Two subjects in the last couple of days have required reference to the Army Standards book that details fits and tolerences etc. at overhaul.
Piston/bore clearances and valve spring length.
However, to my mind the figures given are illogical....
For example...The valve spring compression test states that it should require a poundage of 50-54lbs to compress the spring to a length of 1 15/32". It then states under the heading..'Acceptable at overhaul' a figure of 48lbs is OK.
Ergo...if a pressure of only 47lbs was required the spring would be replaced having been deemed outside of tolerance.(or unacceptable at overhaul)
In the next column under the heading..'Condemnation limit' it then gives a figure of 45lbs.
As the spring went in the scrap bin at 47lbs using the limits defined under the first heading how does that work?
These two sets of limits are quoted throughout the book and consistently show this anomoly..
Am I missing something here?
For reference note that any figures I mention in reply to questions where I use the Standards Book are based on the 'acceptable at overhaul' figures, as 'overhauling them' is what we are doing!...Ian

email (option): ian@wright52.plus.com

I agree, it isn't clear. But one interpretation is that the Army mechanic was under instructions to bin anything not reaching Column 3 standards, otherwise try his best to fettle things in order to reach at least Column 2 standards (maybe that meant putting a shim under the valve spring in order to bring the poundage up from 45 to 48?). Only if he failed would he (be allowed to) requisition a new part.

Or maybe it meant that he rummaged around in the used-but-serviceable spares bin to try and swap his borderline spring for another one which came up to the mark. If so, they must have ended up with stacks of parts falling within column 2 & 3 limits, and I guess they used them up when they had no other option.

Just a thought. Not sure if it helps you in your day job though.

Chris J

That is Ian's day job... Finding all these above 45lb springs Ron

email (option): ronpier@talk21.com

You may well be right in your interpretation Chris...it sound quite plausible. Generally I will go with the column 2 figure, ignoring the 'can we get another 500 miles out of it' line of thinking.
In some cases I ignore the laid down tolerances altogether, when I have concluded from experience that another figure works better.
Nevertheless, most of the standards book is a great reference document and should be on the bookshelf of any M20 owner.
Now where did I put that box of 'only used in three engines' valve springs? ...Ian

email (option): ian@wright52.plus.com