99 YZ400F Bigbore

5 replies to this topic
  • DRZ04

Posted September 27, 2008 - 04:36 PM


I am looking freshen up my little used 1999 YZ400F and would like to increase the bore. What is the easiest way - does Athena make a bigbore kit - can you install a YZ426F barrel & piston on the 400 cases & head???

  • Alternative

Posted September 28, 2008 - 05:45 AM


Yes you can use a 426 barrel but the piston pin is 1mm larger on the 426. I think someone (Wiseco maybe) makes a 426 piston with the smaller pin but if not, you'd need to use a 426 crankshaft as well.

  • grayracer513

Posted September 28, 2008 - 07:20 AM


Check with Luke's Racing for a complete big bore kit.

It can also be done with stock 426 parts, but the 426 crank is not required in its entirety. Have your crank rebuilt using the 426 rod, pin, and rod bearing. Then, you can use the stock 426 cylinder and piston, etc.

Don't expect massive gains by doing this, however.

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  • ericyzf4

Posted October 04, 2008 - 01:35 PM


If you want to go 426 the parts for a 2000 will work, but you will need to spit the cases and use the 426 rod.

check this out


  • DRZ04

Posted October 04, 2008 - 08:30 PM


Thank for the information. Looks like the easy way is using the stock 426 parts.

  • SpAStiCYFZ

Posted October 06, 2008 - 07:02 PM


my bike has a 426 top end. with the stock yz400 rod. it just uses a 3mm
oversized piston from thumper racing/wiseco. it ran real good before i
tore it apart to freshin up the to end. now it wont start. my advice would be to leave it alone. ive had this thing for 2 years and ive been working on it more than ive ridden it.

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