Thumper Racing 430 kit

4 replies to this topic
  • Rando400

Posted November 07, 2002 - 08:54 AM


Has anybody tried the 430 big bore kit on their 400? I have been told by some local riders that boring the 400 creates problems and makes the engine unreliable. Any comments on this?


  • SFO

Posted November 07, 2002 - 10:45 AM


the thumper kit utilizes a cast iron sleeve instead of nicasil plating.
I believe you can make a 400 a 420 and a 426 a 444.
There is reduced headgasket sealing area but proper assembly will yield a reliable machine.
My experience has been with the eric gore 444 kit and it uses a replated nicasil bore, BTW.

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  • ThumperWR450F

Posted November 07, 2002 - 02:32 PM


i have a 97 suzuki DR350 that i put the thumper racing big bore kit in and made it a 435. about 2 weeks later it blew the bottom end. i wouldn't recommend getting any big bore kits cause it's gonna cost ya big time. just my 2 cents

  • Rando400

Posted November 08, 2002 - 06:02 AM


Thanks for the info, I think that I will leave it a 400 and be happy it runs every weekend.

  • SFO

Posted November 08, 2002 - 06:34 AM


This is my favorite part about my big bore kit in my 426.
Before and after dyno runs. 444.htm

The extra ten hp on top is really fun for fifth gear wheelies and 15/42 gearing...
I got 50+ dyno runs on the motor and 1400+ baja miles without a single wimper reliability wise.

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