I replied to the "elmer's engines" thread a while ago with the note below:

C – close fit

S – smooth finish

P – press fit (or Pray it fits) – sometimes PF also used

SF – slip fit

LF – loose fit

B – braze or silver solder

F – make flat

SWT – seal watertight

SGT – seal gas tight

W – weld

H- harden

H&G – harden and grind

These conventions were and are used on some North American tooling drawings as guides for skilled toolmakers. Many old timers I worked with would see specified tolerances on tool drawings as an insult to their skills in making and fitting a machine to work as expected, and preferred to see the notations above used rather than tolerances.

http://www.john-tom.com has free PDF files of plans for several of Elmer Verburg's engines.

When I used to work for a large USA based controls company in their tool design office we came up with several unofficial variations on the above symbols over the years, as below:

RF – rust fit -as in when a steel shaft is turned too small, leave it outdoors for a week, then the rust coating will allow it to fit right

PH – not just a press fit, Pound Hell out of it.

LAMW – loctite, after machining it wrong

GIRD – get it right dummy (it's important) as applied to an important dimension

WWD – weld without distortion (ie perform distortion control measures) but this was interpreted as "weld when drunk" a few times.

Have fun with the Tiny. Elmer's engines are great.

JD