Having adequate thread engagement when installing rod end bearings is critical to safety. For female rod end bearings, a witness hole drilled in the threaded body provides the means for confirming minimum thread engagement has been achieved and, on follow-up inspections, is still present. However, many rod end bearings aren’t manufactured with a witness hole. That’s where you come in, dear homebuilder. You get to finish the job.
What is minimum thread engagement? While a definitive specification from an authoritative source is elusive, it is widely accepted that a rod end must engage its mating part a minimum of 1 to 1.5 times the rod end’s thread diameter. For example, a 1/4-28 rod end must have a witness hole no less than 1/4-inch from its end. A 5/16-24 rod end must have a witness hole no less than 5/16-inch from its end. (As a point of reference, MS21153 female rod end bearings, which are manufactured with a witness hole, have a witness hole 5/16-inch from the threaded end of the bearing whether the thread is 1/4-28 or 5/16-24.)
The witness hole’s diameter need only be large enough to accept a probing piece of safety wire. I drill witness holes to 1/16-inch diameter, but I’ve seen them as large as 3/32-inch. (MS21153 bearings have a 0.070-inch diameter hole.) The hole should pass through both walls of the threaded body.
While visual inspection of a rod end’s engagement is acceptable, I use safety wire. A visual inspection can produce false positives if the hole isn’t inspected straight-on (so the holes on both sides are in alignment and sufficient light is present to pass through) or if the hole has been obstructed by paint or years of dirt or grease. Probing with safety wire provides positive feedback and is far more practical in dark and hard-to-reach areas.
Or you can mark the minimum depth on the male thread with a marker or paint, wide enough to be visible when inserted past the minimum depth.
1.5 times thread diameter is the machinists’ guide for maximum strength. At 1.5 the bolt will break before threads strip (steel to steel).
Great info, thanks!
This is not a technique I’d use for the following reasons:
In the Vans community, it’s common practice to also check the install. Spin the rod all the way to the stop each direction to verify it cannot fall off.