Thanks for the link Rob.
Internal diameter of coil screw thread is 0.2". I counted the threads and measured the depth a little more diligently any yes you are all right it's more likely to be 20TPI.
It's probably my inexperience but the hole just seems too small to accept a 1/4 o/d screw, unless in reality they are less than 1/4...... I'll upload a pic, see what you think.
This is going to be long road but it's actually quite enjoyable. I'm only doing it because I had a G3LS field bike in the 70's!
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By applying a very slightly tapered piece of rod from the inside, I judge the hole diameter to be between 4.95 and 5.00 mm.
The nominal screw core diameter of 1/4 BSW is only 0.1860" or 4.72 mm.
BTW most other screws are 2BA. The finer ones are 3BA (and not M4 !).
Albert
setscrew-2.jpg
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http://archives.jampot.dk/technical/Ele ... C_1955.pdf
Quite a few companies are making repro mag parts inc. SR1. Like parts lists and the small BA screws, of which thread size are questioned here. It still has not been confirmed you have the correct SR1 mag. You did say a lot of non original parts have been fitted. Your 1955 Matchless parts list does not list all the lucas parts. So you need to refer to Lucas parts list for AMC.
'55 to '57 SR1 for G3 was part # 42294A according to Lucas, superseded by 42293. The only difference between the two that I can see relates to the cut-out terminal (kill switch) arrangement, having trawled through the part numbers for them line by line. How many other versions there were I don't know.
Nowhere in Lucas' machine-specific or individual instruments parts lists are these screws described, or even properly illustrated as far as I can see. I think they may be included under 'sundry parts', # 490663 where a couple of bits of what blurrily looks like threaded rod (which is all they are basically) are shown. I'd be surprised if those parts changed over the years, but you never know I suppose, stranger things have happened!
All the fittings are indeed just grouped as one Sundry parts lot. The kill button parts are also one lot. An AMC bike SR1 did not have or need a kill button. The bikes had an exhaust valve lifter to kill the engine. The original covers were Bakelite and the cover gasket about 2 mm thick cork. The cork made a good waterproof seal, Did not need screws over tightened to the point, they crack the covers. Some of the repro gaskets are just thin paper. You will need to bend the contact tab out to make a better contact on the coil. Both need to be clean also the grub screw will only really make contact on a wire plug lead
Thanks for all your help guys. I have no reason to believe it's not the original mag. ID plate says its an SR1 MKIIA 9 55 type 42293A which sounds good for a 56 bike.
My measurements agree with Albert, my problem is lack of imperial nuts & bolts to try fit as precious little small stuff came with the bike. It's time to start investing in some imperial tools and fasteners!
OK I got confused with you stating 55 mag and you have 56 bike. The number you believe ok is not the number for 1955 bike so guess you have checked the SR1 number is ok for 1956.
aobp11 wrote:By applying a very slightly tapered piece of rod from the inside, I judge the hole diameter to be between 4.95 and 5.00 mm.
The nominal screw core diameter of 1/4 BSW is only 0.1860" or 4.72 mm.
BTW most other screws are 2BA. The finer ones are 3BA (and not M4 !).
Albert
setscrew-2.jpg
Just resurrecting this thread since I have the same issue with my SR1. Im trying to remove these screws that hold in the coil, they are just breaking up though. Has anyone found replacement screws available anywhere? I am tempted to try and drill them out...
Thanks for that link, Mick, not sure if they will work though. The original screws are a lot smaller at the top where the slotted end is and do not back up out of the hole, they have to be loosened to remove the coil then screwed back in to remove the grub screw.
