Share Your Secrets

Dear Ralph:

Where can I get the specs for trail porting my engine (Rotax 670)?

Tom Scheideman

Tom:
There aren’t really any instructions, books, or sources that will give you exact porting specs for specific engines. One of the best sources of porting information is “Tom Turner’s Two Stroke Software”, which is a software package that will give you the ability to calculate port specs for any state of tune you may wish. This is pretty much for those with experience in the field, definitely not for beginners.

The art of porting has long been regarded as a “black art”, and those who have spent the years learning the art are not readily willing to give up their secrets. Kind of like a “magician’s silence”.

Many of today’s engines are so close to the edge in terms of “maximum safe” porting that it is very easy to go too far. But, you can never go wrong with matching the flow areas and blueprinting the engine. Matching the ports is all we’re doing here. You want to take a look at what the factory was trying to do on a mass production basis, and “clean it all up” with some time consuming “hand massaging”.

Do not attempt to do an “eyeball” porting job. You will have to have the capability and tools for measuring and cutting accurately. Take a digital dial caliper and measure all of the port dimensions – you’ll find that some dimensions are bigger than others by as much as 0.5 to 0.75 of a mm. Since you can’t add metal to the ports, all you can do is take metal away from the “low” or “small” dimensions so that they match the “high” or “big” dimensions. The port contours are very important, and you do not want to be altering them without good knowledge of why and what you’re doing.

Perhaps the easiest thing to do (that will benefit the long term durability of your engine) is the chamfering of the ports. Factory engines are pretty “raw” in this respect, but are getting better. Take a ball hone and make 2-3 quick passes from both the top and bottom of the cylinder to get rid of all the “sharp edges” on the vertical portion of the ports. The top and bottom edges of the ports are best champhered with a small Dremmel type tool. You want to be able to run your finger along the edges of all the ports and not feel any sharpens. If you’ve never tried any of this before, don’t try it on your new engine. It typically takes several hours of experience and a few failures along the way to gain the ability to do this right. Doing this type of work is where the differences are in having an experienced two-cycle shop do your work as opposed to having an auto-parts store bore your cylinder, or the local “hack” mechanic.