Cylindrical square from a lathe tailstock.

[Toolmaker51]

Lifters have good properties to base a cylinder square, using pushrod pocket as the seat. The circle of contact is ideal, less subject to dirt.
Lifter faces carry a large radius to lessen friction and concentrated wear to cam and lifter; radius causes non-sequential rotations in the bore. Some bores re not centered to cam, to achieve rotation.

https://blog.trendperform.com/inside...-tappet-lifers

[DIYSwede]

Thanks Frank and TM51 for clever suggestions!
I've actually thought about engine lifters too, but for this I need a >1/4"-ish through-hole for fastening.

Also, at least one recessed end, with a ring-like surface (like TM51 says) to ease honing.
We'll see what I can come up with in my upcoming scrapyard tours.

Cheers
Johan

[tonyfoale]

For a cylindrical square that is already very nearly +- .0000 in every dimension get your hands on box of new old stock racing solid lifters for a big block chevy Ford Chrysler they wont be the largest but at about .875 to 1" by 2.25" if you are checking something not very large you can't beat one of them an added plus is they are polished to mirror and the hardness will be off the books

I have no idea where I got it from, nor why, nor what it was off but I found a gudeon pin 1" x 2.75" in my "waiting to be used box". It has a > 1/4" hole as Johan requires. It is beyond use as a square because I had used it for testing a tool post grinder.
A look through Summit Racing's catalog will probably show sizes and engine model for various pins.

[Frank S]

As long as you don't need too many zeros to the right of the decimal point or a high Rockwell hardness new flat or dish top pistons can be a good make do a hole can be drilled through them for rigid mounting the diameter to height ratio is in reverse though but as an added plus by using the gudgeon pin holes 4 of them can be used to make a small lathe or a centering device or many other things.

[tonyfoale]

As long as you don't need too many zeros to the right of the decimal point or a high Rockwell hardness new flat or dish top pistons can be a good make do a hole can be drilled through them for rigid mounting the diameter to height ratio is in reverse though but as an added plus by using the gudgeon pin holes 4 of them can be used to make a small lathe or a centering device or many other things.

Pistons are tapered and oval.

[Frank S]

Pistons are tapered and oval.

Not truck pistons especially the ones used in older engines with rings below the gudgeon pins if they have any taper it would be less than 1 thou over the entire 5 inches of length

[Toolmaker51]

As long as you don't need too many zeros to the right of the decimal point or a high Rockwell hardness new flat or dish top pistons can be a good make do a hole can be drilled through them for rigid mounting the diameter to height ratio is in reverse though but as an added plus by using the gudgeon pin holes 4 of them can be used to make a small lathe or a centering device or many other things.

See how clever we are collectively?
Ordinary folks look at pistons and see pencil cups or ashtrays.

[Frank S]

Ordinary folks look at pistons and see pencil cups or ashtrays.

they work well for those also, really large ones make great door stops

[DIYSwede]

Thanks again, Tony, TM51 and Frank for suggestions!
Starting tomorrow, I'll check the shipyard shops out for a suitable Ø 3-4" by 6-10" tall wrist pin,
then check my farming brother-in-law for used tractor piston parts, for smaller dimensions.
That failing, my plan B is to approach just any truck service center - as I'm privileged enuff to live in a small country
that has been cranking out Volvo, Scania, Atlas, BM and other truck, marine and equipment diesels for over a century.
Most of these have wrist pins at Ø 32 or 40 mm with suitable holes.

Cutting, grinding and lapping these small ones will of course provide no hardening of their new ends,
but that's only a minor problem given their far-from-heavy use.

Using pistons for reliable setup and even an indexer can for instance be seen here:
Steve's Tool-Works

When still an over-aged "whiz kid" I stumbled upon that guy's site, as well as "Frugal Machinist",
Welcome to Chipmakers Metalworking World
in the early days if the Interweb, long before I even considered setting up my own diminutive "walk-in closet shop".
2 decades later, neither young nor promising any more, I'm still diving dumpsters, salvaging and re-purposing pure junk.
-Just an itch I needed to scratch, I guess?

[Toolmaker51]

I'd expect wrist/ gudgeon pins being very square by nature [to length as well], with expectation retaining rings or buttons keep them located. Lapping ends won't degrade hardness, even case-hardened.
Just visualized über simple lap fixture. The proper connecting rod, provided crank and pin end are same width, not twisted of course. Certainly square by design. With a hand motor, relieving [grinding] bottom into 3 pads would be easy.