One of the most valuable guitars in my collection, believe it or not, is the '95 Korean Epiphone. It doesn't sound great nor does it command a high price but that was my first electric guitar; and definitely the guitar I've learned most on - both in music and guitar setup/maintenance.
I've personally changed almost everything on this guitar: pickups, bridge/tailpiece, nut, electronics, knobs, switches, jacks. I've slightly re-positioned the bridge PU to line up with the strings more (ill-advised but I scratched that itch). I've even removed the pickguard and lost it. Point is...we've been through a lot. The only things left to swap out for higher quality parts are the frets and the tuners. And believe it or not the frets are still in good shape for an old guitar.
On a side note, I'm not sure what everyone else's experience has been but for me these Korean Epiphones are built surprisingly well. They don't follow the Les Paul recipe to the tee, but in terms of setting it up it has been easy and reliable.
I decided to change the tuners when I saw the SD-90 set going for pretty cheap on eBay. The current tuners are the Kluson style ones and even though I wanted to get locking tuners for it (purely for the convenience when changing strings) I didn't feel the urge to spend more on it for this guitar. So I settled on the SD-90 which should be a drop in replacement for the ones that were on there.
My beef with the current tuners:
1. Aesthetics - hate that colour
2. They feel cheap, the posts rattle around a lot when there is no string tension on them
Other than that, they were really just fine. Tuning stability has always been fine as long as I wrapped the strings on there properly. Tuning accuracy was definitely in the workable range, although on the bass strings it tended to be a little jumpy. I do not know of the specs on these tuners so I don't know what the tuning ratio is.
The Gotoh SD-90 (my version came lightly relic'd) has the following specs:
Aged Nickel Hardware
Tulip tuning buttons
15:1 ratio
Screws and bushings included
Right off the bat, I like the colour of the tuners better than the original set:
L: Stock Epi, R: Gotoh. Bile green vs Puke yellow?
I could also tell that the tolerances were much tighter on the Gotoh set as opposed to the Epi stock ones. The posts do not rattle around as much and turning the gears gave a little resistance and made it feel more in line with the higher quality tuners that I have in my other guitars. Even the shaft with the tuning button on it rattles around less than the stock ones.
Installation
Installation of tuners can be a mixed bag. If you have planned everything properly and are lucky...everything can go smoothly. Generally though, the surprises are not good and they can range from cross threaded screw holes, parts that don't fit, tuner holes needing to be enlarged, chipped finishes...the list goes on.
In this instance, everything lined up perfectly. The tuners were a drop in replacement however I had to actually remove the bushings that were on there. I had not planned to do that but the SD-90's posts were just slightly larger than the Epiphone ones, and they would not fit in the bushings that were on there.
So, all the previous bushings had to be removed (there are many ways to do this, basically find a tool that fits in the tuner hole but is unable to go through the bushing hole, then push it out from the back of the headstock - take precaution in protecting the finish).
Next, install the new bushings. Again, there are many ways to do this. I just grabbed the closest tool I had on hand, which was an F clamp and sinked the sucker in that way:
Once the bushings were installed, it is clear sailing. Drop in those tuners and line em up with the screw holes that were there previously. I used the old screws from the first set (whichever screw that comes out of it's hole gets put back in the same hole) to avoid creating new threads in the hole. This is advisable but you may find changing screws to be unavoidable in certain installations (say for example if you are changing chrome tuners for gold ones, you're not going to want chrome screws on gold hardware..presumably). If it is unavoidable, don't sweat too much about it - if a screw hole becomes threaded out there are ways to fix that.
Once installed, I added a quick drop of light machine oil, turned the gears several revolutions and I called it a day.
Results
I think the tuners look great on there. the colour of the buttons are a much better match for the guitar in general.
As for the performance, I have no issues with it whatsoever. You may notice a slight improvement in tuning accuracy for the bass strings if you're upgrading from stock tuners, but as mentioned earlier I never really had an issue in this respect from the stock ones.
Tone wise...I think you would be very hard pressed to hear a difference. I know there are people out there that believe that it does make a difference and maybe they are right but I would not be expecting a huge difference. That is not to say it's not a warranted upgrade - I am very happy with my new tuners but then again my expectations are in line with my results.
All in all, for the price ($50 for the non-relic version, $70-$75 for the relic) I would definitely recommend the Gotoh SD-90 if you're looking for a drop in replacement for the Kluson style tuners without any functionality upgrade. As always, do your research before buying the part and make sure they will fit your guitar - note that these are Japanese tuners and as such they may not drop in to guitars that use USA hardware with imperial measurements.
I lost the truss rod cover after the latest rounds of setup/mods...