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In my racing, class rulebook is said for a four-stroke engine:
Chamfering of cylinder ports must not exceed 1.00mm (0.04 in.) at a 30 degree maximum angle.
If I search for cylinder port chamfering, I find mostly/only two stroke engines. As I understand a chamfer could be created at the intake and exhaust port where the manifolds are mounted.
Is this a way to increase flow on a four-stroke engine?
it was an old trick where we used to chamfer the port 50-60 deg for about 3mm to help create a tumble affect. i never could tell you if it ever worked but it certainly ruffled the feathers on the scrutineers,
There could be two interpretations of this.
1/ clean up of the seat transition(s) - a top and/or bottom cut of the (usual) 45 degree seat. The limit being to prevent porting under the excuse of recutting the seat.
2/ more likely it's, as you suggest, regarding port matching of the cylinder head openings to the intake and/or exhaust manifolds - also to prevent excess removal of port material to enhance flow.
Not something I've much experience of but I have seen test results that showed no measureable change until the mismatch was around 50 thou', 1.25mm. There are even those that claim a slight mismatch can reduce reversion.