This here is a can of worms - I know... I am specifically talking about THIS compression testing kit. I bought the kit because it had all the screw in connections, rather than those rubber hold-and-hope jobs that the other kit has. I am testing the compression on my XJ by myself so I had to have the screw in type, and I also don't trust the hold in place types. Needless to say - none of the fittings... uhhh... fit the jeep. I thought that it was initially a SAE/metric issue, but the 4.0L in the XJ uses 14mm spark plugs and I had a 14mm attachment. I read and reread the instructions and it mentions an adaptor for deep well plugs, but it doesn't come with this adaptor. I drive over to Advance Auto to rent a tool - but they don't rent compression testers. Needless to say, they have lost me as a customer. The sales person behind the counter looked at me like I was crazy for wanting to rent one, rather than buy there $89 kit. So I drove to AutoZone. They had a basic kit with the extension for 14mm deep well socket for rent. I used it and it worked like a champ. Super easy to manipulate and use. Tested all cylinders in about 45 minutes. On the last cylinder, I thought that maybe with the extension this Harbor Freight kit would work just fine. I screwed the extension on and tested it - but I got ZERO psi. I kinda freaked because that is bad bad news for compression. I took the Harbor Freight tester off, installed the AutoZone one, and it read just fine. I reinstalled the Harbor Freight one thinking that I must have done something wrong... and it still read zero. So even with the adaptor, the tool failed to work at all. I love Harbor Freight, but I think from now on, I'll stick to the cheap hand tools and free flashlights. I give this product zero stars to match the ZERO psi it wasn't reading. I would be curious if anyone else has had better luck with anything that is more complex than a hammer or a socket.