Great, I can post now we have past that 4000 barrier!
I was worried that it might be like waiting for Anders when we were on#2999! What a relief - a bit like the dreaded 40th birthday, the next day you realise that in fact you feel just the same! I was aware that #4000 was approaching at about #3750, but then forgot about it. I suppose it's a bit like being on Death Row for twenty years - nineteen years drift past, then one morning you are woken by the warden with "Wakey wakey, sunshine - today's the day! What do you want for breakfast!"
You know, I found it really fascinating when you first mentioned the possible use/reading of the draughtsman's drawing as a 'tool' as a part of the process of producing a technical illustration, it just never occurred to me, it should have, but obviously if you are seeking that most 'fundamental' level of information, can there really be a better source/starting point than the engineering drawing, assuming you can accurately interpret the drawing. Which clearly begs the question, do your illustrations adhere to any particular level of tolerance and fit Tony?
One of the things we were taught at college was reading engineering drawings. You have to be able to do it, and at a fudamental level that is what the job is all about - taking engineering drawings and interpreting them in a way that any non-engineer can understand. Sometimes all you have is ED's, the machine/sub-assembly/part may not have been made, but an illustration is still needed. I drew the Ilmor 265A without ever seeing a complete engine, I saw a lot of parts before I finished the drawing, but to start, all I had was a bunch of ED's. This is common, nearly every illustrator can and has worked this way. My aeromodelling was a big help, as I had been poring over model plans for years, probably from eight years old, puzzling over geodetic glider wing construction and so on.
As to tolerence and fit - unless you mean "Can I tolerate doing this job?" and "Will it fit my drawing board?" - as I have said before, engines have lots of bits all in contact with their neighbouring bits, 'fit' is all-important, 'tolerence' not so, and sometimes you need to use artistic license, although I hate it, I'd rather use a bit of lateral thinking to get round a problem.
Seriously though, I realise that being given access to engineering drawings or very detailed technical information beforehand is probably neither common nor necessarily a prerequisite. It could be argued that an initial lack of information is one of the primary challenges you face as a the technical illustrator and you have given us numerous examples of how, in being denied certain information rather than 'make it up' you have instead applied engineering principles and attempted to work out a particular process or mechanism yourself in order to 'get it right'.
It is fairly common, or was, to be given reasonable information. It depends how the job is commissioned. If a sponsor insists on a car cutaway but the constructor is unhappy about it, and particularly if you are not known to them, it can be a bit tricky. If the constructor commissions it, there is generally more willingness to help. You also come across individuals within organisations that are more or less sympathetic! It's great when you are seen to be struggling with a sub-assembly by a friendly storeman who says "I've got one of those in bits in the store - come with me!" Sorry about the change of tense here and there, I sometimes forget I'm not illustrating anymore!
However, on which work do you feel that you were provided with or were able to amass the most satisfactory levels of preparatory information beforehand? And beyond possibly reducing the lead time did this in your opinion necessarily make for an easier process as well as a better final work?
It is possible to have too much information - you only need enough, more than enough is a burden, or takes up space and wastes time. I probably had more information from Ilmor than anyone else, it was a pleasure in more ways than one, and I always hoped that, having been involved in a very small way from the outset, that I might go on to do all their engines, but it wasn't to be. Got a ton of drawings from Benneton, too, for the windtunnel, plus my photographs, but that was a challenge - I have never used a 15mm Nikkor on so many shots! What influences produce a 'better' final work I'm not sure. This is where the art side creeps in, although I will always call it Technical Illustration. Perhaps it's the phases of the moon...