starting 84 250r project, cylinder

[sbjones73]

well i finnally got the time to go to my local machinist. took my cylinder to him to get measures for boring. was told it was 2.7659. that comes out to 70.253mm. my piston is very scored up from what i am told running straight gas with no oil mixture through it.

so here is my question. machinist recomended going 70.70 or bigger. will this make any problems for cooling when done? i will be riding this thing in trails and hopefully (if i can find one, a track) how large can you go before you have any cooling or adverse problems from going to big.

here is a pic of my piston, and cylinder. i don't think the cylinder is that bad. i will be taking it to another machinist for a 2nd opinion. i can't feel any of the scraches with my finger nail at all. love the scoring? not me dang it

[toocheaptosmoke]

Most of the time you should go with as little boring as possible, I would only go up to the next available size piston. I'm thinking maybe the guy told you to go .5mm over, which would be a .5mm over stock 70.5mm which is only like a first or second over bore. I know wiseco makes pistons up to 2mm over for the air foolers so you should still have plenty of meat left if that is the case.I'm running 1.5mm over and have been OK.

[money89tractors]

70mm is stock bore. With yours measuring 70.25, that means its only been over bored once. Wiseco makes a piston up to 72mm for the 81-84R. I am running 71.50mm piston (one bore left on my sleeve) and i run trails for hours and hours. My R was also packed with mud at TF (air cooled engines has a harder time getting rid of heat when covered in mud) and still had no problems with heat at all.

-Phil

[1upfront]

I bought one of those 5.5mm overbore pistons for my 83r just for the hell of it and had the cylinder bored all the way to 75.5 to match it thats 3.5mm larger than it was ever intended to go LOL, never any problems though and i ran the piss out of it at trikefest so don't worry about it getting hot, it won't as long as you have it jetted right and use good oil.

[1upfront]

the top of the sleeve is only about 1mm thick HAHAHA heres a pic

and the bridge on one of the intake ports had to be removed because it was so thin it would have caused heat issues heres a pic

but all in all it woked perfect lol, I didn't think it would hold up to my abuse but it does, here's a pic of the 83R performing at TF08.

[toocheaptosmoke]

Wow, that wall is pretty thin. Where did you get the 5.5mm over piston? Was it actually made for the 250r or from something else?

[1upfront]

it was for a fl250 its a cast factory style piston they go on ebay now and then I wanted a 75mm so I could still get a couple bores but time was closing in on trikefest and I just bought what I could get a 75.5mm and the guy who bored the cylinder had to bore it 4 times to get it big enough lol, we also had to bore a head gasket and I had flotek mill my head to match the piston size it runs on pump gas and castor 927 but I had a new crank to throw in it too before trikefest but ran out of time so Ill have to pull it back apart and put it in before the old one goes its had like 4 or 5 pistons ran on the same rod LOL.

[sbjones73]

thanks for all the help guys. i think i will go with the .70 over. that way everything is cleaned up good.

upfront, love the pic. got any more!?

[sbjones73]

i think i am going to use this thread to up date all of my rebuilding and everything. so here is the first part of it.

i redid the forks finally. i went witht he blue front boots, why? i don't know. but man the mud that was all in the tubes was crazy. but they are done. also took the bent up stock bars off and put some lower profile bars on. then i took the head light assembly apart (cut it apart that is) because it was badly cracked and bent and rewelded and put all back together.
next will be the rear, while the engine is being bored.

here is my pic of the front end.