How thick is the bass body? You want long ferrules on the back, to reduce the overall string length required, and short ferrules on the top so that you've got a decent thickness of body wood to take the pressure from the rear ferrules. You don't want the two sets of ferrules touching in the middle as the top ferrules don't provide any real structural support.
I'd be wary of the term 'top mounted' in a Chinese description. Here I think 'top mounted' means that it's surface mounting type because of the lip, and not the countersunk, without a lip, type.
Front mounting ferrules only need to be slightly wider than the string, whereas rear-mounting ferrules need to be a lot wider to incorporate the ball end. This makes their diameter significantly larger. The dimensions of those top mount string ferrules make them rear-mounting types in my book.
I've suggested before, and I'll suggest it again: Get some rear mount guitar ferrules and drill out the holes big enough for bass strings. Shorter and smaller all round. My Peavey bass is 1.6" thick. There simply isn't the thickness to get two sets of 1" long ferrules back to back! You could of course cut down the bass ferrules for the top, but that would still leave a very wide hole with no support for the string as it leaves the hole through the wood.
Last edited by Simon Barden; 03-10-2018 at 04:46 PM.
Just got a set of the long ferrules. They definitely look more suitable for the back. I have a set of short bass and a set of guitar ferrules. I sort of like the idea of drilling the guitar ferrules out for use on top. May still use the short bass ferrules on top, because I have no idea how to drill one out.... The guitar ferrule holes would be super small for bass E and lower strings....
I'd just clamp a ferrule in a vice, then stick a drill bit in from the top and drill away. The ferrule is tapered at the bottom so that should keep the drill centered. You'd probably need to clean the hole up with a round needle file and then maybe some rolled up fine grit paper to get rid of sharp edges.
The main internal diameter of my Telecaster rear ferrules are 0.25" (1/4") in diameter, so more than capable of taking a heavy 0.135" low B string on a 5-string with room to spare. Your enlarged through-hole needs to be bigger than the bottom E string diameter, and also allow for the thicker wrap at the ball end, as some of this will probably have to go through the hole. So check on the E string wrap thickness of your bottom E string from your normal manufacturer and have the hole a bit bigger than that. On a Peavey 0.100" bottom E string, I measure the wrap thickness at 0.125". So maybe a 5/32" (0.156") or 4mm drill. I'd drill out all the ferrules with the same sized hole.