I can't think what prompted me to do it, but I just took a look at the inside of the transfer port using a 10X lens. Shock horror, at first I thought someone must have started to cut an internal thread in it, but a closer look shows that the row of grooves about 0.1 - 0.2 mm deep are not helical, but in series going most of the way down the port (I wish I could photograph this). Another check found that the 3.7 drill, which will not fit into the entry end (see my earlier post) goes about half way through from the exit end.
I know that a polished surface is not considered a must for the tp, but the turbulence as the air passes over these deep grooves must be immense, so much so that it may be the cause of the gun struggling to make full power?
So what should I do now? I don't have the engineering facilities to sleeve the port and as far as I can see the end plug is not available separately as a spare. Should I try opening the port to 3.8, which should get rid of most of the rings, or do I give up and buy a whole new cylinder? I don't like the idea of just leaving it as it is