so far the hydraulic lifter oil feed holes I've measured have fallen in the range of 0.080" to 0.090". For instance, the Crane hydraulic roller lifter has two oil feed holes, 0.060" each, which are equivalent to one hole 0.084" diameter.
Since the hole in the lifter bore bushing is enlarged in order to feed the lifter adequately, the subject of oil control to the valve gear must be considered. The bushing shall prevent excessive oil lost in the clearance between the lifter and the lifter bore, but an orifice of 0.080" to 0.090" may not restrict oil flowing to the valve train adequately.
My solution is to restrict oil flowing to the valve train downstream of the hydraulic lifter, using the push rods. A set of push rods can be ordered with an 0.040" or 0.060" restriction in the tips. It may be confusing that I've recommended two different orifice sizes so I think a quick explanation is in order. The issue is not oil supply, the smaller orifice supplies plenty of oil to the valve train. The issue is clogging. Some engine builders believe the smaller orifice is so small that it is too easily clogged by debris, therefore they prefer to use a larger orifice to reduce the chances of clogging. I'm not going to tell you what to do, I'll leave the decision up to you.
A similar way to restrict oil flowing to the valve train with hydraulic lifter applications is to order a set of 5/16" push rods with 0.120" wall thickness. The passage in the midle of such push rods is only 0.072" diameter, making the push rods an 0.072" orifice that is over 8" long. They're nice and stiff too, the way a push rod should be.
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