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merry x-mas all ...i was wondereing if you can put a 91 piston in a mc7 ..i have brand new 5 over wisco ..its got blind pin ..stepped skirt ..i think its for a 91.

If your Mc7 is 9-ported, it can work. If you're running the original third port configuration, don't use a 9-port piston. TJ

Steve,

The 9 port style piston will fit your motor and run fine if block off the back side passage that bypasses the reeds. It can be done easily with a home made gasket or you can make a new reed plate using the standard plate as a pattern... just don't cut out the slot that feeds the back passage.
Another option is to use any of the manifolds that route all air intake through the reeds. The Mc 45/75 version looks just like the original 7 except it takes a pyramid reed cage.
Steve O'Hara

thanks steve ,that sounds good to me.. since someone alredy hogged out stock reed plate and put in pyramid reed cage .thanks again .looks like i have couple choices for what i can do ..

Steve,
Great, you already have a Mc 45/75 intake. That is all that they did at the factory... took the early manifold and bored a bigger hole in it so the Go Power cage would drop in and then added longer studs to allow for the extra material of the reed cage.
With that intake manifold the "piston valve" portion of the intake tract in your Mc 7 is effectively eliminated and so you could gain a bit of performance by filling in the back side passage with epoxy to "stuff" the motor a little bit more. The next step would be to elongate the three holes that are no longer used to charge the case into "9 ports" as they have been called over the years.
The change in performance would be modest and reliability might even be improved as the "9 port" layout can provide some cooling and lubrication benefits to the piston and wrist pin bearings.
If you choose to create the "9 ports" make the top of the ports the same height as the transfers.
Or, you can just leave it all as is and run either type of piston with no worries.
Regards,
Steve O'Hara