Pictured, above, are 5 "J-Head" hot-ends. The top two hot-ends are listed as being "RPW". These were made by myself and I have them labeled as per my company's name, Reifsnyder Precision Works or RPW. (Reifsnyder Precision Works sells hot-ends via http://www.hotends.com.) The middle hot-end was purchased from MakerFarm, Inc. The bottom two hot-ends were purchased off of E-Bay and originated in China. I have colored bars, on the right, indicating which hot-ends are made to the proper specifications and which were not. I do need to purchase a Mk V hot-end, from a company in the UK, that appears to be made to the proper specifications and review it at some point in the future.
Externally, you'll notice that the top 3 hot-ends all have 5 rows of cooling vents and the Mk IV-B hot-ends are the same length. The Mk V clone has 4 cooling vents, a push-to-fit connector that could be nice for a bowden extruder, an over-sized nozzle, and the vents are not milled to the rounded contour of the core. (This will negatively affect cooling.) The Mk IV clone is shorter than a true Mk IV and has 4 cooling vents.
After I made the first picture, I took each hot-end apart so as to display the internal differences.
The first hot-end, a Mk V-BV, is made to the proper specifications. It has a hollow set-screw, bearing washer (to protect the liner from being distorted by the hollow set-screw), a PTFE liner, PEEK nozzle holder, and brass nozzle.
The second hot-end, a Mk IV-B, is also made to the proper specifications. This hot-end has the hollow set-screw, PTFE liner, PEEK nozzle holder, and brass nozzle. Note that the PTFE liner is longer than that of the 3rd hot-end. This is because this Mk IV-B was made to the original Mk IV-B specifications.
Note that the first two hot-ends have red PTFE liners. Since the Mk II hot-ends, I have made all hot-ends with red PTFE liners. This is not-so-obvious way to tell that you may have a hot-end that came from my shop.
The third hot-end, also a Mk IV-B, is also made to the proper specifications. This hot-end has all of the same components as the previous hot-end. However, the PTFE liner is made out of white PTFE and is shorter. The later Mk IV-B hot-ends had a shorter liner so as to have a proper melt chamber. This hot-end is made to the newer Mk IV-B specifications.
The fourth hot-end is completely missing the PTFE liner. It also is not bored out so as to install a PTFE liner. As a result, it will not work with PLA. I have also noted that any internal nozzle clearances cannot be compensated for that it is quite possible to have an internal gap between the nozzle and PEEK. I have serious doubts that this hot-end would work, for long, with ABS either.
The final hot-end is supposed to be a Mk IV clone. There is no hollow set-scew and no way to retain the liner. This will result in a rapid failure of the hot-end. To compound problems, the nozzle is threaded all the way to the end. This will result in the nozzle not seating properly and has been proven to result in early failure of the hot-end. In addition, it is a little shorter than the Mk IV specifications.
The top 3 hot-ends are made to the proper specifications that have been proven to work successfully. The bottom 2 hot-ends differ so much from the J-Head specification that I really can't think of them as J-Heads hot-ends.
It appears that the fastest way to tell the difference between a quality J-Head and a cheap clone is to count the rows of nozzle vents. If there are 4 rows, short-cuts were taken and it will be of questionable value. If there are 5 rows, the proper care may have been taken to machine the rest of the hot-end. Of course, if you buy it from me, or one of my re-sellers, I will guarantee that it is free from defects and readily replace the rare hot-end that fails due to a manufacturing defect.