[dsraven]

if you have a vernier caliper you could check the od of both parts and compare. sometimes it comes down to how the thread tolerances were machined. as with new fasteners there are some that are made to tighter tolerenances and when new they are hard to spin a nut onto a bolt by hand. others the nut would spin easily. possibly the new part was made to a tighter tolerance to allow for the wear that could occur from a worn out steering box or lack of maintenance over the years. anyway, I would also do the wire wheel trick as suggested. since you have placed the parts next to each other and had the threads mesh together well I would have to say possibly the threads on the new part are cut not quite as deep as the old part was so they are tighter. if the old lock nut spins onto the new part and vice versa look for a slight diameter discrepancy in the OD of the barrel, meaning the new part is slightly larger, or a burr somewhere in the threaded area of the new barrel. you could also try some layout dye on the threads of the barrell and see what rubs off when you try to screw it together in the housing, that will show where the tight spot acrtually is. if the dye is rubbed off in the bottom of each thread then it would indicate the barrel is overall slightly larger. if it is rubbed off consistently on the leading or trailing edge of the thread then possibly the thread was made a bit off.