There are two different sizes of shaft tip diameters for steel iron shafts. One is a .355 taper tip and the other is a .370 parallel tip. The .355 shaft features a tapered tip that gradually gets smaller. Tapered shafts are made in lengths ranging from 36.5 inches to 39.5 inches. The .370 shaft has a parallel tip which stays the same diameter. Parallel tip shafts are all made at the same length which is typically 41 inches.
Golf club companies design the bore diameters in their irons to use either .370 or .355 steel shafts. During the assembly process tapered shafts only need to be trimmed from the butt end to the preferred length, where as parallel shafts must be trimmed from both ends.
It is possible to install a .355 shaft in an iron with a .370 bore by using a shim to insure a secure fit. A .370 tip shaft will not fit in a .355 iron unless you have the hosel drilled out to a .370 bore diameter. Doing so will slightly change the swing weight because you are removing weight from the club head.
If you are not sure what shaft size is in your set of irons, you can look it up by visiting www.golfworks.com and clicking on the OEM Shaft Replacement Guide