Cylinder Porting And Piston Mods

dieseldave

New Member
Jul 4, 2008
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I've read about guys enlarging ports in the cylinder and grinding on the piston to enlarge intake. Can anyone give more precise details on the port sizes? Any help on piston mods? I am trying to get these little engines to run at their potential. Every little thing I do helps, But I would love to hear about others attempts at this.
 

Goat Herder

Gutter Rider
Apr 28, 2008
6,248
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N.M.
The thing I left out When I first said to shave the piston skirt. Is if you have an older Kings Motor Bikes ch or such I swear by Kings starting to think a bushed enging is better [somtimes] their all good.:rolleyes: Alot of these motors come with the intake below the piston skirt with the piston at top dead center [top of cyilinder]. If you find when you take carb intake off at TDC your a staring at a partial piston skirt that section of it needs to be remooved. If I had a camera right now I sure would post a pict. Somebody could beat me to this. NORM?:eek:
 
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velardejose

New Member
Sep 9, 2008
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Lima, Peru
Hi
Is a common practice with two stroke engines to leave part of the piston blocking the intake?
Or is it to limit the power output?
Thanks in advance
 

tyrslider

New Member
Sep 26, 2008
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RainCity
It is mainly for longevity and cost that the piston skirt is left true. There are a # of engineering reasons as well. There are programs that allow you to simulate porting before you mod. But it is mostly hypothosize, trial and error otherwise. As you get into porting you realize balance is key. Mod the intake and you have to mod exhaust, compression and jetting (possibly carb size as well) or no benefit. Tighter tolerances are in order if you mod the piston much or you get piston slap (rocking in the cyl). Clearances (piston to cyl) on my rd were .0004"-.0005"; thats right, ten thousandths. But they were forged pistons in a honed straight (.0001") bore, not cast, and I had 36mm carbs instead of stock 28mm. I also had the crank built for 13000 rpms chambers built for top end and the heads milled.

Truth is if you have to ask how or what you're probably not prepared to do it yourself. It's better left to the pro's unless you have someone to help/show you how, and what tools to use.

Some things you can do are take your cyl off and polish all the transfer ports (the ports that go from bottom end to top). You can make sure your head & cyl mating surface is flat. To do this get a scrap slab of granite (its really flat) and stick some emery cloth to it (2 stages 220 grit then maybe 320 - 400 grit). W/ even pressure rub in a circular/orbital motion breifly until the entire mating surface is effected. Then get a tuned pipe (for the type of power you want i.e. low, mid, top range) and rejet.

W/o rebuilding the crank this is the most mod I would make on one of these motors. The fact that the pistons and cyls are interchangeable indicates that they are probably at least .002" different. Which means piston slap and seizure are likely w/ too much mod.
 

Goat Herder

Gutter Rider
Apr 28, 2008
6,248
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With 4000 thousand + miles I'am experiancing no probs ran the ch darn hard . with up 275 ponds on all the time!
 

young grease monkey

New Member
Sep 20, 2011
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Chicago
I actually polished the ports on a model nitro engine for a RC car and it runs great. All I used was steel wool. And also I think the part of the engine that makes the most disturbance to flow is where the exhaust pipe meets the port because the pipe is a small diameter pipe that meets a larger rectangular port with no taper or slope, just a cutoff. I am working on making a manifold out of cast aluminum that I will make myself. My other hobby is a small foundry for casting aluminum in sand molds, so once I build a proper furnace I might sell manifolds on ebay
 

Tohri

New Member
Aug 28, 2010
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People's Republik of Massachusetts
My experience with porting is mainly that the biggest danger to these motors is themselves. Here's what to look for, basic theory. You can't really screw it up much worse than it already is.
Exhaust port: Take off any slag, leave everything nice and smooth, but not polished. Leave the port ceiling Alone, perhaps widen the port a few MM. Not much more than that.
Intake: same as the the exhaust, widen if you feel like it. The real flow restriction is the manifold, get the Manic mechanic manifold.
Transfers: The hidden menace. So much slag on the ceilings it's amazing the machine runs at all. Correct the ceilings with a dremel right angle attachment and a sharp carbide burr. Use a tiny straightedge and aim both charges 3mm below the tip of your spark plug, and 5mm behind it (Towards the intake) this part is challenging, but you'll get most of your power here. Widening the ports is tricky, you'll need to do port tunnel work for that.
Piston: Either relieve piston skirt for better Top-rpm power, or JBweld the intake port ceiling down to match the piston skirt at TDC. This will raise low end power by altering your timing. Feel free to experiment with boost ports fed from holes in the piston.
Crankshaft: Balance, or have balanced your crankshaft, and 'stuff' the crank pin and balance pad recesses with ultra grey RTV.
Cases: match your cases to the cylinder and fill in any hard dimples in the transfer port areas.
Besides that, I always reccomend replacing all the load bearing hardware with rated stuff and all the gaskets with Fel-pro and lapping the cylinder mating surfaces.