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350SXF/XCF-W to 390

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Price: $775.00

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Thumper Racing 350 to 390 Big Bore Kit

FITS ALL 2011 - 2015 KTM 350 MODELS  




Fill in the soft bottom end power of the 350sxf and 350xcf-w with Thumper Racing's 350 big bore kit. There is no down side to the kit:  stronger bottom and midrange without hurting topend and overrev.  Our 350 to 390 kit gives you the ultimate motor.  Better everywhere is what our pro testers as well as weekend riders tell us.  Better torque off idle, you will now have the power to pull the same gear all the way through a corner.  The 350xcf-w big bore kit does not take away from the ridability of the motor.  Shift less ride more.

Thumper Racings 390 kit includes: Cylinder resleeve with our patent pending aluminum "Cool Flow" designed sleeve which is then nicasil plated and honed to exact tolerances.  Performance piston kit at stock compression, topend gasket kit and uploadable FI program designed to give the best performance possible. We offer CDI reprogramming, ($65 with kit purchase) or we can email the program to you for upload (must have the KTM User Setting Tool in order to upload program to your CDI)  

Don't want to be down for the two weeks it takes to build the kit?  We have exchange cylinders available for a fully refundable core charge of $400.

Please call or email for availability of core cylinders.

Still not sure? Read what the owner of Carson Motorsports said after installing the 350sxf big bore kit on his bike:

"First off, I want to say that I have been very apprehensive about doing ANY mods that could mess up my 350. I really liked the bike alot right out of the box and have been careful about doing one change at a time and testing each one to make sure I like it. That being said, I should have installed this kit months ago! In my opinion, it's the perfect mod for this bike. The bottom and mid range boost is pretty surprising for a 40cc increase. It gives the bike the ability to pull each gear from a much lower RPM and still run all the way through each gear if you choose to. My main concerns with any big bore are vibration and increased engine braking.
On the 350, I also had a concern about having to deal with getting the EFI mapping right. None of those are an issue with Thumper's
390 kit. The 390 does have a bigger power pulse, but I didn't notice any odd vibrations vs. stock. The decel is also not an issue. You can chop the throttle and it acts the same as stock, no crazy decel nose dives that can upset your suspension. The ECU map (installed through the User Setting Tool) that Travis included with the kit was spot on, No flat spots, great throttle response, no decel popping, etc. It even starts quicker than stock with no throttle input. It was fairly warm yesterday and the bike showed no signs of running hot, although I did install a water pump kit in the bike, just in case. The fuel consumption seems to be very close to stock. Before the 390 kit, I wanted to gear the bike down one or two teeth in the rear, to help it pull through the gears better. With the kit, it pulled that same gearing with ease and feels like it could easily pull two less teeth out back, or one more tooth up front.
In my opinion, there are no downsides to this kit."



  • 350 Big Bore Piston Kit


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