Big bore kits?

Hi all. I just jumped off superminis this year and moved up to the big bike. I know there are threads similar to this all over tt, but there wasn't one that i thought suited my question.

Well, i just found a good deal on a lightly used 09 kx250f with 10 hrs on the revuild. Well, its bored 40 over making it a 290. My question is do bbk's change reliability? My dad was always a believer in keeping the stock bore, but now that i've found this bike, and i'm paying for it, i want to know about big bore kits. It also has an aftermarket pipe, if that changes anything.

40 over means .040 inches over the stock bore. There's a big difference between that and a big bore kit. Unless they have proof it's a kit, it's just been blown up bored and replated in the past. Either option shouldn't effect reliability if it was done right, but if it's" lightly used" but .040 over, it may not have been taken care of.

Ha, sorry. What i meant by lightly used is the new rebuild has been lightly used. He said he only used it as a practice bike and uses a 450 for his racing. Wouldn't a racer take care of their stuff? I mean every racer I know always did. So will rebuild frequency or valve problems not be effected by the boring?

anytime I see practice bike, and 250f, I would run the opposite direction.

From experience, I wish I put a 270 kit on my duel injection kx250f rather then cams and what not as it will yield a bigger difference but I wanted to keep it legal. I take core of my bike but i've spent probably 1200$ in motor parts at cost basically to run it for 100 hours (racing and practice). Every practice 250f I've seen is pretty well hammered as they are HIGH hour machines. Yes racers maintain their stuff, but the degree of maint on a race bike is goign to be a lot higher then the practice bike. I'm not saying its a piece of junk but if it was ridden for 5 hours a week 25 hours a year for 3 years the thing has well over 350 hours on it. Thats a lot of time on a 250f regardless of how well it was maintained. Heck at 100 hours my bike feels quite a bit different with a fresh motor rebuild bottom to top including valves and seats then it did when it was new.

If the big bore was done properly, it shouldnt effect the overall longevity of the motor to much. Bear in mind you're adding mass to the piston by the increase in size, therefor adding some additional force on the lower rod bearing which may increase wear some, just as putting a high compression piston in increases the forces on the lower rod bearing. Valve wise it shouldn't make a difference if it still has stock cams as they are opening and closing as they would normally with a stock bore.

All i'm trying to say is is that if you do look at it pay VERY close attention to the minor stuff. Play in linkage, true wheels, taught spokes, suspension. I've seen some high hour dlc coated showa forks that come on these bikes that werent maintained well with fresh fluid at the proper intervals where the DLC on the lower legs is starting to wear right off.

Lastly for what its worth- unless somebody has receipts to prove the work that show when the parts were purchased, and when they were installed (by a dealer) on official dealership type paper you can not take peoples word that it was rebuilt. Notice every bike for sale has a "fresh top end", fresh to me is put together and started not even ridden. Fresh to some means it had a piston the beginning of last season and at the time the valves were about spent but we put it together anyways.

Edited by dmm698

Thanks man. I think i am going to go see it. If i do i will check everything you said. He claims he has receipts from the shop and his maintenance routine, if true, is pretty flawless. I will remember what you said.

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