That is basically what the prop extensions are  - spacers.
The ones I have made I knurled the front surface.
If they get long enough that you start to run out of thread, then you need to make a new nut and front washer.
Longer still, and you need to make them like the Fox ones are where the prop nut extends all the way through the prop and into the spacer.

Pictures "borrowed" from here:
https://www.ebay.com/itm/274055507444

Old (Froom?), VECO, K&B-VECO and Fox shaft extensions were either aluminum or iron 1/2" hex bar stock threaded full length 1/4-28 (prop nut size) then turned down to 5/16" outer diameter for most of their length. The prop and front prop washer, of course, had to fit 5/16". The spacer front was not usually knurled. Simple. NOTE: Good idea to have to spare drilled and balanced props handy in case of breakage.

The largest size on usual 4-way wrenches fits 1/2" (flat-to-flat) nuts that fit 5/16-24 nuts, or similar metric sizes. A spacer, as in the image, can go between the engine's prop drive washer and prop washer or spacer.

Kevin, talk to B. Paterson about making you a proper spacer.

If you use one of these , mean likely drilling the prop oversize . or the front half of the prop oversize .


Theres 1`/2 & 3/4 Extnensions . Fox & Veco !

People with lathes do a intregal spacr / backplate . Simplify & add lightness .

As Yr 1/4 , likely trim the nut shaft shorter . Ive found misalaneous Auto ( junk Yard ) spacers  .
Alternator mounts , seat belt bolt spacers ( fit the 5/16 shaft driver - as noseweight  Fwd of Prop )
G M Stuff tends to have machined thick washers here'n'there . Reasonable quality mmild steel . Often Plated .
The 70s / 80s stuff anyway .
Any small engine outfit or ancient auto parts store might have something . A EVEN number of turns of masking tape or whatever , if needed . Centralises .
But often needs peeling off and re doing , if removed .

You need engineering Standard washers , not stamped sheet ones . Tho a BIG Stamp can do good ones by the million . So dont discount. Get the Vernier  out .
Engineers supplies etc might have a ' too fit ' , or a M'Cycle wrecker or Auto ditto may have buckets of them . Might need hand cleaner here ! .   =======

Save a bit of faffing about . andl keep the C G Back . M;Cycles more liable to have usable alloy ones . 2 minutes to machine up one at your local airport for A / C quality Alloy !!

One Fing . The ST isnt LIGHTER than the thinggo 40 , you could move the .46 BACK ! for balance .

SHORTENING the Prop Nut thing , so it only goes say into the FRONT Half Depth , keeps the props ' multi Fit . So still useable on other engines . Allowing the shank on the ST is 7 m.m. ,

Easiest way ( that ) is often a tenth the hassel ! .

Youd need to drill your 1/4 spacer 7 m.m. bore . FIT ALL THE BITS UP , see if there how much thread protuding , and cut Extn. type prop nut for sufficent engagement , say 7 threads min. With it washer accounted for.

Or Try That , and if its no good , whack right into it . When I have , I knok out the nut plates , Wash with thinner above bearers & holes , tape or fit flat wood held with wedges , pour aryldite into the oldbolt holes
and a skin of it across the dressed / block sanded / or Filed Clean & washed - motor mount face .

If Ive a pice of alloy plate the right width , thatll drop in . Maybe flat it all off with good epoxie , drop the plate in , mask edges to retain glue . then Clamp plate lightly . SO THE NEW FACES ARE ALREADY TRUE.
Or just dress the suckers after the stuffs set or kicked off . Theres some give till its cured . So the Clamped ( release agented ) plate can be a late fit . After kicked off- before fully cured .

I use say 1/16 steel with 1/8 N F nuts wider & longer on a ' re-do ' like 1 1/2 x 5/16 , or 2 x 3/8 if theres room . Carefully , Pilot Drill the new bolt holes . If there not in clean wood . A pilot templates essential
as they tend to wander .

So theres say three stages , remove fittings - Thorougly clean up & degrease - reform / reface mounts - redrill & refit for new set up . Contract seems to have escaletd . That Makes FOUR  . In for a bit of effort.

A trimmed fancy prop nut'd be a tenth the effort .

The Alumn Brodak Spinner Nuts , go through the FRONT WASHER , for max thread engagement . a steel one done like that would a be a the beast , eh .

Thank you Jim and Air Ministry. Great information! 👍