I tried an M6x1.00 bolt from the hardware store today and found that it threads into my carriage-shifted Royal 10 further than 1/4"-20 but will not get all the way through the threads without forcing, which could damage the typewriter.  This metric bolt may still be workable by stacking washers on the underside of the desk to get just the right depth to engage the threads without needed to turn too far to need forcing.

Converted to metric, the original 1/4"-24 bolt is M6.45x0.94, a 1/4"-20 (course) is M6.45x0.79 and 1/4"-28 (fine) is M6.45x1.1, which makes the metric bolt the closest of commonly available bolts but not a perfect match.  Being undersized in bolt diameter allows the M6x1.00 some room for the thread mismatch.

For my desk mounting, I also tried to use an undersized bolt with a nut on the backside of the threads.  My machine doesn't have enough room for the nut but this may for other machine with a more open design, especially later machines which used stamped sheet-metal instead of cast iron.

Studs or bolts could, in principle, be made from readily available rods with a readily available 1/4"-24 thread-cutting die but the die I found is $50, plus more for the tool to hold it.

I considered drilling the threads out and tapping slightly larger modern threads but this risks cracking the cast iron as it will be thinner after drilling.

I finally decided to buy a 10-pack of Harley Davidson bolts with the correct thread, which ran about $30 shipped.  I need two, PM me if you need a pair, I'll have extras.  Hopefully this does the trick.
 

M. Höhne wrote:

I mounted a Royal Model 10 using M6x1 bolts just fine. I got it from someone who had jammed 1/4 x 20 bolts in there and maybe this forced the threaded holes loose enough that the metric bolts worked OK. It is tight now and solid. i always wondered why a metric thread in a US made machine, especially of that era---well maybe they weren't metric originally.

Did the metric bolts thread all the way through reasonably smoothly?  Were any of the threads missing from the 1/4x20 attempt?
 

Hi PCC

Just did a quick Google search for ¼" - 24 machine screws and found this site. Just wondering if there's anything here that might help. All the best,

Sky

The proper bolt is probably 1/4 x 26tpi, which is a BSF thread. 6mm is very close to 1/4, and the pitch of 26tpi is also very close to metric 1 thread/mm. That said, the depth or finish may be off just a bit, enough to make the bolt snag. British threads are rounded on the crests and in valleys, metric are usually sharp. I used to work in a bike shop that specialized in bicycles from the late 1800s to pre-WWII, a time before the adoption of one of the two remaining prevailing standards. Most bicycles made in the USA at the time when the Royal 10 was being made were using BSF or Whitworth threads.
Phil Forrest

Hi Phil

I was wondering if this machine was old enough to use a Whitworth, Whitworth fine or some other long forgotten thread, but figured that with America wanting to sever all ties with England after the Revolutionary war, they would have got rid of anything that harkened back to British engineering standards. Looks like I stand corrected here. All the best,

Sky