Not only do you match the rod weights, you also balance them. Here’s a good article written by an experimental airplane enthusiast:
http://www.eaa.org/experimenter/articles/2010-01_howto.asp
@Mustangman They only gave away the $10 trophies when they ran out of the $5 ones.
The same is true today. A good friend of mine races in the “thumper” class at a local dirt track. I don’t know how many thousands of dollars he has spent over the last 25 years or so to get a whole shelf of two footers. I know he spent over $5000 on the engine in his present car. It’s probably not strictly legal internally.
Ya gotta love it. You wouldn’t do it unless you did.
BTW, I really liked @Caddyman 's answer to the original question. What was it again?
Blueprinting can include the setting of clearances to maximize power by way of reduced friction. That could be minimum outside diameters for main and rod journals and maxmum inside diameters for main and rod bearing inserts. Also cylinder diameters can be bored to maximum per drawing (blueprint) and piston diameters set at minimum. Blueprinting means that engine dimensional specifications are optimized for maximum power, yet staying within the maker’s specifications so the engine can be described as “stock”.
With today’s computer controlled assembly line and computer controlled machining of parts and with some extremely accurate measuring instruments that weren’t even available 30 years ago…the tolerance differences are far far closer to a blue-printed engine then engines of the 60’s and 70’s. In fact - in many cases you couldn’t get those close tolerances using the 3+ decade old technology. Pistons weight differences is now down to grams…not ounces. Measurement differences is now measured in Microns instead of Millimeters.