120 to 80 grit cloth backed emery type, not the paper backed, IF I was to try this, however the idea of all that grit coming loose sounds bad. The shade tree method I have seen to get out of trouble on a assembled engine was to braze a couple of pieces of ground tool steel to an old valve, at least then you are only dealing alu. shavings.
You can rent the exact tool from Isky to do what you want!With the sand paper you are talking about taking forever and a day times eight or sixteen.
The product we use is from Norton. It's called Norzone R821. It's 36 grit. It will cut fast then you could finish off with a touch of 80 grit. Any quality 36 grit should do fine. The grit is not going to come off of this type of paper. But you still face the problems of the aluminum grit. Again I would put shaving cream or grease around the piston to keep the grit from getting down the sides.
I have done this before with the 36 and did a finish touch with 80. I was never brave enough to do it without a complete disassemble and scrub job although I have heard of those that have........
Why not buy this tool Here at http://www.etsupply.com/edgetools/index.html scroll down to you see I N H E A D P I S T O N C U T T E R S you can use a drill to do this
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