My 300 has them and every two stroke I ever remember working on has had them. You don't want to screw up all that time on the grinder by misaligning everything. I think the 300 uses the 250 dowels. If I remember correctly the 300 cylinders were machined out to accept them.
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1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike