Originally Posted by whenfoxforks-ruled

6 speed straight, or high, low? Nice bike! What is the squish band now? From the edge of the piston to the lowest point of the head chamber?

Originally Posted by whenfoxforks-ruled

High compression piston, what, the stock one is flat? Got a pic of the head?

Originally Posted by whenfoxforks-ruled

There is an optimal distance between the top of piston and the edge of the head, the squish band. I do not recall the amount. Your problem is going to be getting the port timing back to where it will run properly. I can not see the if the wrist pin location is the same, but the ring lands are not! You need a professional engine tuner is an understatement. May be that Rich knows a guy??????????

Originally Posted by thumper135

the wrist pin location is the same, the ring land area is somewhat different, also the newer piston uses a thicker 2nd ring.

port timing is exactly whats keeping the build progress at the moment.

Originally Posted by SHSPVR

Is there any why you can't get the rigth OEM piston for it?.
Ok you said ring land area is somewhat different but dose both ring keep clear of the timimg ports?.

Base of what you said it look like you need shave and mill the cylinder base and case base at less 1.90mm that will bring your timing back in to stock spec.

Can you post a picture with 2 piston on the Wrist Pin or a picture with the old piston intall in the cylinder like other one.

Originally Posted by SHSPVR

What about the old piston that what I wanted to see

Look at this screenshot below
Are you saying that the rings are in exactly same place as the old piston has its with the Wrist Pin install for side by side view if that the case then your timing port shouldn't change and don't need shave and mill the cylinder base and case base at all.
Is that old piston a flat top?.

Originally Posted by helio lucas

what was the deck clearance with old piston?
simply shave the base to get the same amount and port timing will be the same and you´re done... this answer your question.

now the big question? why does the two pistons look different? old was high performance? not the same model?
are you not able to get one like the old? because later you might need one like the old one.

Originally Posted by whenfoxforks-ruled

No professionals yet. My guy figuring says shave the bottom also, the same amount as the difference. Then check the head. The width of the ring lands are the problem though, you can get 1 side to match port timing, but, how to make the other stroke match? Vintage Bob

Originally Posted by SHSPVR

What he ref to is where ring gap is going when it going up and down in the cylinder bore is should contacting the cylinder wall at all time not near any port passages see below screenshot becuase sometime you have pull old pin out and move it in new place by re-drilling a new hole that holds the ring in place.
Also it necessary to chamfer the port edges. Rounding of the sharp edge prevents premature wear on the piston and ring assembly. Using a rotary burr to make the initial chamfer, and finishing it out with a sandroll, is a good way to get the proper angle and finish.
Take look here
http://atvconnection.com/Department...nder-Boring.cfm


This will not hurt at all in fact it should give you better fresh fuel/air mix transfer

Originally Posted by SHSPVR

You need pull that pin and move by re-drilling a new hole in new place with out it crossing into any port

Originally Posted by thumper135

and leaving it as it is will cause problems? worth a risk?

the new hole can be drilled anywhere?

what about the old hole, will it have to be refilled/closed or just let it be there?

Originally Posted by whenfoxforks-ruled

You figure you have the port timing from the bottom up, back to what it was. What is the distance from tdc to the head? What are you going to do about the different port timing on the way down? The lower ring will hit sooner?

Originally Posted by SHSPVR

Leaving it as it is will cause ring to broke "BANG NO MOTOR" that why you do not want the ring gap crossing over any open port.
Yes it can be any where but as I said you do not want the ring gap crossing over any open port.
What has to be done is replace it with a littler bigger aluminum dowel pin then CNC or Mill Turning the ring grove like should be as if it wasn't there that way the ring will seal then drill the new hole and install the steel dowel.

You know for what it worth Wiseco, Ross Racing Pistons, Arias Pistons, JE Pistons and min other can make you a custom pistons just for bike have thougth about ask them?.

Originally Posted by helio lucas

thumper135, where are you located?
what is the diameter of the pin in piston?
i will look if i can get you another piston to fit. i bet there is many that fit that bike and are easy to find...

Originally Posted by thumper135

I get your point completely, the new pin on the 1st ring groove will be moved to a location where it contacts no port passages.. also making sure the old pin is cleaned like it never was there.
now since the 2nd ring located end pin is on the right side, i should get the new pin tapped on the left side? since it goes approx 90 degrees away from the 2nd ring pin.

Originally Posted by whenfoxforks-ruled

Because the ring land from top to bottom is different, it is going to effect the port timing in both directions. You have SEEMINGLY fixed 1 direction. For all I know, all that material should have been taken from the top? The ring centering pin, showing up in the port, thats a first, and an issue also for sure. There has got to be street bike people out there who have messed with this issue already? Vintage Bob

Originally Posted by SHSPVR

My frist guest would be he from India
That bike is for the asian markets only there all base a round RX100 up to RX135 or in are case an old Yamaha Enduro 2-stroke from 70/80 in USA look up DT125 which by the way is being sold in min 3rd world countries.
Min 3rd world countries are start to ban 2-stroke in fav of more eco friendly 4-stoke road bike.
I belive in the asian markets is being replaace it with Yamaha YZF-R15 "150cc", FZ and Gladiator Type SS "125cc" / RS "150cc".

Originally Posted by whenfoxforks-ruled

You think that the distance swept up is going to be the same. You have estimated that the piston difference removed from the bottom will fix it. You have not taken into consideration the new bottom end volume? Also, the distance from the bottom of the lower ring to the top of the upper is greater. This will alter the timing in both directions, fixing 1 is not fixing both. It may well be impossible to get exactly correct. Now if you had a software program, to plug in your parameters, you could see before you do it. Did you try any of the pistons from the models SHSPVR brought up? The cylinder is not toast yet. Vintage Bob

Originally Posted by SHSPVR

Bob I belive the volume didn't change that much look like rigth around 2% or less which not enough to make that much of a over all diff to it.
When he dose get this all done the biggest change will be in the Compression ratio which should jump to about 8.5 due to dome piston vs his old flat top and that should push the hosepower up a bit.

Originally Posted by helio lucas

well, the dt 125 oversize will fit but i don´t think will be easy to find there. try polini, vertex or pro-x if you wish so. before 88? has two windows on the inlet side, after that just a plain skirt. and has the two ring pegs right on the place.

relocating the pin is a very hard work since has to be done correctly. if the pin comes out will damage the cylinder and the piston. has to be a very tight press fit...

the volume of the bottom end hardly change anything on that very low performance bike.

one thing to be very aware is the increase in compression ratio. that bike is air cooled and since it does 180km\h it will go wide open for a lot of time. while there is a nice increase in torque it may overheat. if that bike will see very long time wide open i do not increase the CR at all. just put a thicker base gasket and get the cylinder port timing a little agressive, towards high rpm power and overrev... it may return the lost timing because the overboring.
if is used only for small burts like in the city, go for it. it will be fun... maybe less top speed but lots of low end torque...

Originally Posted by helio lucas

well, the dt 125 oversize will fit but i don´t think will be easy to find there. try polini, vertex or pro-x if you wish so. before 88? has two windows on the inlet side, after that just a plain skirt. and has the two ring pegs right on the place.

relocating the pin is a very hard work since has to be done correctly. if the pin comes out will damage the cylinder and the piston. has to be a very tight press fit...

the volume of the bottom end hardly change anything on that very low performance bike.

one thing to be very aware is the increase in compression ratio. that bike is air cooled and since it does 180km\h it will go wide open for a lot of time. while there is a nice increase in torque it may overheat. if that bike will see very long time wide open i do not increase the CR at all. just put a thicker base gasket and get the cylinder port timing a little agressive, towards high rpm power and overrev... it may return the lost timing because the overboring.
if is used only for small burts like in the city, go for it. it will be fun... maybe less top speed but lots of low end torque...

Originally Posted by whenfoxforks-ruled

You forgot the almighty GT750. At least it would have an excuse for blowing white smoke. Vintage Bob

Originally Posted by SHSPVR

Bob I belive the volume didn't change that much look like rigth around 2% or less which not enough to make that much of a over all diff to it.
When he dose get this all done the biggest change will be in the Compression ratio which should jump to about 8.5 due to dome piston vs his old flat top and that should push the hosepower up a bit.

Originally Posted by helio lucas

no mr. here, i wish...

it´s pretty consensual from some 50 years that mirror finished ports don´t work. they are better finished smooth, but not polished. from my point of view it´s worse on four strokes but it´s really a "no no"!

thats the first cylinder i heard of ring gap pass a port. i´ve seen ring gap on exhaust side but not right on a port...

the squish clearance is mostly based on squish velocity. if you don´t have the means or knowlege to calculate it the best way to get around the problem is to get the compression ratio right. if you really want to go on squish clearance and forget CR, 1,5mm is a good start, i assume, for that engine.