Thump is correct. The 426 has a connecting rod with a larger piston pin. It tends to hold up better than the earlier 400 rods, which were know to break near the small end without much notice.
You can rebuild the old 400 cranks with a 426 rod if you use a 95mm 426 piston & barrel or a suitable Wiseco piston for the 426. This is a very good mod for increased torque and as a bonus improves reliability. It's well worth the effort
My choice of words was poor and made the whole thing far too confusing. :eek:
Wiseco only makes 92mm & 94mm pistons for the stock 400 rod.
If you go to the 426 rod you have to go to at least a 95 mm piston (either OEM or aftermarket). You can't bore and replate a YZF400 barrel to 95mm (94mm is the limit) so a 426 barrel is also required with the 426 rod.
I haven't found any piston company building pistons for the 426 rod smaller than 95mm.
The 94mm setup works really well in the old 400s but if you are going to run at high rpm you have to make sure you have a fresh rod in the bike because the 94mm piston is much heavier than stock and will stress the rod a lot harder. If you have to rebuild the crank anyway you might as well just buy a 426 rod and barrel and do it right. ;)