So I installed a big bore kit on a China doll. It's 49 mm piston. My question is when the Piston is at bottom dead center I can still see a very small amount of intake left. Isn't it supposed to completely close itself off? Also when the Piston is at top dead center there's almost 3 mm of the top of the Piston showing at the exhaust port
Big bore
- Thread starter khodge9
- Start date
Street Ryderz
Well-Known Member
You can see above or below the piston for intake? I don't understand the exhaust port with piston @ tdc how could you see the top of piston? The exhaust port should be fully closed with piston at tdc, the intake also should be covered with piston at bdc.
Karl Snarl
Well-Known Member
That little bit of the piston at tdc, you can use a marker to mark. then take off the jug, grab the piston off the con rod, and trim it off. You can ramp the exhaust side of the piston if you want it to open soon and wider. Thing is I recommend reading about ramping and porting before you try it. Understand what you are doing and why. Then decide if you want to port and/or ramp. The trimming of the tiny bit of the bottom of the piston at tdc blocking the intake port, isn't going to hurt anything but will give ya a little more power from better flow.
DO NOT CUT ANY OF THE PISTON SKIRT OFF THE EXHAUST SIDE. YOUR ENGINE WILL NOT RUN IF THE EXHAUST SIDE OF THE PISTON SKIRT IS REMOVED.
DO NOT CUT ANY OF THE PISTON SKIRT OFF THE EXHAUST SIDE. YOUR ENGINE WILL NOT RUN IF THE EXHAUST SIDE OF THE PISTON SKIRT IS REMOVED.
Chainlube
Well-Known Member
Don't do any of that, get a thicker base gasket. Ramping pistons isn't recommended, it creates uneven burns.
It's better to trim the top off the cylinder and use another base gasket to maintain squish, NOT A NEWBIE CHORE!
Karl Snarl
Well-Known Member
Thats why i told him to read about it before trying any of it. Clearing the little bit that hangs in front of the intake is just fine though, won't do anything but give just a tiny bit more power.
I'm sorry that's my fault you guys for my typo. I meant to say when the Piston is at top dead center there's still a little bit of the intake open. And I've already tried to add a thicker base gasket actually. I added a 1.2 mm thick aluminum plate that I cut to fit
Karl Snarl
Well-Known Member
Adding a larger gasket makes the piston hang in the opening more. You can trim the intake side. Dont touch the exhaust side of the piston skirt at all.
Cisco
Well-Known Member
I thought these big bore kits were only for long rods.
Karl Snarl
Well-Known Member
No idea. I use mainly the pk80 40mm stroke from cdh. That's what i put a jug on. The other steel sleeve i purchased all in one. I know that cdh steel sleeve jug will fit perfectly on a cdh bottom end. They sell whole steel sleeve engines too.
Street Ryderz
Well-Known Member
At tdc the intake should be fully open or some of the skirt showing, it is best to trim the skirt if that's the case it aids in flow and adds duration to the intake. Also lowering the cylinder not raising it will uncover more of the intake since you will want to keep squish, piston to head clearance to a minimum lowering it it's the better choice or decking the cylinder once port opening to stroke is matched up.
