Here's a clipping mod I just did on my OD855 build, though it could be used with any number of pedals that use diode clipping to generate signal distortion. The mod uses a little different approach to a switchable clipping configuration that you may not have seen before, though it's by no means original on my part. This set-up takes advantage of the difference in forward voltage threshold (FVT) values between different kinds of diodes. Here, I'm using 1N914 silicon diodes with a FVT of about 0.7V and 1N34A germanium diodes with a FVT a bit over 0.3V.
In a nutshell, what I did was to solder a pair of 1N914's together head-to-tail, trim one of the diode leads off each end of the pair, and then solder the remaining leads into one of the two diode positions on the PCB (doesn't matter which one). Next, I took an on-off-on DPDT toggle switch and soldered a single 1N34A across the opposing lugs at one end, and a pair of 1N34A's oriented in opposite polarities across the opposing lugs at the other end. Lastly, I ran a pair of wires from the two eyelets of the remaining diode position on the PCB to the two middle lugs of the DPDT switch.
So how does this work? When the toggle is in the middle "off" position, the 1N914 pair on the PCB provides the signal clipping. Switching to the single 1N34A position puts this diode in parallel with the 1N914 pair on the board. But because its FVT value is only about half of the 914's, it will activate first and clip one half of the audio sine wave at ~0.3V while the other side of the sine wave is clipped by the 914 of opposing polarity at ~0.7V. So this position gives rise to asymmetric clipping equivalent to what you would get if you had a single 914 and single 1N34A mounted in opposing polarities on the PCB.
Switching to the other side of the DPDT switch with the pair on 1N34As causes them to take over all of the signal clipping, i.e. both sides of the audio sine wave. Again, this is because their lower FVT value allows them to clip before the pair of 914's that are connected in parallel.
The difference in FVT also causes a volume and signal compression difference between the three switch positions. The pair of 914's sound loudest and least compressed, because their higher FVT causes them to clip off less of the signal. The pair of 1N34A's is the softest and more compressed, because the low FVT clips more signal. The asymmetric clipping with one of each diode type falls in between.
Keep in mind that for this particular switchable clipping technique to work, you need to (1) use diodes of significantly different FVT values, and (2) the higher FVT pair should be mounted in the permanently connected position of the PCB, with the lower FVT diodes connected to the switch. If the 1N34A's were on the PCB and the 914's on the switch, the clipping would occur through the 1N34A's no matter which switch position was selected.
If you find the above descriptions confusing, you might find R.G. Keen's excellent article, "A Musical Distortion Primer
", to be helpful in understanding the key concepts of signal clipping.
Below is a "gut shot" photo of the pedal, where you can see the pair of 1N914's mounted in the outer diode position on the PCB, just below its upper right corner. The switch wiring is connected to the inner diode position from the back side of the board, and the switch itself is mounted through the side of the enclosure to the right of the footswitch. Also shown is a diagram of how the 1N34A diodes are connected to the on-off-on DPDT switch. The pair of diodes are at the bottom of the switch in the photo, and the single diode is at the top.