50cc Cylinder head porting

50cc Cylinder head porting


Product Code: 50CCOAE1133

Product Information

This info is from my experiments with a 139QMB cylinder head. There are much more advanced and technically correct forms of head and port work, but this method can be done by most people with patience. I did notice a definite increase in power after performing this work.

The outlet of the stock carb is 18.8mm, so my plan is to make the intake and intake port of the head as close to 18.8mm as I can and make the path more direct if possible. Sorry, but I'm not going to remove material in one area and build another up etc... Just trying to get good flow without days of work. 

Sorry, the pics suck.

Stock intake port, 16.5mm diameter. I should have been smart enough to remove the valve before taking the before shot. Oops.

1hr45min later, 18.8mm intake port. The intake port really narrowed down inside, but I opened it up. I mostly worked the side that would create the more direct path to the valve. I left the surface kinda rough... well, not rough but not smooth.

The pile of metal dust should give you an idea of how much material was removed.

The exhaust port started out at 16.8mm. The passage is incredibly small on the exhaust side and not much direct flow area.

You can see that there's a nice improvement. I opened the port itself up to 21mm to match the stock header that I opened up as far as I could. The header quickly gets funneled to a smaller size though, because the exhaust pipe is only around 5/8 to 11/16" inside diameter. The exhaust port isn't quite done because I wore out some dremel stuff and I'll have to get more tomorrow.

I opened the intake up on the bend and a little all the way through. I port matched the intake, spacer, and head.

I opened the exhaust port up all the way to the lip that holds a gasket and then mirror polished the port the best I could. I couldn't get to some areas with the tools I have. After the grinding with stones and carbide bits I used coarse drum sanding bits, fine drum sanding bits, steel wire wheels, brass wire wheels, 600 grit paper all the way up to 2000 grit paper, then a course compound little buffing discs and cones, then a fine compound on a loose disc and cones. There's a little bit of time in this head. 

You can see the reflection of the inside of the port and all the lights. ;D Not that it matters, it'll be covered with soot soon enough.

I ended up burning a valve in this head because I knicked the valve seat slightly. Be very very careful when porting and polishing or you could ruin the head. If you do put even a small knick in the valve seat, you can try lapping the valves with a compound and a suction cup used to hold the valve and spin it against the valve seat. If that does not make the valve seat look like new, take the head to a machine shop and show them the problem.

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