Obtaining Slimline Frame Drawings

You are right . Now is the time to correct any faults .Check down tubes for straightness. They get bowed back by frontal impacts , bottom tube pads  get worn from the contact with centerstand and bent up from kickstarting on stand with  missing bolts. Top rails get twisted in frontal impacts and some were built that way ( very difficult to correct) .silencer brackets and rear suspension mounts crack . Rust can weaken the bottom tubes at the rear, Stand pivot points wear .Front engine mounts can crack . Swinging arm mounts can be worn .Someone will come up with the dimensions but i have heard that some drawings  have incorrect steering head angle.

I think a good start with checking frame alignment is to assemble an empty crankcase and gearbox shell in to the frame.  If all the bolts/studs fit easily then there's a good chance the frame is in good shape.
This tip was given to me many years ago by the local "Motoliner" guy.  This was his first check unless other damage was obvious.
 

I read somewhere when AMC had to make frames they measured a Bracebridge St one and did the drawings ,However the  transfer of measurements was incorrect and so the new frames were a bit different to what went before ! , Wonder if its true?.  Head angle ?.

There were at least 4 versions of the Slimline Frame made and not all by Reynolds. The Mercury version came constructed in Italian steel. You also had a couple of racing variations such as the Lowboy.
Another not well-known fact being that the first designs for the replacement of the 1950 Featherbed, later known as the Slimline, were assembled in 1957. But signed contracts for years of frame production delayed the use of this frame until late 1959.
Finally...........the original Featherbed Frame drawings used Imperial measurements. 

My bike's PO told me nothing was really straight/parallel/square etc. The two lugs that project in to attach the side faces of the back mudguard aren't in line.  They caused a slight zig-zag in the side of one mudguard that eventually caused a crack as it 'breathed' over bumps.  I corrected it by filing a slight angle on the face of the spacer tube.
Robert's points are well made - especially the result of a head-on bump.  It's not unknown to find Dommies where the front mudguard hits the frame under full compression.  That has to be worth checking.
But I suspect that if you try to 'blueprint' your frame according to drawings, you'll have a hard time.  I'm sure it's been written up that the first frames were made in a jig or jigs made by or for McCandless, and the drawings for later models built at Plumstead were done after measuring frames, rather than the other way round.  So fames were made before drawings existed...can anyone confirm this in case it's a myth that becomes true after it's repeated?

Your measuments detect a slight variation in the rear.  Your skills should be able to equalise that alignment to give you the fundamental set up of an inline chassis and drive train.  Your goal should be to get those elements working in unison so she runs straight and true at both extremes of suspension movement.
 

AMC didn't make Featherbed frames. Ken Sprayson stated in his book that all Featherbed type frames, for road bikes & racers, were made at Reynolds. As for the Italian connection, some frames were made in Italy around 1974, but they were Commando ones & are well documented.
Does any one know for sure that Mercury frames were made in Italy?

Frame drawing sent to Mark today.

DSN Pg 60 details a lot you need to know about the Fbed. Yes by all means check dimensions and straightness, but considering the years and mileage these frames might have seen then checking the known areas of wear/bend etc might be more useful.

Mark ,this should'nt bother you, just some unsubstantiated recall. Further to Robert's post, th stated steering angle for road frames was a "transpositional error" after measuring a frame, from "one degree away from th 25 mark" to "one degree after th 25" i.e.26 stated ,rather than th 24 for th Manx. All F-beds are supposedly made at 24. Yours may have been "adjusted" to th transposed 26 by another praiseworthy OCD builder. This should make th bike even more stable. Are you familiar with th F-bed riding performance? That previous engineer may have already corrected th entire frame. This opinion of mine on th s.h. angle is based on an writing in R-H magazine only a few years ago by someone close to th time,at th factory ,investigating warranty claims of dented front guards at their rear by th downtubes.

Lots to check, the headstock angle changed in 1963 from 26 to 24 degrees, according to drawings, so be aware of this one. From the drawings we have it suggests that there many different frames, 1959, 1960, 1963 1965 ( same bar one bracket) 
However, a frame must be damage free but not necessarily 'straight' not so much for the handling but more for safety. Irrespective of the frame condition if the wheels are vertical and in line with each other it will perform as a frame. Now even if this is the case and the frame has been repaired and cracks not identified then the frame would handle correctly but could fail on the next road imperfection - it's straight but not damage free 
A bent frame free of cracks that keeps the wheels vertical and aligned is better than a repaired 'straight' frame carrying hidden faults. In the last 9 years I am aware of 3 Commando frames that have been straightened but had cracks that were only spotted at the coating stage. 

Its strange how Featherbed frames will handle well even if noticably bent. My 99 is definately twisted enough to give concern about chain run and wheel alignment ,all else is in order with good forks and weight distribution ( with panniers permanently fitted I can adjust that ) .The bike handles very well and happily out cornerd sons MT09. He also rode the 99 in the IOM for many weeks and liked the handling. The Atlas is perfectly straight and handles like a loaded truck in comparison to the 99 .. Something to do with the weight distribution ?, A heavier flywheel ,Fatter tyres , the later revised head angle, riding position, ( or all  together !)) .  A certain Mr Degens suggested I reposition the swinging arm to compensate the 99's issue and expressed doubt that the frame twist could be  practically resolved as it was likely  built that way.( he had straightened many a bed to build Tritons).