If you are in the market for a source of 6-flute spline drivers, you want one that is described as 0.096 across the major diameter to work on the R390A. It may be the same size on the R390. The actual size is closer to 0.0955
Where to purchase https://www.mcmaster.com/products/bristol-splines/ | 0.096" | 2" | Short | 1 7/8" | 5/8" | Black Oxide | 7048A18 | 4.48 | | | 0.096" | 3 1/4" | Long | 3 1/8" | 5/8" | Black Oxide | 7048A31 | 6.88 | Purchase a couple of spare unitsThe following is how I work on any of the many R390A fasteners with this style key, set in a regular screwdriver shaft and handle. 1) I used a diamond cut-off wheel in a Dremel tool to cut off a ~3/4 inch length chunk, wear eye protection and a facemask 2) Use a grinding wheel to carefully grind a very slight taper in the first 3/16 inch or so of the hardened key, your only removing a small amount of material. You can chuck the 3/4 inch length in a drill-press, a 0.002 brass shim sleeve is optional when chucking it up. We are making a slight taper over the first 1/10 inch so the key shaft starts in the hole. 3) Purchase a ~4 to 5 inch long shaft screw driver of modest steel quality, with a 1/2 to 3/4 inch diameter handle.4) Cut the screwdriver tip off using a hack-saw. You do not want a fully hardened spring steel shaft, chrome moly steel is ok. 5) Two options, drill a #42, 0.093 centered hole down the center line of the end of the screwdriver shaft, it will help to make a small brass guide fixture for this, a slight taper or off-center is OK meaning a good eye and hand drill will work. The diameter of the hole is about the major diameter of the key, less ~0.002 or so. The depth of the hole should be at least 3/8 inch, but not over 1/2 inch deepordrill #41, 0.096, and then use a hammer and strike to squish the hole so the drill bit will not reinsert, you do not want to mash the shaft just pinch it 0.005 or so, use your drill set as a gage of your technique. What we want is the hardened steel 6-flute spine driver to have a good mechanical interference grip into the softer steel screwdriver shaft. 6) Insert then firmly tap the taper end of the short section of key into the hole, force the key into the softer steel shaft hole of the screwdriver with some firm taps. This will hold a lot of torque if done correctly. 7) Over time and with use the end of the driver will wear, use a 400 grit wheel and reface the end for sharp edges, and debur the end. 8) this worked pretty good 20 year ago and is still working fine Enjoy working on the receiver 6-Flute Spline 6-flute spline drives have six sharply angled grooves for a tight grip. With more points of contact than 4-flute spline drives, they withstand more torque while preventing the driver from slipping and damaging the drive. Also known as Bristol drives, they're often found on set screws in exceptional equipment such as the R390A and R390, R392 etc. Tighten with a 6-flute spline driver John Harvie On Tuesday, June 16, 2026 at 01:28:53 PM EDT, Renee K6FSB via R-390 <[email protected]> wrote: might i suggest next best thing to Bristol spline would be torx which are not too difficult to obtain and much less chance of cam-out than allen. Renée, K6FSB On 6/16/26 10:13 AM, Jacques Fortin via R-390 wrote: > Hi Fred, > IF EVER you found a valid source for the Bristol ones, let us know ! > Allen head ones are more easy to find... > Use the largest head recess size (3/32"). > But resist the temptation to buy the Stainless Steel ones: these are less > "tough" than the black oxide coated steel ones... > And for the square nuts... I already "made" some from standard 1/4" hex > nuts. > File or grind those on three sides. I prefer brass to the steel ones > however... > > 73, Jacques, VE2JFE in Montreal > > Aha! I'm looking for six to ten of the 4-40 x 1/2 inch Bristol socket head > cap screws for the BFO and BW shaft clamps. One square nut (if available) > would be helpful as well. If not, I'm sure there would be Allen head screws > at my local hardware store. > But What didn't work for the ham I got his from was a mix of slotted and > Phillips screws -- the heads wore out quickly. > -Fred > > ______________________________________________________________ > R-390 mailing list > Home: http://mailman.qth.net/mailman/listinfo/r-390 > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
