400 exc bored out to 520

Guy told me his 01 400 exc 400 was bored out to 520. Is this possible? Could it have been a big bore kit with a new jug? I don't know anything about these motors. I have a lc4. Thinking about buying it. It has tons more power than my 400 lc4. 3rd gear wheelies without trying. Much more power than mine.

I have 2 friends that have done big bore kits, and both of them sold their bikes within 6 months.


One was a 2010 Yamaha YZF 450, and the other was a 2004 Honda CRF 450. The Honda was impossible to kick start, and the electric starter burned out. The guy with the Yamaha hated it - after spending $2000 on the kit and install.


My opinion (for what it is worth), is if you want a 500 / 520 / 530 buy one that was built that way. You'll have a lot less hassles.


Good luck either way!

I think KTM are one of the strongest motors. I ride snowbikes in the winter and the big thing right now is turbos. KMS, a very good builder of turbo kit for bikes refuses to boost YAMAHA, HONDA, Or Suzuki because their motors are so weak. He says KTM, and Kawi, are the strongest motors you can get. So if you want a big bore I'd say you're pretty safe. I'm also willing to bet that the 450 and 500 Motors are pretty much the same below the cylinder so I believe that a 450 could easily handle 520cc. I'm taking my 500 to a 600 next month and I'm not worried at all that the motor can't handle it. Find out if he did it him self or had it done. I would also do a top end as soon as you get it and check the rod and crank bearings while you're in there. Also make sure he opened up the cooling system. Big displacement means more heat to get rid off.

Allen up in Idaho just built a boosted 450sx and he's running 10-12psi. These are strong motors!

ImageUploadedByThumper Talk1395373212.116015.jpgImageUploadedByThumper Talk1395373339.660012.jpg

Thumper racing makes a kit to put a 520 size piston in a 400 ....it's a custom made piston. But very popular it comes out to 453 cc

I did the kit on my 05 400

Thumper racing makes a kit to put a 520 size piston in a 400 ....it's a custom made piston. But very popular it comes out to 453 cc

I did the kit on my 05 400

That's the guy I'm using to push my 500 to a 600

I bought it today. It's plated. It's fast. It's mine. At least now I have a model bike I can find parts for. My other one was near impossible to find matching plastics for. He hadit since new. Only rode it one one 60mi trip each summer. Took it in to the shop last year to get the top end done and the guy told him he could make it a 520 for him if he wanted to spend a little more $ on the kit. New water pump too. This is more of a trail worthy bike than my 400 RXCE. Gonna miss my speedo and real working turn signals till I acquire that junk again.

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