Here's my scoop and see if it has any logic to it. I know many guys have had luck with top pinned used pistons. But let's look at the root of the problem in the first place.
Mercury is like NASA. They give the job to the lowest bidder not nessesarily the best. Case in point: cheap poor low silicon cast pistons in the promax's etc. I've had the wrist pin pull right out of these for no apparent reason. The pin coming out is the least of your worries. Case in point 2: the 2.5 block used to be Nicasil coated by Mahle in Germany. The pistons were Mahle's also. Never had a pin issue. (there are some mahle's out there that have given pin problems, BUT they are 12 year old pistons! They SHOULD fail!) In 1994-5 Mercury in their infinite wisdom had Wieseco build the pistons and US Chrome do the block. Not because it was better..it was cheaper. The early US Chrome block had the plating peel off as they got up to speed with the process.
Two strokes have been using side pin pistons for the last 50+ years. All of a sudden we have the pins fall out. Honda doesn't have a problem with pins, niether does Yamaha (outboard or anything else they make), or any other manufacture I know of. Nope just the piston made in the good old USA. Funny how that works out. A high silicon piston (one which doesn't expand massively) is very hard on tooling and machining. Very hard. (US mfg's are all about the bottom line)Hence why Wieseco uses less than 6%. Also why they rattle until they heat up. Very nice. A quaility (foreign) piston runs a nice piston to wall clearance the is sometime half of the wieseco. It fits better from the start. A high silicon cast piston is worth some horsepower because it fit better. (2 horse per hole on a Merc) that is why a cast piston is all you'll find in any high perf 2 stroke from any mfg minus mercury. 60hp 250cc moto crossers...cast pistons (think about the power per cc compared to our pitiful 2.5s)(think about which would have more "stress" as the bike runs 14000rpm). If your worried about breaking one, cryo it. In over 150 engines over the last 3 years I've had ZERO failures. Must be something to it.
A piston is a engineering marvel. It is an alloy. It has calculated expantion characteristics. If you change the material in a certain spot, you change this..ask anybody that MAKES pistons, they will tell you as much. You cannot buy a filler made from the alloy of a good quaility piston. A piston has a coating on the outside. If you have ever seen a piston that lightly scores, this hard anodized surface (for lack of what it really is)is what seems to spread itself on the side of the piston when you lightly seize it, giving that "wiped" look. It is the piston's first line of defence. I'm not making this up, I have been told this by many different manufactures. If you glass beed the piston you effectively destroy this coating. A used piston that has been heat cycled many times has much less of this intact. Why all this talk....because there is no reason to put a used piston in any engine that you've taken the time to tear down. Pistons have a limited life. The original Merc v-6 was designed to run 5500rpm. We spin the 7-8-9000+rpm with essentially the same components. I almost laugh when I see guys question why the pins came out of their '97 2.5. They should have! The thing is 8 years old! Then they buy one new piston and top pin the old ones! Throw them in the trash and buy new ones.
I've used pistons from Vertex (Italy) and ART (Arias Racing Tech, Japan) and some from Perfect Bore (England). They all are lightyears better than the Wiesecos or the low bid promax parts. The Vertex and ART parts are 1/2 of the merc and you can buy a piston and ring set for 6 holes for less than Merc charges for a ring set to do the same engine. The perfect bore stuff is more and we DLC (diamond-like-carbon) coat those. The skirt has to be micro polished to accept the DLC.
All this is my opinion, but the pistons get more abuse than any part in the engine. Why stretch their life past the due date.
Randy
