Some excellent points made by WoodallP above. As they point out, placement at the rewind end rollers is not as critical as at the wind end. Within limits, as the tension is applied any slight variation in first curtain length, or second curtain ribbon length will not impact the dimensional relationship between the curtain ends. What is critical is the length and attachment point of the first curtain ribbons to their pulleys either side of the second curtain drum, and the length and placement of the second curtain onto the drum.
To answer your question: "Is it very hard?"
No, I do not see it as very hard. Intermediate. Certainly some mechanical dexterity helps, and some practice. You have more room to work in a Barnack than some other camera designs, but if you are a bumblefingers it won't end well. A sound basic understanding of how a focal plane shutter works is important, I think. Also important is a means of assessing the shutter accuracy, even if it is just a CRT television set (which, if you spend some time practicing with it can tell you a *lot* about the relationship between the curtains, the amount of exposure and the acceleration across the gate). (If you don't have some means of assessing your adjustments to the curtain relationship, I simply cannot see any point even trying to adjust a fp shutter, let alone fit new curtains.)
It's essential that you understand the difference between balancing the curtains for *consistent* exposure across the whole gate, and, adjusting the slits for the *correct* amount of exposure. Balancing the tension on each curtain spring, if you have done an adequate job of servicing the mechanism and have serviceable curtains, should give you an identical amount of exposure across the gate. If it is under or over spec, curtain tension alone will not generally dial it in. The solution is *not* simply to keep boosting tension to shorten times, or reduce it, to lengthen them. (Within a sensible initial range, yes, if your curtains are running too slow they may need to run faster, and vice versa, and, yes, this will impact the gross amount of exposure. But say you've added the standard 1.5 turns of tension to the second curtain but your exposure is far too long. Setting 2 turns of tension is one thing. Setting 4 turns is quite another, and ill advised. You need to start the second curtain earlier, *not* make it run faster, to shorten exposure duration.) But it is pointless calibrating a initial slit that drifts off spec across the gate. Thus—curtains are balanced, to obtain a *uniform* amount of exposure. Timing is calibrated, to obtain the *nominal* amount of exposure by correcting the width of the slit. This is a fundamental principle that unfortunately is much misunderstood by many camera owners. One of the things that, (grudgingly, at first) finally sold me on the Leica shutter is the fact that from the IIIc at least there are actually some dedicated slit adjusters which really do give you a fighting chance of actually getting something like your nominal speeds. Compared to certain other maker's designs it's endearing...
Some well framed reference images of the original curtains prior to removing them are worthwhile. If you have never installed replacement curtains before, they may subsequently help you find the answers to questions you didn't know existed. I cannot overemphasise the value of making some precise measurements of the positions of the curtains, also. Eg where their ends rest relative to the rewind end of the film gate. How much the end of the second curtain overlaps that of the first: in the released (rest) position past the gate; immediately prior to the end of the gate; and at the centre of the gate; is invaluable reference data. From memory you want around 2mm of overlap just before the second curtain caps off the gate. IIIc and later Barnacks have eccentric stud slit adjusters on the curtain latch, so that the precise slit width can be fine tuned for 1/1000 and also 1/500 and slower (these are your two most critical speeds because they are most demanding to get right). In theory if your slits come out marginally too tight you can adjust them wider. I find that more often the slits are a little too wide, and you need the adjustment to close them up. So I try to get just over 2mm overlap. Say 2.5mm The idea is to fall nicely into the stud adjustment range. If you need to set the eccentrics to minimum slit with your brand new curtains, how will you have any scope for future adjustment to maintain shutter accuracy?
As the first curtain is usually fitted first, (unless you will also be fitting new ribbons for that), dialling in the basic relationship between the two curtains (which is critical to obtaining a shutter that is in the adjustable range for accurate times), then, is contingent on second curtain placement on its drum. This will determine the curtain edge overlap visible during the cocking stroke or at the released position, the amount of which informs the basic relationship between the curtains. Get too much overlap and it might be impossible to obtain any exposure at all over the first 1/8 or 1/4 of the gate (in effect, the slit becomes so tight, there is *no slit*, initially. Not enough overlap and you might get an impossibly slow shutter (Eg 1/1000 gives 1/400 and resolutely defies all attempts to shorten exposure) or perhaps even, risk fogging the film during the cocking process if light can sneak between the curtain edges.
So—gently scribing the position of the second curtain end on the drum is never a bad idea at all. Assuming your original curtains have maintained their dimensions after wear and ageing, there is a lot to be said for cutting replacements to the exact same dimensions (particularly the second curtain) and attaching them at exactly the same locations. If your shutter has its factory curtains, and you have replicated them precisely, your shutter performance may not be bang on. But it shouldn't be grossly out, either.
What can be frustrating is when you have gone to pains to do exactly this, but establish that the shutter is fairly out of spec. In my experience (with lots of fp shutters generally, not only, Leica) more often they'll run long, not short. When this happens, you have to rely on your logical analysis, experience/intuition to decide how to rectify this. In the case of a Leica, it might mean peeling your newly glued second curtain off its drum and jinking it along a 1/2 mm or so. If you're not confident, you can try taping the new curtain on, initially to see what you get.
In order to get satisfactory results at 1/1000 in particular (a very tight running slit) it is absolutely critical that your curtains are cut precisely parallel and that your ends are at right angles. Be pernickety about measuring and marking the cloth. Seriously. Measure, mark, and measure again. Spend 5 or 10 minutes with a good steel rule and a square. To the nearest millimetre isn't good enough. You may start with a 2mm slit at 1/1000 (or if you are doing a IIIa with much lazier curtains and 1/1000 setting, maybe a little over half that). If your curtain end varies 1mm in length from top to bottom...you could produce a 1/2 stop inconsistency in exposure across your negatives top to bottom. Even 1/2mm may be around 1/4 stop. So measure the outlines really precisely with a fine pencil nib and use a steel rule and real sharp blade to make your cuts. Corner angles must be right so that when you fit your curtains exactly parallel to the film gate, your curtain ends produce a truly parallel slit which will then result in consistent exposure across the top to bottom of your negatives. If your new curtain is in fact a slight parallelogram shape, even if you fit its end parallel with its curtain drum or roller axle, it may go askew as it runs. Quite apart from any slit irregularities this can produce, the curtain may billow or wrinkle in the process. They must be perfect rectangles, fitted plumb, so that their spring rollers pull them evenly, and keep the fabric taut top to bottom, and end to end.
It is important to mark some crease lines for your curtain ends, and to compress these folds evenly and precisely. A set of pliers with non-serrated jaws can be quite useful for this task. You need to do this so that when the folded ends are slipped over the ribbon bars, you have a curtain end that is really crisp and well delineated, and glued ends which will lie flat against the backs of the curtains.
Like WoodallP I am a believer in applying a small dab of strong adhesive over my ribbon ends if they are to be retained. It's quick and simple to do, so, silly not to.
At the risk of stating the obvious, you must be capable of disassembling your camera without damaging it, reversing this and then making certain adjustments apart from the shutter (the rangefinder must come off, its screws are tight, and then it will at the bare minimum need its vertical calibration reset). More could be written, but that is a start.
