Porting: Tools Of The Trade, By Eric Gorr

Eric Gorr

Engine Builder
Jun 29, 1999
Tools of the Trade
There are two main types of tools used in porting, measuring and grinding. Here is an overview of how these tools are used.

The basic measuring tools include a dial caliper, an inside divider, and an assortment of angle gauges. The caliper is used to measure the port height, the divider is used to measure the chordal width of the port, and the angle gauges are used to measure the roof and side angles of the ports. Calipers and dividers are available from places like Sears or industrial supply stores. Angle gauges are fashioned from cardboard and specific to individual cylinders.

The most common grinding tools are electric powered. They consist of a motor, speed control, flexible drive shaft, tool handle, and tool bits. The power of these motors ranges from 1/5th to 1/4th HP with a maximum rpm of 15,000. Popular manufacturers include Foredom, Dremel, and Dumor. Each company sells a full compliment of accessories for all sorts of hobbyist activities. The most popular source for cylinder porting tools and accessories is CC Specialty in Tennessee (1-800-762-6995).

The tool handles and bits are the secret to porting. There are two types of tool handles; straight and right angle. The straight tool handles are used for machining the port ducts. The right angle tool handles are used to gain access to the port windows from the cylinder bore. Over the years I've tested hundreds of different tool bits and arrived at some simple materials and patterns for finishing the different surfaces of a cylinder. The materials of a cylinder range from aluminum as the base casting material, to a cast iron or steel liner, or nickel composite plated cylinder bores. Here are the basic tool bits used for porting; tungsten carbide works best for aluminum, steel, and cast iron, stones are best for grinding through nickel composite. The tungsten carbide tool bits are available in hundreds of different patterns and shapes. The diamond pattern is the best performing and the shape of the bit should match the corresponding shape of the port. Stones, or mounted points as they are termed in industrial supply catalogs, are available in different shapes and grits. The grits are graded by the color of the stones. Gray being the most course and red being the most fine. The finer the grit the faster it wears but the smoother the finish.

Making Ports Bigger
Generally speaking, if you're trying to raise the peak rpm of the powerband with an aftermarket exhaust system of clutching on a snowmobile, the ports will probably need to be machined in this manner; widen the transfer ports for more time-area and raise the exhaust port for more duration. Most OEM cylinders have exhaust ports that are cast to the maximum safe limit of chordal width. Often times widening the exhaust port will cause accelerated piston and ring wear. In some cases the port will be widened so far that it breaks through into the water jacket. The internal casting on some cylinders is so thin that it prevents tuners from widening the exhaust port. Transfer ports should be widened with respect to the piston ring centering pins. The ports should have a safe margin of 2mm for the centering pin. The height of the transfer ports is based on the time-area of the exhaust port above the transfer port opening height. That is called Blow-down. The exhaust port has to evacuate the cylinder bore of burnt gasses before the transfers open, otherwise backflow will occur into the crankcase. That can cause a variety of dangerous problems like blown crank seals, chipped or burnt reeds, or in extreme circumstances a fire that can extend out of the carb. The angles of the transfers are important too. Generally speaking when the side angles direct the gasses to the intake side of the cylinder, or the roof angles are a steep angle 15-25 degrees), the porting will be better for trail-riding. When the side angles direct the gasses to the center of the cylinder and the roof angles are nearly flat (0-5 degrees), the porting will be better suited to drag or lake racing.

Making Ports Smaller
Ports are purposely made smaller for several reasons. One or more of the ports could have been designed too big, or a well-meaning tuner may have been overzealous, or a customer may have asked for more that he could handle. There are performance gains to be had from smaller ports, for high altitude compensation or for more punch for trail and snowcross riding. Simply using a thinner base gasket or by tunring-down the cylinder base on a lathe. Cometic Gasket Co. in Mentor Ohio makes graded gaskets from .25 to 1.5mm and even custom base plates for stroker engines. ( Another method is by welding the perimeter of the port, although that entails replating the bore. Transfer and intake ports can be made smaller with the use of epoxy. Brand name products like DURO Master Mend or Weld-Stick are chemical resistant, easy to mold to fit, and can withstand temperatures of 400F. Master Mend is a liquid product and Weld-Stick is a semi-dry putty material. The epoxy can be applied to the roof of the ports to retard the timing and reduce the duration. It can be applied to the sides of the transfers to reduce the time-area, and it can be applied to the transfer ducts to boost the primary compression ratio (crankcase volume).

Porting for Big Bores
WISECO offers big bore piston kits for most popular snowmobiles. ( The average increase in displacement is 50cc per cylinder. This requires that the cylinder be over-bored 4-8mm. Because the ports enter the cylinder bore at angles, when the bore size is increased all the ports drop in height. The steeper the port angles the greater the port height will drop. Lower port heights mean retarded timing and reduced duration. The exhaust port gets narrower and the transfers get wider. A larger displacement cylinder will require more port-time-area. Normally the exhaust port needs to be raised higher than stock to compensate for the compression ratio. If you're adding a set of performance pipes at the same time as the big bore, you'll need to compensate the port timing to get the best gains from the pipes. It's a complicated thing. Sometimes tuners use thicker base gaskets to compensate for big bores, but ideally the port-time-area needs to be calculated before any serious porting changes are made. If you are strictly trail riding at high altitude, you can just have the cylinders bored and replated because the porting will inherently change to suit that type of application.

Computer Design Software
The best-kept secret in high performance tuning is the use of computer design software. These products became popular about six years ago when Tom Turner adapted the SAE programming code and added in his own empirical data from his career as a motorcycle drag racer and tuner. Tom's products are named TSR software ( and available in MS-DOS format for PCs. The programs cover individual engine components. The most popular program for porting is PORTTIME 2000 and it sells for $200. It features target specs for all sorts of vehicles including snowmobiles. Basically a tuner types in engine spec dimensions like the bore and stroke plus all the individual port measurements. The program runs mathematical calculations in order to provide a simulation of how the porting changes will affect the engine's powerband. TSR's programs will get a tuner 90% to the engines potential. The next level of programs includes 1D and 3D gas dynamics simulators. Dynomation is a 1D simulator that sells for about $500 ( It enables tuners to combine all the engine components together to simulate dyno runs on the computer so they can save time machining metal and swapping parts on actual dyno runs. Dr. Gordon Blair programmed the 3D simulator named Virtual Two-Stroke, the most well know two-stroke engine researcher. His program is marketed through Optimum ( through lease programs. Priced at $12,000 a year, it's intended for use by engine manufacturers.