Hello ,I have a few questions,I am planing on rebuilding my old wheeler an I don't know what size I should go to would like to get it close to a 300ex my questions are
1}where to get the kit?
2}what size to go to?
3}what should it cost?piston kit an gaskets
4}what cc motor would I have going 20 over /40 over?
5}can it be bored without sleaving?an how big?
6}or should I just stay at stock size?an just replace the top end ,she runs fine but 12 years of hard riding I think it's time ,she is loosing some power an smoking a little

 

Here are a few answers to your questions:
1) The 250X and 300EX have the same bore but different stroke - 74X57.3(mm) for the 250X and 74X65.5(mm) for the 300 and 300EX.
2) Yes you can bore without sleeving and Wiseco pistons go up to .080" oversized for the 250/300's, but if you plan on keeping the quad for a while, I would recommend not boring anymore than you have to to clean up the cylinder - you have that more top end rebuilds available as time goes by. This time you could probably get by with a .020" overbore.
3) Can't tell you CC's, but for each .020" add .5mm to the bore (STD=74, .020=74.5, etc)
4) As far as what parts I would recommend for a top end rebuild and what I sell them for in my shop are {trying to give you an idea of cost, not trying to sell parts, someone else may know of a more inexpensive supply }: Wiseco piston/rings/wrist pin/ circlips ($85), cam chain ($48), both intake valves ($20ea), and a complete gasket set ($50). The average cost of cylinder boring is $35-45. If the exhaust valves are in sad shape, they are the same as your intake valves, but usually the exhaust valves only need lapping.

 

So,if I go 1st bore .020 over with a wiesco I should be where I need to be, I just want back the power that I have lost over the last 13 years,I can't belive the motor has lasted this long with out ever being rebuilt,I do keep the oil changed an rode the hell out of it/now that I have the 400ex to ride an I can work on the x/I would like to rebuild it to like new for the kids for them for the next 13 years of riding the hell out of it.
where is the best place to get the valves from?

 

I apologize, because I told you the exhaust valves were the same as the intake valves, and they are not , please forgive me.

For some of my OEM type parts, I use Vince's Cycle at www.vincescycle.com, he has competitive prices, sometimes even less expensive on his aftermarket stuff than I can get from my "dealer only" parts suppliers. Most of his parts listed for the 250X are a little less than what I sell them for.

 

stock (74x57.3)= 246.4 cc
.020" over (74.5x57.3) = 249.8 cc
.040" over (75x57.3) = 253.1