1999 yz400f rebuild

7 replies to this topic
  • OldDudeWithYZ400

Posted September 18, 2015 - 08:49 PM


After some searching on this site and others, I am still not clear on the best method to rebuild my yz400f top end. Sorry if I simply missed it. The bike is still running good but the time is near. I'm looking for better but reliable performance.  So:


Rebuild w a Weisco or JE piston at 12.5 or 13.5? It runs hot already.. Hotter @ 13.5 I assume?


Is the 450 piston in a 400 case a reality? Do I need a new cylinder jug or just a bore to do do this? What is the resulting compression ratio? 


Any recommended cams for slower speed use? Yes I know it was a motocrosser, but I like that (what feels to me to be a ) 1200 to 12000 linear power band. 




  • Goonyz400f

Posted September 19, 2015 - 12:52 AM


Um I don't think its possible to bore the original bore out to 450 as its already quite thin,but since a 426 bore can fit a 400 that means a 440 big bore kit for a 426 should fit too if you wanted to be safe.

  • knuckleduster271

Posted September 19, 2015 - 08:20 AM


Nothing wrong with a Yamaha stock piston- je,wiseco, vertex, cp, all of them are good. Probably time for a new jug as well.
Honestly your cheapest route is probably a cylinder works kit. I got mine with a complete top end gasket set for $380 I think. Comes with a new cylinder, vertex piston in your choice of 12.5:1 or 13.5:1.
When I was calling around the cheapest I could find on getting a cylinder replated was about $225 in itself.

  • grayracer513

Posted September 19, 2015 - 09:15 AM


You can't bore it to 450 because the 450 was a bored AND stroked 400. 


You can, however, either bore and replate the 400 barrel to 95mm, or use a 426 cylinder and end up with a 426.  To do this, you will either need to change to the 426 crank (or install a 426 rod on yours) so as to use the 426 piston, or use a piston from an early ('03-'04) 450 on your existing rod.  

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  • OldDudeWithYZ400

Posted October 02, 2015 - 04:46 PM


grayracer513, Sounds like you know these engines well. Although I have  been a mechanic in a previous  life, this sounds complicated. Do you think I would be better using a kit as suggested by knucklebuster271?  Seems like this should be fairly common hop up. Like hot rodding old small block chevy's. Yes I am that old. ;) But after some thought, the 95mm bore and 03-04 450 piston is appealing. What are the drawbacks to this approach?

Edited by OldDudeWithYZ400, October 02, 2015 - 04:59 PM.

  • Goonyz400f

Posted October 03, 2015 - 01:00 PM


I think the only downfall would be the bottom end would get worn faster as there's more stress on it

  • bk29

Posted October 04, 2015 - 03:53 PM


I had a 99 back in the day and I loved it. I would rebuild with stock parts. One mod I would defiantly recommend is a fly wheel weight. Helps with the flame out issues these bikes are known for. It doesn't make sense to pour a bunch of money into this bike. Any hop ups will shorten the life of the big end bearing. It's a week spot Yamaha addressed when they upped the displacement to 426. Improvements were coming quickly during that era. Rather than spending a bunch on that bike I would look for a nice 05 or newer Honda or yzf.

  • Tbahr

Posted October 05, 2015 - 05:08 AM


Old dude, I just finished a cam swap on a 1999 yz 400 f for an old friend. I used the hot cams exhaust cam with the built in compression release. Also a cometic thicker gasket set to lower the compression. Changed this bike from a hot running, hard to start nightmare to a one kick from the seat, cold or hot sweet trail bike. Make sure your hot start valve is working. I would also recommend the flywheel weight as bk29 suggests. We just got done riding the Red Bird Crest Trail in the Daniel Boone national forest, Ky. It worked flawlessly.
Tbahr another old dude!

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