I have been upgrading these to late model piston and belvelle spring for years as the early skinny ear spring, a couple of the ears will snap off and piston will over travel and the piston seal comes out of bore and sticks the forward clutches on. The new style stronger spring has to be used with the new style, thicker piston. the 623 part# piston is .830" thick and the 834 part# piston is .780" thick. If you use a old style piston with new style spring you will have the same problem as if the ears broke off an early style spring. Made that mistake, piston over traveled and seal came out of bore and stuck forward clutches on.