you can't tell by looking at them
"Both hydraulic and mechanical lifter types look the same from the outside, with both having pushrod seats held in by a retaining clip. In a hydraulic lifter the seat moves by means of a hydraulic valve and oil pressure within the lifter. The mechanical lifter does not have a valve and is solid. The pushrod seat in a solid lifter sits upon an internal step inside the lifter body, preventing it from moving. The hydraulic lifter, on the other hand, has a pushrod seat that sits on top of a moveable hydraulic mechanism which acts like a tiny hydraulic pump. Below this mechanism are a valve and spring that produce an upward force, moving the seat up against the pushrod when the lifter is on the base circle."
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