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· Another old dirt bike guy
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Yes the KX85 cylinder can be bored out to the 52.5mm size and then replated. Basically that is what a KX100 cylinder is anyway. In the past many of the top builders preferred to use the KX85 cylinder when doing big bores due to the better exhaust porting on the 85, but the late model KX100 cylinders now have the same exhaust port dimensions so this isn't such a big deal now.
Now to achieve a 103.5cc you will have to install a Hot Rods 2mm longer stroke crankshaft and the 52.5mm piston.
To achieve the 105cc you will need the 52.5mm piston and you will have to have someone stroke a crankshaft for you, as it requires a 48.5mm stroke, and no one is making a pre-made 48.5mm stroke crank for the little KX's. This 105 set-up will require machining of the crankcases for rod clearance, where as the 103.5cc set-up will not require any crankcase mods as the Hot Rods 2mm over crank is a drop in ordeal.
 

· Another old dirt bike guy
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566 Posts
How about the head? Will I need a 100 head?
You will have to have the head re-cut or buy a KX100 head in order to run the 52.5mm piston. If you have a stroker crankshaft installed, you will have to have a head re-cut not only for the bore size, but for piston clearance and correct squish band too.
Actually Eric Gorr does a big bore kit using the Wiseco KX100 pro lite replacement piston, which is readily available just about anywhere. Eric is very reasonable on his prices and he can do the cylinder boring, port the cylinder how ever you want it done, machine the head, have US Chrome replate the cylinder and supply the top end kit, all for around $400 - $450!

It would cost that much just to get a new KX100 cylinder form Kawi, and then you would still have their **** plating! I have US Chrome replate all of my little Kawi cylinders at the first piston change, because the stock plating is complete crap!
 

· Another old dirt bike guy
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566 Posts
I have only heard of making a KX100 into a 105cc, it is basically the same cylinder casting as the KX85. Keep in mind fellas, that when going big bore by that much percentage larger than stock cc's, it normally requires stroking too. You can't make that much of a cc change with boring alone, the cylinders aren't thick enough, and besides that, having a bore that much larger than the stroke would be a crappy running engine anyway. I think they may make an over bore piston for the KX100, if so you might be able to get a little bigger than 105cc but I am not for sure. Another thing to keep in mind is the fact that there is more to making power than just cc's! And a big bore without supporting port work will normally run strong in the low end but fall flat on it's face in the top end. I guess what I am saying is that it is a package deal to do it right.
 

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Contact Eric Gorr at Forward Motion, Eric ships to the UK.

lol do you know how much a phone call will cost me lol, Ill try get a email addy for him unless you have one Griff?? is it worth goin to 310 or sticking to 285. Im only thinking about this at the moment but i have the cash to do it just need to find out if its worth it or sell my 250 and get a thumper.
 

· Another old dirt bike guy
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How about the head? Will I need a 100 head?
I’ve got a 2003 road legal kx85 in the uk and have the kx100 head swop with piston
just with the extra 15% I’ve noticed the bike dose around 12 mph faster but each time I hold back and don’t wanna push so far haha anyways I’ve had 82.4 I didn’t have a speedo with the 85 head on but I’d say it did around 60-68before
but I’ve noticed after the swop my rings have last a lot longer Evan tho I trash it every ware currently 4 years on the same rings since the start and I use it for around 3 hours every couple of days I know they need swooping and it’s a ticking time boom atm but it’s no different at all no sounds starts first kick Evan if it’s been stood for weeks
I’m now been looking around and don’t want to waste somthing that’s already good but next upgrades will be the kx100 swing arm or anything alse that fits the needle from a kx100 carb and maybe a ke100 flywheel for the running gear to get rid of the battery on my bike and a vhm head
My main consern is it’s fast fast enouth you got to just change though 1-2 and 4 and all these other upgrades will only shorten them more but will I get it faster is the question am looking for I’d be happy if I got it to do 90 but can the gearbox handle the heat and all that.
so for people looking the 85-100 head swop is straight forward just gotta to some carb adjustments to get it to run
 
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