 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. (www.cometic.com) 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. (www.wiseco.com) 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 ar