Smitty's "Porting 101"

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Beginners' Course in Moped Cylinder Porting
By: Smitty

Porting a moped cylinder is no where as easy as people seem to think! A perfectly good cylinder can be turned into a boat anchor in a second. You HAVE TO know what you are doing, and what you are hoping to achieve. Don't just have the attitude of "well, Smitty ports and polishes these things all the hard can it be, I think I can do it myself" There are several "keys" in the above sentence! Yes, Smitty does port and polish these things all the time, and has for YEARS. Truth is, it IS pretty hard to do, especially to get them "perfect". It only takes a second to completely ruin a good cylinder. I always suggest that for your first couple of attempts (as it is almost inevitable that you ARE going to mess-up the first time!) be done on a junk cylinder that you would never try to use anyway. If you ruin the port-job, no big deal, you weren't going to use it anyway, but IF you happen to get it right the very first time, then you have a pattern to go by.
It's not all about "opening-up" the ports. Again, a lot of it has to do with improving the "flow characteristics". This is why port timing and port shape are more important than port size in a lot of respects. If you go in and just grind-out the ports, opening them to the max, the engine will be even weaker than it was to begin with.
My very first successful porting job was achieved because I actually took over a week just studying the piston and cylinder, and the "flow" through the engine. I used a sharpie marker to make reference marks, measured the port windows, the port timing, etc. It took me over a week just to feel confident enough that I REALLY understood what was going on with just these two parts, the piston and the cylinder. Then I studied the piston and cylinder, along with the intake manifold and header pipe. It was at this point that I realized that the "holes" don't normally line-up perfectly from the factory, thus disrupting the "flow characteristics" of what I was attempting to achieve. So, I also, again with a marker, made reference marks to "match" the intake hole of the cylinder, with the mating hole of the intake manifold, and the same with the exhaust flange.
When it finally comes time to do the actual removing of metal, I suggest using small files and sandpaper. Yes, it takes a lot more time, but you take off a lot less at a time this way. You'd still rather go back and shave a little off the end of your board that you're cutting, than to hunt for the ever-illusive "board-stretcher" when you've taken off too much!!! Same principle.....once you've over-done it, it's too late, there's no putting the metal back in the port, it's ruined. Tie a rope to it, and put it in your fishing boat for an anchor.
Again, I can not say enough how important it is to pay VERY close attention to detail, and what you are about to do Exclamation

I'll use piston-port type two-stroke engines that have the intake manifold bolted to the cylinder here as a guide, as they are a little simpler than reed-valve type engines, and engines with the intake connected to the crankcase.

"Port-Matching" ...
...probably one of the most important things that can be done to improve performance is to make sure that all of the ports match what they are connected to. By this I mean intake port to intake manifold, transfer ports to engine case, and exhaust port to exhaust header flange.
The appropriate gasket, and a marker is the best way to "match the ports" to one another. Use the gasket as a template, line-up the bolt holes with the holes in the gasket, and use a marker to trace the inside of the gasket hole around the port. Remove the gasket, and then you have a guide to go by in order to get the port-holes to line up. Grind / file carefully, just to your mark, be careful not to grind past your mark. You will need to repeat this process for the intake, exhaust, and transfer ports. Do the same thing to the actual intake manifold, the exhaust header-pipe flange, and the engine crankcase *provided that you have completely disassembled the the crankcase, and removed the crankshaft, bearings, and seals. You do NOT want to get ANY grindings or metal shavings inside of the engine!!!
After all of this is done, you have successfully "matched" you engine's ports.
What you have accomplished up to this point will definitely allow your engine to run smoother, stronger, and more efficient.
Now, you can either be satisfied with everything at this point (recommended), or, you can choose to get more detailed and do some more in-depth porting.

Port Reshaping

Now this part gets a little tricky.
You can't just merely go in and start grinding away at the ports! The following process needs to be performed with the upmost care and particular attention to detail.
One of the first things to check and keep in mind is the depth of the bolt holes, as you do NOT want to open the port so much that you expose the bolt hole passage inside of the intake or exhaust ports. Not only would this disturb the "flow" that you are working to achieve, but it also creates the likelihood of an air-leak past the bolt threads. So, trace the bolt hole path, and depth, and make sure that you do NOT get too close to this limit!
Cleaning-up the rough edges of the port windows inside of the cylinder bore is really all you need to do to the port windows, as ANYTHING further, and you're messing with the port timing (keep in mind, we're out to build a little two-stroke screamer, NOT make a boat anchor out of your cylinder!). You also need to keep in mind the piston rings transition across the port windows inside of the cylinder bore.   If the port window is sharp, or too wide, etc., the rings will not have a smooth transition path across the openings, and thus will very likely result in ring breakage.   Reshaping the port mostly consists of "ramping" the port walls to the port-matching that you have accomplished up to this point. If you have a round intake port, then you need to make it as "round" as possible. If you have a rectangle-shaped intake port, you want the intake manifold and the intake port to match each other. Also, sharp corners are not good for efficient flow. The corners need to be rounded, but remain "matched". The exhaust port outlet usually needs to be as round as possible, to match the header flange.
For better atomization of the fuel mixture, intake and transfer ports need to be "roughened". This aids in breaking-up fuel bubbles, and ideally will turn wet fuel into a vapor, as the more atomized the fuel mixture is , the better combustion will be, and the better the fuel mixture will expand. The exhaust port on the other hand, needs to be as smooth and slick as possible. The smoother the exhaust port is, the more freely the spent fuel mixture is dispersed, and the quicker the exhaust is expelled from the cylinder. This will aid in the "scavenging" in the combustion chamber, and will actually help to pull more fresh fuel mixture into the cylinder faster. Another benefit to a smooth exhaust port is that carbon won't build-up in the exhaust port as quickly as it will when there is roughness for unspent oil to catch in, and bake and burn into carbon.

After you are finished with all of the port work, Arrow ALL PARTS MUST BE THOROUGHLY CLEANED!!! When you think that you have everything cleaned really well, CLEAN IT AGAIN!!! It is very important that ALL metal shavings, grindings, and metal dust be completely cleaned from all of your parts. Failure to clean away ALL traces of grindings WILL RESULT IN PREMATURE FAILURE OF YOUR ENGINE Exclamation
I hope this has been helpful in explaining how "porting and polishing" is done correctly.

If you are not comfortable performing the above work, then by all means, DON'T!!!
There are those who say that my methods of porting are unprofessional.   These same people also want you to believe that you have to have tens of thousands of dollars of machinery, equipment, and tools to do a "professional" port job.   I'm here to tell you that these things are a myth!!!  A little patience, attention to detail,
and pride in your work are the most important.  

I also plan to eventually add a video to this site, showing all of the details of me porting cylinders.  In this video I will go over from start to finish just exactly how and why I perform the cylinder porting that I do.   I'll show the tools and techniques that I use. I will also explain the "do's and don'ts", along with my theory of port-flow, fuel atomization, scavenging, and more.
  Of course, most of You are already aware of the results  ;^)
This video will also most likely be made available on DVD.
So stay "tuned" as I am getting ready to teach You how to port your own Moped cylinder(s), and acheive the same great results that I have for years!

The above provided by:
SmC mopeds & Customs