I had a senior mechanical instructor to drive it into my head.
Always check and measure the dimensions of seals and bearings.
Compare the new part in the box with the old one that you have removed.
The senior man explained it this way. In a factory environment, machine downtime has to be accounted for. The factory manager is relentless about uptime and efficiency. If you measure first and find a problem, you get a free replacement from the parts room and the parts room is assigned a portion of the machine downtime for the wrong part. If you install without measuring and find a problem, you may get sympathy from the parts room but you (young mechanic) are assigned all the machine downtime.
For the second time you have to disassemble, remove a part and reassemble with new part.
The unit supervisor is unhappy because one of his machines is down and the factory manager is frowning.
The machine operator is delighted because he/she can spend more time in the break room goofing-off.
