Sorry guys, YZ250s from 90 to 98 had the same stroke. in the mid 90s, Yamaha experimented with the long rod kit with good results. This became the major upgrade of '99...The Long Rod kit worked well, I built about 6 myself, using the rod from an earlier model YZ & a 5.63mm spacer plate.YZ & YZ (WR) 250s of the 90s had the same motor, with the exception of a wide ratio gearbox...Hope this helps the debate...
In my humble opinion...
I was not clear last night... The basic point I was trying to make is that the 95 YZ got a minor update, and the 96 YZ got a significant update, but the 95 & 96 WRs were the same bike as the 94 WR - by 96, the YZ & WR were quite different bikes, simply due to the lack of updates to the WR (Bold New Graphics don't count...). That
may have allowed a difference to creep in, but it wasn't a designed-in difference between the two - just a by-product of Yamaha's neglect of the WR.
In 1997, the two were brought back into line again.
Trainspotting:
The 95 YZ250 motor was the same basic animal as the 94 YZ & 94 WR. The main change to the 95 YZ motor was the updated barrel, which had a different power valve (added a resonance chamber on the LHS side of the barrel), different exhaust pipe flange, and tweaked porting.
I'm 99% sure that there was also a tweak to the cylinder head too, to try to avoid the 94 motor's tendency to ping.
The 95 & 96 WR motors missed out on those updates.
