Originally Posted by
FBinNY
If the post will insert part way, odds are that it's the right size. However there can be tolerance issues, or most likely with a welded frame some distortion that will call for some limited reaming to correct.
Yes, this is a common occurrence. The top tube is fixed at both ends, and expands when heated. Steel at brazing or welding temperatures is soft, and the expanding top tube can easily distort it. This also can happen at the head tube end, which is one reason why head tubes are reamed and faced before installing the headset. Most builders will run a reamer through the seat tube to fix this issue. The OP's LBS may have one to do the job.