I was wondering what seems to be the preferred aperture size for people that use the Williams TK/5d receiver sight?

I think that you would be better advised to get a range of aperture sizes and select the one that best suits "the conditions of the day".
A few variables come into the equation: eye relief, lighting conditions, your eyes, whether or not you use prescription lenses, amongst other things.
Rather than a fixed aperture size, perhaps consider a variable.

Drill out the 0.050” diameter aperture to 1/16” works in most light conditions

What do you guys think about those adjustable apertures?

get a merit adjustable rear aperture. works well.

Merit apertures are great. But they’re out of production.

I shot with my Merits yesterday and out of curiosity I slipped a numbered drill bit thru the hole until I found the aperture size I shot with yesterday. It was a a #57 drill bit which is 0.043”. We shot in full sun conditions. Black targets on white steel bullet traps. Less contrast may be better with a slightly larger diameter.

So, the “target” aperture supplied with a Williams FP is 050” should be great in full sun. A person could buy extras and experiment with numbered drill bits to find what works for their eyes for different conditions.

There are people buying Gehman adjustable apertures that are made for Olympic rifles and using an adapter to screw into the Williams.

I'm using Gehman's on all by Lever Guns. I think the adjustable hole is more a true circle than the Merit. Only thing to be concerned about is because the OD is large they could interfere with the hammer depending on how high the rear sight is for normal shooting. On two of my guns I needed to build a higher mounting post for the Williams receiver sight.
As far as the thread goes you can still buy them with the correct thread that's the same as in the Williams post.
ppkny

ppkny wrote: Mon Sep 05, 2022 1:44 pm I'm using Gehman's on all by Lever Guns. I think the adjustable hole is more a true circle than the Merit. Only thing to be concerned about is because the OD is large they could interfere with the hammer depending on how high the rear sight is for normal shooting. On two of my guns I needed to build a higher mounting post for the Williams receiver sight.
As far as the thread goes you can still buy them with the correct thread that's the same as in the Williams post.
ppkny

PH510 is the part number.

you have to call merit to order apertures now. i got some the first of the year on a phone call. the phone number is 518-763-3011.

I've been using a Gehmann adjustable aperture on a Williams sight, but have noticed a fair amount of "slop" in the vertical direction with the threaded aperture portion of the Williams rear sight. That is, with the Gehmann adapter screwed into place, the entire adapter and threaded receptacle on the Williams are free to move up and down. The movement is more than I'd like to see, and there is no provision to lock the L-R adjustment in place, like there is for the vertical adjustment via the Gibbs screw on a Williams sight.

Has anyone worked out a solution to minimize this vertical play in a Williams "target" style rear sight?

After I had the vertical mount made to raise the Gehman up high enough to clear the hammer I had to build a side bracket to prevent excessive movement due to the only think holding the sight and post was the screw and the slide. The 'L' bracket is slotted so you do have some L/R movement.
ppkny

PPKNY : That's a beautiful solution. My machinist skills are probably not as good as shown in your photo, but I think I'll have a go at making a copy of what you've done there.

Thank you so much,

Steve M.

PPKNY:

Side note - I like that target style knob in place of the gibbs lock screw for the vertical adjustment! Another innovation.

Steve.

Glad I can help. You can get a piece of aluminum angle iron at Home Depot
ppkny

Yes - I manage a business that includes a machine shop. I still like to make my own bits and pieces, even if I'm not as skilled as the real machinists on staff. Thanks again for sharing your work. There's more to that piece than meets the eye.

I assume (we all know what that causes), that the piece you drilled and tapped to accept the Gehmann eyepiece is also drilled and tapped (90 degrees offset from the eyepiece) to accept the Williams 40 TPI windage screw?

Steve.