I'm new to Sevens so can't comment specifically, but did some porting(both two stroke and four stroke bikes) and Minis. Without taking much meat out and reshaping, it is possible to improve flow by cleaning the passages up with a die grinder like a Dumore or similar. Theories may have changed in the thirty...oops forty years since I did this sort of thing. Matching the manifolds to the ports is important, as well as the gaskets. The other trick we used to do was to not grind to a fine polish, but to leave some scratching at right angles to the flow, to create what we called laminar flow, though that term is usually applied to fluids. Anyway, the idea is to reduce friction and small consistent eddies allow less friction than a polished surface. At least that's what the theory was and I used it racing sailboats as well. Gains were never big, but on low powered motors, a mile an hour on the straight was sometimes enough to make a difference.

Location: Seattle, Washington, USA

Thanks for your input Eric.

I've done this type of work before on two strokes and BMC A series engines where there has been some accurate information to go on to determine what needs to be done to get a desired result.

Unfortunately in this case I am not familiar with what I can do to clean up the porting without compromising the performance it should have in a standard stock state.

If those in the know think that it would be best to leave things as is then I'll not touch the block and leave it as made.

Cheers,
Stephen

Location: Calgary Alberta Canada

Hi Stephen, I'll let others respond regarding how much to take off. However, I would think that matching manifolds and gaskets to the ports, without removing much material would be beneficial. I can see that the ports are not symmetrical. You won't be doing much, but you would be reducing turbulence and increasing flow. The former is good for a combustion chamber, but slow in the passages into and out of the chamber. I wouldn't much call this porting but just cleaning up casting flash and that sort of thing. Like blue printing, matching weights of pistons and rods, always gave a bit more power, without actually changing engine dynamics. Sort of like Vincent motors, the Rapide was not as balanced as the Shadow though their tuning was not very different. Better balance means less friction, more power and ultimately more longevity at high RPM. Its what made my Aermacchi motor spin to 9,000 without sending out the rod.

Location: Seattle, Washington, USA