Back in the day BSA by & large used steel tooling that traversed a fixed distance between two stops or till the turret tripped the cam and rotated the tool holder . And most likely plain high carbon steel to boot as tool steel was expensive back then and carbide only justified for defense work .
As such, the tool movement was the same , hopefully , each & every time .However this tooling did not take into account things like heat build up in the job & tool nor tool wear nor tool pick up or even the ambient temperature and particularly in the UK stillages full of metal parts that have been sitting in the open courtyard all night so are at 2 deg when clamped into place.
Thus no two parts were ever identical which is why each & every part was gauged and usually stamped.( what does this funny star mean ? )
Now when assembling the bikes the parts could be blueprinted for want of a better term to match over sized shafts to over sized holes thus hopefully yielding a motorcycle with everything within reasonable levels of fit that today would have the bike sitting in the line at the mechanical rectification station. And I seriously doubt the go-nogo gauges would have been measured to an accuracy of +/- .001" let alone +/-.0001"
Also remember they were built to a price not to a quality standard , one of the contributing factors identified in the decline of the UK motorcycle industry.
And where they were built properly ( eg Some Gold Stars ) then they either costed substantially more or were sold at a reduced margin or even a loss as again more than one historian has pointed out .
While it can be beneficial to repair parts to a tollerance far tighter than BSA did , the burning question is of extra cost vs utility.
Now if I was considering to do the WA to NSW cannon ball run or I am a person such as Mark Parker who's thing is to push the envelope way past what BS A intended then it may be a worthwhile use of my time .
However as most of us do the odd rally & occasional Sunday ride with friends/club members then we have to justify weather cutting seats with a very expensive & time consuming Sunnen machine ( or similar ) that we use so infrequently that it takes a full day to do a 10 minute job against the surface finish from a set of Newey cutters that require very little skil and even less time would be just as good for our purposes ?
One only has a finite time on the planet so one has to choose weather spending a large chunk of it learning a new skill that will get very little use is justified againts being out there riding the bikes .
As for You Tube being populated with shaved monkeys with over active egos & under active brains , well yes I have to concur that the bulk of what is on there is trash which makes life difficult if you don't know any better and were actually trying to get proper instruction.
Mechanics ( in NSW at least ) used to get basic training in machining , heat treatment , metallurgy & surface finishing , for a few years I taught the odd class in some of those subjects and to be quie frank it was a waste of their time, my time & taxpayers money .
They used to get basic foundry practice & blacksmithing too but both had been dropped before I got there in the early 80's .
TAFE courses change to reflect what is the actual shop floor practices & by the 80's it was just measure & replace where needed .
Shane & I found his out when we were trying to find a maching skills course to enrol in.
Not any more it is water jets, plasmas , erosion & CCM
Then there is the question of precision tooling vs the calibrated wrist.
I can usually set mower engines to .004" valve lash +/- .001 by feel as it is a daily task and can tell if they need to be reset by the sound of the engine cranking & running . If you go back & look at the British Pathe' movie inside the BSA factory note the assembly production worker tightening the A 10 con rods. no tension wrench
, pure feel & experience. Note also the book end piston ring compressors & the fact they cranked the pistons up into the cylinders as distinct from trying to drop a heavy cylinder on the pistons squarely as most people do. Which would you call the better praqctice ?
Then there is the difference between doing it for fun and doing it to eat
The old addage of " I can do it good , I can do it cheap , I can do it fast, - but only one at a time " is God's own truth when you are trying to make a living from mechanical repairs
I can remember spending hours polishiing microscope specimens down to 0,5 µ cleaning all the old diamond dust off the cloths, before starting & between samples suiting up in disposable PPE with hair caps in uni lab sessions to take perfectly focuses micrograps without a blemish, then I went into industry where it was 5 minues a hit . one 2 oz tube of paste per 100 samples and specimes like rail crossings , just so long as what we were looking for could be identified &/or measured as these had to be costed and some one had to justify that cost .
The trick as I have learned in being a mechanic ( well tech actually because I was not apprenticed ) is to know what the customer wants
Good, fast or cheap and in most cases it is cheap so cheap dictates that stuff I do not regularly do or can get done cheaper, like rebuild Hydro drives gets farmed out the person who does them all day every day & I get back a ready to instal tranxaxle for less than the wholesale price of the rebuild kit and 1/4 of e price of a replacement.
Before it became illegal to straiten motorcycle frames down here I have had 5 SR 500's bent back into shape
3 by the Aldersons who did the whole thing by eye using long levers and 2 at a workshop using one of the Motoliner with jigs & rams
Dennis did the job same day for something like $ 100 while the motoliner was around 3 times the price & took over a week.
The proof was in the riding & the hand done frames rode a lot straiter than the jigged frame did.
If a workshop down here was to have done what you did to your Ariel then the bill would have been close to $ 50,000