Can someone please explain to me how this fella accomplished this feat? A 125 to a 200.. I guess maybe 3rd over (.75) + 125 = 200?..?

 

assuming constant stroke and that the 125 is a "square" engine - meaning stroke and bore are same - the diameter and stroke of the 125 would be:

D' = (125 X 4 / Pi) ^1/3 = 5.4 cm

using 5.4 for the stroke in the 200 cc engine would imply a bore D" of:

D" = [(200 X 4) / (5.4 X Pi)] ^1/2 = 6.9 cm

pretty hard to imagine boring out a 125 to the tune of 1.5 (6.9 - 5.4) cm
 

This guy is just spouting "urban legend"...  based somewhat on fact.
Yamaha intelligently designed their late 1960's 125's to literally be downsized 175's...  some people would call the 175 models (CT, DT175, and IT175) a "factory overbored 125"... I would call the 125... even the YZ125 of the era... a "factory sleeved down 175".
Eventually Yamaha went to a longer stroke/smaller bore 125/175 and then a longer stroke/larger bore on basically the same engine platform as the early AT/CT models.  This resulted in DT180's, RT180's, DT200's, IT200's, WR200's, Yamaha Blaster 200's...  many variations of the same basic AT/CT design...  some air cooled, some water cooled...  but basically the same DNA as the early Enduro 125 and 175.

The WR200 design could be thought of as a "cousin" to the AT1 and YZ125's...  I wouldn't call it a "factory overbored 125" though... 


 
 

Mark - your knowledge of these bikes blows me away, very cool!

I know right..Mark is a library..Lol

DT175 is 66x50mm & a DT125 is 50x50 but WR200 has a 57mm stroke--well a 66.8 bore rather than 66 if that's right, but mainly stroked to 200. Anyway a 125 on 0.75mm over i'm guessing is still a 125 as only 123 or 124 std. Not sure what i'm gettng at but never mind.
www.motorcyclespecs.co.za/model/yamaha/yamaha_wr200r%2091.htm
www.ultimatespecs.com/motorcycles-specs/...a/yamaha-dt-175-2004
www.motorcyclespecs.co.za/model/yamaha/yamaha_dt125%2080.htm