See also Hydraulic Lifter - Disassembly / Inspection / Assembly in the EVO section of the Sportsterpedia.

There is a small amount of pressure on the hydraulic lifter plunger holding the top retainer clip in place on a dry lifter.
It can be pushed down with your finger to install the clip.
But with a lifter half or more full of oil, it's harder to work the plunger with your fingers as the pressure behind the plunger is greater.
If you install the lifter into a holder, that frees up both hands and gives better leverage on the plunger.
The holder below is fabbed from a steel sheet and a welded bushing to hold the lifter.
A 2×4 block of wood with a 1“ hole drilled into it can also be used to hold the lifter.

Below is a fabbed steel holder.
The block is used to press the plunger down. A spare pushrod or screwdriver can also be used to depress the plunger.

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A doglegged pick works good for removing the retainer clip and removing the internals. A small flat head screwdriver works also.
Push slightly on the plunger to remove pressure on the clip and use the pic to pull the clip away from the groove in the top.

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