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Piston help again please??
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Posted by: 400ex18

my question is what is the difference between the 86 and 87 pistons, are they just a lil taller?? is that it? cause the company that i got it from said i will have the same proublem? so im pretty much lost, first time rebuilding an r!

Posted by: C-LEIGH RACING

quote:
Originally posted by 400ex18
my question is what is the difference between the 86 and 87 pistons, are they just a lil taller?? is that it? cause the company that i got it from said i will have the same proublem? so im pretty much lost, first time rebuilding an r!

Yes, pistons are different. 85/86 are the same & 87/88/89 are different.
The 86 model you know has the cut out at the bottom on the intake side + the wrist pin hole is low down in the piston because the rod is 5mm shorter than a 87/88/89 model rod.
87/88/89 model pistons, wrist pin hole is higher up in the piston because of the 5mm longer connecting rod.
87/88/89 model pistons have a full skirt on the intake side & a window about half way up on the intake side.

Whats the problem your having with the engine.
Neil

Posted by: 400ex18

the proublem is when i got it back togather the piston was hitting the head and would not let it crank over, so i took it back apart and realized what was going on!! i have an 86 r but is it possable that they have but the longer rod in it??

Posted by: Dave83

Yes,it probaly has along rod setup then.Look on the rod for #s.

Posted by: 400ex18

i could not find numbers on the rod no were!! it might not even have a 86 motor idk!! i know the piston that i took out of it was for an 87, but when i order my piston i dident know there was a difference between them, so i order the 86! so now i got one comming for an 87 250r so hopfully it fixes it!

Posted by: C-LEIGH RACING

Now, the next thing is, what year cylinder you got.
85/86 models have a big oval intake window in front of the reeds & a 87/88/89 cylinder has two vertical bridges in the intake.
Neil

Posted by: 400ex18

im pretty sure its the 87 to 89 but not sure will check tomarrow! does it matter what year cylander i have?

Posted by: C-LEIGH RACING

Yes,
Generally people know its ok the use the old 85/86 model pistons in either year 250R cylinder, just a matter of using a spacer plate for it to work with a long rod crank, but, its not normal to use the 87/88/89 model piston in the old 85/86 model cylinders.
The design of the new model piston could lead to desaster if that full skirt happen to get caught in the big intake window.
Neil

Posted by: wilkin250r

Let's give you a little background information to help you out.

In 1987, Honda changed the connecting rod on the 250r crank, making it 5mm longer (a little less than a quarter inch). Ordinarily, this would cause the piston to sit 5mm taller. To compensate, Honda also changed the piston so that the wrist pin is 5mm deeper.

The piston isn't actually shorter from skirt to crown, but it SITS shorter on the rod. With the 5mm longer rod, but 5mm shorter piston, the piston crown is at the exact same height as previous.

As you have found out, if you use a long-rod crank with the 86 style piston, the piston will sit 5mm taller than normal. It is certainly possible to run the combination, it just requires a 5mm spacer plate to raise the cylinder up to the proper level. In fact, it's a very common setup, because the 86 piston was available in more overbore sizes.

Posted by: 400ex18

Hey guys thanks for all the help!! I got my new piston in today and its gonna work great!! But I do have one more question tho its like 30 outside will it hurt to break in the motor been that cold??

Posted by: wilkin250r

quote:
Originally posted by 400ex18
Hey guys thanks for all the help!! I got my new piston in today and its gonna work great!! But I do have one more question tho its like 30 outside will it hurt to break in the motor been that cold??

Nope, just remember 2 things.

Cold air contains more oxygen, so it needs to be jetted a little richer. Also, make sure you give the engine plenty of time to warm up before you start punching the throttle.

Posted by: 400ex18

Well finally got my 250r back togather, just wanted to say thanks again for all the help!!

Heres a pic

Posted by: Jer250R

well, this helped answer my question too, so thanks for the post.

I have an 87 frame so i assumed i had an 87 engine, but i was wrong. just came back inside from trying to install my new piston and rings...turned it over before i tightened everything up and the piston was hitting on the crank. looking at my old piston it is quite different, with a lower pin and a much deeper cut-out on the intake side (wish i had compared them before i went outside

)

man i wish people wouldn't mix and match frames and engines

Posted by: 86 Quad R

its all about education.............. you are learning. the next time you wont make the same mistake.

knowing what to look for is important.

most have encountered what you are going through.

Posted by: Jer250R

haha, very true...the sad part is i've never really installed a piston, rings, wrist pin and clips before so i took my time and did everything exactly perfect..i was very proud of myself...right up to the point where i realized it was all for nothing lol. next time should be even easier i suppose