Just went and measured a couple of customer K2Fs to compare with your measurements Hans.
From the circular raised register on the drive end flange to the end of the threaded section (when all threads are there as yours are) is typically 7/8ths of an inch - 22mm in metric. The thickness (depth) of the collar on the inboard side of all the 4 Beesa-spec atd units I have here is 5/16ths from the rear flange of the centre section, give or take a gnat's cock as we say - or 8mm.
Your pic seems to show 19mm sticking through from the register - which is 1/8th or 3mm short and more than enough to cause the problem.
As to 'why', it is hard to say. I rather doubt Dave Lindsley would have put bits in the wrong places on the inside, but there must be some problem with the internal fit - or something unimaginably horrible has happened to the armature in the past resulting in a loss of length which couldn't be rectified without replacing the whole armature.
It is just possible the oil seal isn't sitting properly in its recess behind the drive end bearing race, or that the race somehow didn't go fully home. If the latter, you'd notice because the oil seal would be a loose fit. It is also possible that there is shimming on the drive end (behind the bearing's inner race) which would shorten the bit that sticks though to the atd, but I can't think why and 3mm is inconceivable.
It might be worth taking an HT pick-up off and seeing if the brushes are dead-centre on the slipring at the other end though, which they should be. If things aren't aligned correctly there, that would tell you something.
Although your mag might (probably) take a simple sprocket or gear, with fixed camring mags designed for atd use, I think most of us do a trial fit with a known good ATD after assembly to be sure one will go on and stay on. I have a spare timing side crankcase lying about which I use just for this purpose. It's not totally foolproof and the problem of folk finding their own atd won't go on isn't unheard of, to be honest - Beezermac at Priory Mags has a brilliant bit of text on his site about what to worry about and what NOT to worry about with the things, which I quote frequently to people who are concerned. But a desperately worn male or female taper is a problem you can't get round in a hurry.
If you are going to have a look yourself, bear in mind that it could affect your guarantee . . . but, that said, it would only take 15 mins to have a look if you wanted:
*contact breaker off
*pick-ups off
* earth brush (next to the drive end flange on top) off
* SAFETY spark screws out - diagonally opposite, 2 of, at HT pick-up end
* cb end housing off - just a few screws
Then you can pull the armature out. The magnetism won't run out all over the floor . . . so no need for keepers on these ones.
Then, you can look at the inside.
First, check the cb end bearing outer race is flush with the housing it sits in. It should be near as dammit. It can't really have sunk too far in to cause the drive end to be 'short' - but funny things do happen.
Then, look down the body at the drive end. You should see the outer bearing race flush or near-as with the alloy it sits in. If it's well proud, there's something wrong. Behind the race is an insulating washer to prevent return HT current passing through the bearing, and the oil seal. The oil seal should be dead central and not at all loose. If it's skew, there's a prob as it will have stolen valuable space in getting trapped out of line. If it's loose, then the bearing race, or the insulator, or something isn't fitted correctly. Sometimes, the housing will have been messed up in the past if (for example) someone has tried to 'pop' round the race to retain it better, or if it has at some point spun in the housing. But it probably hasn't.
If all is solid but the race is sitting proud, then you need to investigate further, which can be a tricky one depending on how well fixed the bearing race is. Sometimes heat and a bit of pressure on the oil seal will get it out without hassle. Other times it won't - and excess force will destroy the seal for sure. In that case, either a one-off tool - we use the thing shown here -
www.brightsparkmagnetos.com/workshop/BitsAndBobs/index.htm which is quite simple to make if you felt adventurous. Or, a bent instrument with a sharp edge to lever at the race to see if it will start to budge (fiddly though as hard to get in there). Worst case, folk have been known to drill 2 tiny holes from the outside to bear on the race and knock it out with a hardened pin . . . at your own risk! Heat, though, in all cases . . . a good dunking in boiling water can be good, or a heat gun applied carefully. Blow lamps . . . hmm! Last resort and only if you have a new seal available!
There should be nothing behind the race except the insulating paper washer and the seal, and the race should go fully home as stated. The race should have a depth of 9mm if it's an 18mm id bearing (post about 1951) or 8mm if it's a 15mm bearing (early K series only). It's highly unlikely there'll be anything funny about the bearing itself but if there are any shims behind it packing it out, then questions questions . . . Shimming should normally only be done between the slipring outer face and the inner race of the bearing at that end, with possibly some large shims between mag body and cb end housing. Typically on a K2F, there'd be approx 0.5mm next to the slipring. Sometimes there's nothing, sometimes there's a bit more - but the shims come in all thicknesses and it is usually possible to get the end float controlled to a 1/1000th of an inch without trouble.
The worst thing would be if there is a weirdo aspect to the armature itself. The drive end steel shaft should be absolutely solid in its brass end piece, and no signs of damage or repairs for choice. The coil in there will be the one that was fitted before you had it rewound, and ditto the other brass end piece - they live together, and are always best kept together unless there has been a mechanical disaster which has forced a mix 'n match approach.
Mechanical disasters happen when the drive end axle goes sloppy. When the coil has been so messed up it can't be rewound (rare but it happens). When the earth brush track on the drive end has worn such a groove or dimple in the brass it can't be trued without actually going through the brass (that happens too). When a mag has seized due to insulation leakage gluing it solid and things have got busted or bent. In all such instances, mix 'n match may be necessary, or a complete replacement armature in better condition needs sourcing. That is usually possible, but harder with the early 15mm shafts than the later 18s. In all such cases, it makes life harder for the repairer, and the result may not be quite as good as when the thing left the factory. But there are always answer . . . just a Q of figuring out where your 3mm got to.
(If only all problems of length were just 3mm ones - Audible Sigh! - but that would be for the 'ealth and 'apenis part of the forum!)
