An "86" piston will usually produce more power than the "87-89" piston but the life of the 86 is much shorter. The "86" piston is the same as the early CR 250 pistons.
Honda went to the long skirt and long rod in 1987 to increase the life of the piston. The 85-86 ATC 250R and the 86 TRX 250R used the piston from the earlier CR 250s.
Most motocross guys that bought CR250s expected to replace the piston every 10 hrs or less while many of the new guys that bought ATCs and TRXs were new to the world of motorsports. They expected their engine to make as much power as the CRs because they also had a 250 Honda engine. They also expected for the engines to last for a hundred years regardless how often they cleaned their air filter or what type of crap they dumped into the fuel tank.
Honda was receiving a lot of complaints from the new owners of things that had more than 2 wheels so they changed the piston and connecting rod design with the intentions of increasing longevity and not power. Honda also designed a new head that would run 6-month-old 87 octane fuel. The 87 and later TRX 250R engines had more power in stock form than the 85-86 stock engines.but it did not come from the long rod and full length skirt, it came from improvements in the port design. If you put an 86 piston and spacer plate in a 87 to 89 stock engine, it will produce more power. It is because of the absence of most of the intake skirt on the 86 piston and allows the intake to breath a little better.
