340 oil pump crank clearance

I am putting togrther my 1970 340.I installed a brand new oil pump and when I turned the crank to check the clearance i noticed a very slight touch that the crankshaft was hitting the oil pump.I could'nt see it but,I could feel it in the wrench I was using to crank the engine with.I removed the oil pump and it went away.I then re-installed the oil pump and pulled it slightly away from the crank and it seems OK.My question is.Why is this tolerence so tight?