The day-to-day work involved in keeping a vintage car on the road sometimes means chasing parts with dimensions or capacities that are hard to determine. Fitting the proper part is essential to the safe, proper and efficient operation of a car. In a numbers-correct restoration it can affect the bottom line when the project is done.
There are times when certain dimensions aren't commonly available or necessary to handle the job. With a power brake booster, the engineers at General Motors give their requirements to the engineers at the parts maker. They would be asking for a certain vacuum pressure, the right mounting points and the right amount of travel for the pedal system they have designed. None of those requirements provide a reason that the auto engineers would have to know the diameter of the part. All they need to know is that it fits into the available space, which is not exactly the same thing. The dealerships and other repair technicians only need the correct part numbers to do their job.
When a particular specification isn't available or wasn't created by the automaker, the only recourse is to check a trustworthy secondary source. Preference should be given to those sources that get their information from both the auto companies and the parts manufacturers. Repair manual publishers and reputable, established parts store chains both make their livings from having the right information.
According to NAPA Auto Parts at napaonline.com, the brake booster offered for the 1972 Chevrolet El Camino and GMC Sprint by original-equipment supplier Bendix-Delco has a specified diameter of either 10 ¼, 10 ¾, or a range of 10 ¼ to 10 ¾ inches. This suggests a nominal diameter of 10 ½ inches as the original specification, but that for maintenance purposes, any diameter within the specified range will perform properly for this application.
In the case of a serious restoration, it can be impossible to locate consumable parts such as brake boosters with the correct numbers. In this case you need to match the shape, appearance and finish of the original part as closely as possible while assuring that the part functions properly. In terms of value, an operating car is generally worth more than a nonfunctioning one with missing parts, so a certain amount of judgment is called for here.
