Variable Capacitor Fixing Bolts - UK Vintage Radio Repair and Restoration Discussion Forum

E93AFAN · 3rd Jan 2023, 7:59 pm

I recently bought a couple of 370pf single gang miniature variables recovered from equipment in first class condition and of good quality, with no identification markings other than "JAPAN", but neither have mounting bolts, which is a pain as I had hoped to use them for a project.

I have spent a considerable time combing the internet trying to establish what bolts might be suitable, I have also been through all my "might come in handy one day" stash, which has thrown up two suitable (badly mangled) bolts, but goodness knows what they are or where they came from. The threaded holes in the caps are a clearance for M3 bolts which just slide out on pulling, M3.5 seem to be the right size, but after a full turn bite and stop. I thought M3.5's might do the trick as many of the caps I have use this size/thread, which I eventually discovered by trying a standard electrical socket backplate screw in desperation.

I realise its a long shot, but does anyone out there have any idea what other thread types the Japanese might have used during (a wild guess based upon the construction/condition) the 1960-1980 period? I would be very grateful for any suggestions and thank you in advance for your help as always.

emeritus · 3rd Jan 2023, 8:24 pm

I thought that Japan used metric threads, but if intended for the US market, it is possible that unified threads might have been used. The diameters of both 6-32 UNC and 6-40 UNF are almost exactly 3.5 mm. 4BA is 3.6mm diameter.

Is there enough metal to run a M3.5 tap through the holes? M3.5 is not a first choice thread and is seldom found in basic tap sets, but electrical suppliers sell threading tools consisting of a long shaft M3.5 taper tap permanently mounted in a handle. I got mine from Screwfix.

60 oldjohn · 3rd Jan 2023, 8:33 pm

I wonder when JIS standard was introduced ? The heads are usually marked with a single Dot. I think this may be a staring point of your search.

John.

FrankB · 4th Jan 2023, 10:47 am

If you can determine what size SAE (U.S.) screws you need and the length, I can drop a couple in the post to you if you can't find them there. Typically they are 6/32 and 8/32.
Or just retap the holes, as was suggested.

I have run into this problem in reverse here in the U.S., but I generally have a good source of metric screws. If you decide to retap the hole, if there is room, go the next size larger and be sure you don't run the tap into the plates. ( Don't ask)

retailer · 4th Jan 2023, 11:42 am

This shows that in the early days of Japanese manufacturing they were not afraid of copying even the thread size and pitch.

Restoration73 · 4th Jan 2023, 12:45 pm

JIS standard was not metric until the mid-1960s.

It might be worth trying 5BA - I have found these fit some early tape decks.

E93AFAN · 13th Jan 2023, 2:30 pm

Thank you all for your kind support and advice. There wasn't enough "meat" to recut with an M3.5 tap so I have bitten the bullet, drilled and tapped the holes to M4, bit of a dodgy job but managed it without causing any damage.

Thank you for giving me the spur needed to just get in with it again and all my best wishes.

3rd Jan 2023, 7:59 pm	#1
E93AFAN Tetrode Join Date: Mar 2022 Location: Chippenham, Wiltshire, UK. Posts: 71	Variable Capacitor Fixing Bolts I recently bought a couple of 370pf single gang miniature variables recovered from equipment in first class condition and of good quality, with no identification markings other than "JAPAN", but neither have mounting bolts, which is a pain as I had hoped to use them for a project. I have spent a considerable time combing the internet trying to establish what bolts might be suitable, I have also been through all my "might come in handy one day" stash, which has thrown up two suitable (badly mangled) bolts, but goodness knows what they are or where they came from. The threaded holes in the caps are a clearance for M3 bolts which just slide out on pulling, M3.5 seem to be the right size, but after a full turn bite and stop. I thought M3.5's might do the trick as many of the caps I have use this size/thread, which I eventually discovered by trying a standard electrical socket backplate screw in desperation. I realise its a long shot, but does anyone out there have any idea what other thread types the Japanese might have used during (a wild guess based upon the construction/condition) the 1960-1980 period? I would be very grateful for any suggestions and thank you in advance for your help as always.

3rd Jan 2023, 8:24 pm	#2
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,345	Re: Variable Capacitor Fixing Bolts I thought that Japan used metric threads, but if intended for the US market, it is possible that unified threads might have been used. The diameters of both 6-32 UNC and 6-40 UNF are almost exactly 3.5 mm. 4BA is 3.6mm diameter. Is there enough metal to run a M3.5 tap through the holes? M3.5 is not a first choice thread and is seldom found in basic tap sets, but electrical suppliers sell threading tools consisting of a long shaft M3.5 taper tap permanently mounted in a handle. I got mine from Screwfix. Last edited by emeritus; 3rd Jan 2023 at 8:33 pm.

3rd Jan 2023, 8:33 pm	#3
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,988	Re: Variable Capacitor Fixing Bolts I wonder when JIS standard was introduced ? The heads are usually marked with a single Dot. I think this may be a staring point of your search. John. __________________ My favourite text message "I'll be there in five minutes, if not read again"

4th Jan 2023, 10:47 am	#4
FrankB Heptode Join Date: Oct 2005 Location: Olympia, Washington, USA. Posts: 664	Re: Variable Capacitor Fixing Bolts If you can determine what size SAE (U.S.) screws you need and the length, I can drop a couple in the post to you if you can't find them there. Typically they are 6/32 and 8/32. Or just retap the holes, as was suggested. I have run into this problem in reverse here in the U.S., but I generally have a good source of metric screws. If you decide to retap the hole, if there is room, go the next size larger and be sure you don't run the tap into the plates. ( Don't ask)

4th Jan 2023, 11:42 am	#5
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: Variable Capacitor Fixing Bolts This shows that in the early days of Japanese manufacturing they were not afraid of copying even the thread size and pitch.

4th Jan 2023, 12:45 pm	#6
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Variable Capacitor Fixing Bolts JIS standard was not metric until the mid-1960s. It might be worth trying 5BA - I have found these fit some early tape decks.

13th Jan 2023, 2:30 pm	#7
E93AFAN Tetrode Join Date: Mar 2022 Location: Chippenham, Wiltshire, UK. Posts: 71	Re: Variable Capacitor Fixing Bolts Thank you all for your kind support and advice. There wasn't enough "meat" to recut with an M3.5 tap so I have bitten the bullet, drilled and tapped the holes to M4, bit of a dodgy job but managed it without causing any damage. Thank you for giving me the spur needed to just get in with it again and all my best wishes.