ATC 250R, 2.00 overbore

[jwisniew]

Hi, im restoring a atc250r bored 2.00 over. I was wondering if there were any precautions i should take with a bore this large? Where can i get a complete gasket kit?
Thanks

[dreamintheR]

id look in denniskirk.com or rockymountain.com eh you don mean 00.02 so eh you got yourself a pretty big bore job , if it runs ok with the bore leave it alone , you might want it back to stock though :beer good luck

[jwisniew]

yea its bored to its max. Its 72mm, do you have any idea what the displacement would be?

[dreamintheR]

eh , hard 2 tell , is the sleeve bored out? or the jug itself ?

[Lots_Of_Nothing]

boring doesnt add much cc's at all, i see alot of people that always think this for some reason, such as "my R is bored .40 over so i have a 290R", this is extremely not true, your not gonna gain much at all, except alot of heat, most likely you will get 1-5 cc's, but this does not neccesarily mean you will feel a difference, because now the engine is going to heat up more, and that little bit of power gain will be lost, so it wont feel much different at all.

also, theres a few people on ebay that sell gasket sets, thats where you get the best price.

[deathman53]

what year r are you dealing with, with the 85-89 69.50mm is a about a 272 and I think a 72 would be a about 300.

[TimSr]

2.00mm = .080" Since you said its a 72mm bore I can assume its an 81-84 model.
85-89 are 66mm stock bore, and its not rocket science that you cant bore 6mm off of them!

Displacement = .7854 x bore x bore x stroke

Stroke on 81-84 model is 64.4mm so that comes out to 262cc. Stock bore is 248cc.

Boring is not a performance modification. It is a method of cylinder repair. The only practical way to gain significant displacement is to go with a big bore kit which is basically a custom oversized cylinder, or by changing the stroke which is done by customizing a crank.

[ChrisD]

The 250 cylinder is a 273 on the largest bore. I think there is some confusion as to which size piston he has. If you'te talking 2.00 over, then your piston is 2mm over stock, which is a 68mm piston or a 261cc motor. You can go to www.wiseco.com and look up the piston sizes.

You shouldn't have to worry about anything. If it runs, then great, leave it alone and enjoy the extra displacement. I run a 69mm (the second to last bore size) on one of my bikes and the thing rips. Heat has never been an issue. What I would think about is making that thing breathe more (39PWK, reeds, K&N, porting and a PT pipe) You can also use the stock gasket kits all the way up to 270cc.

Good luck

[TimSr]

ChrisD, He didnt mention the year of his 250R but the 81-84 R has a 70mm stock piston, and Wiseco makes up to a 72mm oversize wghich is why I was assuming thats what he had.


Another note, Wiseco lists a 3.50mm oversize as max for 85-86 ATC/TRX 250R, while the 87-89 TRX250R maxes at 2.00mm like the 81-84.

[ChrisD]

Good point. Sorry about that. If so, then the larger piston will create a lot more heat too. Funny how everything changes when you go to the aircooled models. Then I change my view.....look for a 300kit on Ebay....

[dreamintheR]

bore jobs that big arent that good 4 the motor anyways , its probably puttin alot of stress on the connectin rod :Bounce run it till it blows up , then buy the 310 kit :beer
:TrikesOwn

[ChrisD]

I personally wouldn't worry about the piston size and stress on the connecting rod. I have run the 265 in my 85 R since 1997 (now that I think about it...that's a long time) and my motor is fine. I've changed the piston once, but that's about it. It's always been 'ol reliable.

On my 86, I run a 330 kit with a shaved counterbalancer and that thing hasn't blown either. That motor is completely out of balance and works great.

Just my experiences.

[jwisniew]

The atc is a 82'. Just the sleeve is bored. I've been looking on ebay for gasket kits but most are just top end gasket kits and i need a complete set for the engine because i split the case. I plan on getting it ported and i believe it already has aftermarket reeds.

[threewheelin-feelin]

i get what jared is saying chevys 396 was bored at the factory .30 for some cars. so they called it a 402 my uncle bored a 427 out .60 and called it like a 480.

[poppanitrous]

If you have an 85-86 sleeved it can be 72mm (doesnt have to be an aircooled model), if you have an 85-86 with a "square motor" 72 bore on a 72 stroke( 293.2 .3 liters or 18"(which they call the 295cc kit before last bore which they call a 310cc ) vs 66 x 72 which is stock (246.3 cc stock-0.2 liters or 15").

So in summary 72 bore on a 72 inch stroke is 293.2cc to be exact.
All the big bore and stoker kits claimed sizes are generally rounded up to sound cooler for sale appeal:)
poppanitrous

[dcreel]

If you have an 85-86 sleeved it can be 72mm (doesnt have to be an aircooled model), if you have an 85-86 with a "square motor" 72 bore on a 72 stroke( 293.2 .3 liters or 18"(which they call the 295cc kit before last bore which they call a 310cc ) vs 66 x 72 which is stock (246.3 cc stock-0.2 liters or 15").

So in summary 72 bore on a 72 inch stroke is 293.2cc to be exact.
All the big bore and stoker kits claimed sizes are generally rounded up to sound cooler for sale appeal:)
poppanitrous

Thanks for clearing that up... 7 years later.. :lol::D:lol::lol:

I think there are a few thousand other really old posts that need your summarization... :lol:

Oh cheebus you made my day.. Thanks man.. :D