennay]

I rode a pretty fire breathing 6mm on a 61x cylinder on Sunday. I was very impressed, but considering who built it, I'm not surprised.

 

[Matt_E]

:biggrin:

 

[waxhead]

Yeah I agree with you on that machining the cylinder will move the port timing around and I have had to do it on a few of the stokers I have had.

 

[masterblaster]

whats the best cylinder setup for a long rod 5 mil stroker to keep aggressive freestyle port timing on a 61x cylin.how many mil plate top or bottom,without having to press out the sleeves?