'Measure twice, cut once'...That's the old engineering saying meaning always check and double check everything...
I've been putting together a set of parts to build a spare engine for my B33...
Today I picked an oil pump out of my pile of pumps, selecting one that, unusually, turned freely...
A bonus I thought...Perhaps I could get away with a clean up of the exterior and a flush through...
The first problem was when I noticed a hairline crack in the exterior of the pump body...The age deterioration these zinc alloy castings suffer from can sometimes cause these surface cracks..
That bothered me though...even after examining the body closely with an eye glass I couldn't be absolutely sure that it was just a surface defect...
Nothing for it I thought...I'll have to strip the pump just to be certain...
That done I wasn't totally convinced that the worst crack didn't travel to one of the bolt holes, although the line indicating a possible crack wasn't clear...I also found out one reason why the pump was rotating so freely...One of the return gears was missing!....
In the end it was back to the spares box for another pump..
So, 'Measure twice' and be sure....
....Ian
email (option): ian@wright52.plus.com
Wise words of advice that still pass the test of time............
Nowadays, I've learned from my blunders of the past............such as cutting out and drilling 4 headlight brackets and then discovering that two of them were an inch short.......despite me having the measurements to hand......
Must have got distracted somewhere along the way.........
Or, cutting a new brake rod after much measuring, threading it, etc, only to discover that I needed a 1/4" rod, not 3/8ths........ 
Anyone who has never made similar errors hasn't lived ! I blame the better half myself for distracting me...........
I have always found "tighten until thread strips and back off half a turn" most useful, although it has to be admitted that eventually you have to replace with a metric nut and use a pipe on the end of the Tommy bar to whang it on.