piston diffrences

[89250rhonda]

what is the differnce between the 86 and the 87-89 prox piston?

[wilkin250r]

It has to do with the crankshaft.

In 1987, Honda changed the crankshaft on the 250r. They made the connecting rod 5mm longer. This doesn't change the stroke, how far the piston travels up and down, it just changes the placement.

Think about it, if all other things were equal, this would push the piston up 5mm, correct? Since there is only about 1mm space from the piston to the head, this would cause severe problems, right?

So to correct this, Honda changed the piston a little so the connecting rod sits 5mm deeper into the piston. So the rod is 5mm longer, but the piston is effectively 5mm shorter, so it ends up the exact same spot (*note: the piston isn't actually shorter from top to bottom, it's just shorter from top to piston pin).

Spend some time on here, and you'll hear people refer to "short-rod cranks" and "long-rod cranks". They are talking about that change from 86 to 87.

[wilkin250r]

Now, it IS possible to mix and match some combinations. If you have a long-rod crank, with the taller piston, then your piston will sit 5mm taller than normal. It's possible to get a spacer plate on the bottom of your cylinder to push everything up 5mm and bring it into correct alignment. It's actually very common.

But you cannot run the short-rod crank and the shorter piston. This would bring everything DOWN 5mm, and you cannot make that work in any fashion.