How to fix rebound cartridge on Marzocchi forks

Most of the problem people have when drilling the cartridge is not knowing about the piston at the bottom of the cylinder. If the piston moves above the bottom hole, oil can’t get to the rebound valve so your rebound won’t work. Also as the shock is compressed the oil will have to pass through the hole you drilled and a hydro lock will occur on the bottom of the travel.

Extend the cylinder and drill any where below 13mm from the bottom of the cylinder, (I recommend 8mm because the piston is 13.12mm long.).. That will let all the nitrogen out. Push the cylinder down and drill through the piston, using the same hole. (Use a long clamp to hold the cylinder down.) Then you need to tap and put in a set screw, bolt and nut with lock tight or use a roll pin to stop the piston from moving up and down. If you use a roll pin or bolt drill all the way through. Decide what you are going to block the piston with and drill the hole size appropriately.

Next drill a 1/16inch (1.5 mm) or smaller hole 30 mm from the weld on the bottom of the Cartridge. Make sure you extend the cartridge to its maximum length. This way you won’t damage the rebound seal.

Also drill a hole 12mm from the top of the cartridge (no more that 12.5mm no less than 11.5mm) with a 1/16 inch bit, smaller size is better this is to let the air out.
Avoid drilling to deep so you don't nick the shaft, 2.5mm deep will put you in the seal groove but do not drill deeper than 4.75mm you will hit the shaft. If you do it right you should be able to see the seal or the bottom of the seal groove, this allows air to escape from the top of the cylinder.

Now reinstall it, compress the fork all the way then fill the fork with oil till it covers the top of the Cart. Move the shaft of the cartridge up and down to suck in the oil,
fill as needed (170cc in my case) and your read to go. Finish putting you fork back together.
P.S. up the oil weight to compensate for the extra flow, try 10 weight.