These Hagon Progressive Fork Springs are designed specifically for the Yamaha Bolt and SCR950 models and they really improve handling and ride quality. The progressive winding means that as the suspension becomes compressed the spring rate increases. These are 15-20% firmer than the OEM units. Changing your fork oil is recommended when replacing these springs, however some people have done the upgrade by carefully sliding out the old ones and installing the new ones.
If you are ordering fork oil, you generally need around 3 bottles (48 oz) to replace your fork oil.