Thoughts about stroking the 66cc china?

[midwestmayhem]

So I've been thinking about this for a while, and since I finally pulled the 66cc apart to true up the crank, I'm wondering about it even more.

Piston covers up a fair bit of the transfers at bdc. One can mod the piston crown; But if the crown is cut far enough down to completely open the transfers, you'll probably run into the piston ring.

Cylinder can be raised to open up the transfers, but this reduces the compresson height and the compression. Top of cylinder can be decked to gain some compression back, but the compression height is still less than stock so overall compression will still be less.


Hears my idea..
So what if one was mod the piston crown a reasonable amount. Then figure out (measure) the stroke needed to fully open the transfers and keep the compression height and squish band the same as stock. Crank would be stroked to that, cylinder would then be shimmed up the amount the stroke was increased by. In the end one would have a motor that wouldn't sacrifice compression for transfer flow. I did a little math and a stroke somewhere around 43mm should be just about right. (stock is 40mm)
66cc crank's have been stroked using puch cylinder, build's not done yet though. http://motorbicycling.com/showthread.php?t=40181&page=2

Any thoughts on this?

 

[fx-2]

We stroked the crank to 44 mm from the stock 38 mm .The was to keep the port timing about the same as the Puch . The project has gone to Turkman , I've moved to Tucson ,Az. Last I talk to him he has been working on the race bike with the old motor an not had time to do any work on this one . BTW there are cranks that have 38 mm and 40 mm stroke stock . To use the Puch piston you need a 12 mm piston pin the stock is 10 mm so you must use a brass bushing or change con rod . Arrow bike strokes all there pro class motors .

 

[midwestmayhem]

We stroked the crank to 44 mm from the stock 38 mm .The was to keep the port timing about the same as the Puch . The project has gone to Turkman , I've moved to Tucson ,Az. Last I talk to him he has been working on the race bike with the old motor an not had time to do any work on this one . BTW there are cranks that have 38 mm and 40 mm stroke stock . To use the Puch piston you need a 12 mm piston pin the stock is 10 mm so you must use a brass bushing or change con rod . Arrow bike strokes all there pro class motors .

Thanks for the info. I planned to use china girl gt5-a cylinder(wider intake) with a custom reed, with the majority of the reed inside the intake port to reduce the drop in primary compression once the boost port is made. I'm curious, what would the change in port timing be using a stock china cylinder once it is raised to accomodate the longer stroke??

 

[mew905]

Thanks for the info. I planned to use china girl gt5-a cylinder(wider intake) with a custom reed, with the majority of the reed inside the intake port to reduce the drop in primary compression once the boost port is made. I'm curious, what would the change in port timing be using a stock china cylinder once it is raised to accomodate the longer stroke??

depends on what jug you're using. for example my jug (Grubee GT5) has higher ports than the Type D motors (like Dax F80's). but the exhaust would be about the same, the intake would be longer, and the transfers will be slightly longer as well, as far as I can guess off the top of my head (though I'm not good with estimating degrees so dont quote me on this). I'd love to have a 44mm stroke crank but I doubt anyone makes them for consumer use and I lack any fabrication skills. Designing's not an issue, but when it comes to additive fabrication, I cant do it (I can do limited subtractive fabricating, such as cutting, grinding, etc. but... anybody can)

 

[midwestmayhem]

sounds like it would be best for me to wait untill I get the gt5-a cylinder before deciding how much to stroke the crank. I doubt 3-4mm would hurt too much, but it would suck to go 3mm just to find out 4mm would be better.

I have a metal lathe and the know how, it's just that tech school is draining my wallet at the moment.