Way back when they used ridge reamers so that you could remove the piston without damage to same. If your pistons came out without a problem then there is probably no reason to use the reamer. However, in some cases-not all, there can be interference between the small radius at the ridge & a new top ring which could cause problems. For me I would use the reamer if you can catch your finger nail under the edge of the ridge just for safety. As for compression loss: The slant 6 is a low compression engine so my guess is that the power loss would be undetectable. If you are re-using the old pistons check for clearance at the top ring groove. Insert (don't install) the new ring in the groove & see if it will rock excessively. You are checking to see if the ring groove is tapered (wider at the outside than the inside).
