Silicone in Yonex handles - howto
You can use this method for just about any brand of racket and I prefer doing it this way because it's more accurate and able to be tuned.
1) Work out how much silicone you want by using a test trial of lead on the outside of the handle and spreading it evenly. eg 20grams.
2) Divide that amount (20g) by 4 to get 5grams per squirt of silicon.
3) Drill 4 evenly spaced holes into your handle, 2 on each side of the harpin.
4) Use a paper clip to push some cotton into the top 2 holes, so that the cotton is toward to head of the racket. Use a chopstick to push some cotton into the buttcap opening so it finishes just before the bottom 2 holes - stop pushing when you get just before the hole (by measuring distance on the chopstick).
5) Go ahead and squirt 5grams ofsilicone (or hot glue, which I like because I can measure exact weighting of 5gm) into each hole. The cototn will prevent the silicone from going up to the head or down to the buttcap.
6) Counterbalance the weight you just put into the handle with a little lead on the head and/or throat. You'll have to experiment for each type of racket as to how much you need.
Tip: I found that putting ANY weight in the bottom 2 inches of the handle (ie where you grip it) just serves to makes the racket feel heavier and has little benefit to me, so i ignore that area altogether and start 2-3 inches up the handle.
Tip: One side of the hairpin is narrower than the other, so you may not be able to fit as much silicon in to it, just squirt the excess into the other side instead. Its important to even up the silicon lengthwise, for balance purposes.
Tip: 20grams is quite a lot, you can plan for 10grams instead. ie 4 x 2.5grams squirts.
The result, the racket feels very plush and solid, allows you to handle even the heaviest balls from your opponent.
The downside: overall weight increases especially with the lead on the head. Also, you probably have to increase your string tension, because you've suddently acquired more hitting power and will notice the strings feel looser/springier than normal. Main disadvantage is that its near impossible to reverse this mod, you can drill out some of the material but not all of it.