Hey, I had this question, I am currently running a pentium 4 631. Ive heard this processor can reach 4.0ghz easily, the problem is my motherboard will only give fsb options from 200-300 when using a 200 mhz processor and will often choke at 240 mhz. I've heard from people using my mobo that a BSEL mod is the solution for this. Will I be able to BSEL my pentium 4? and if I can, will it start up at 1066 (without changing the multiplyer).

If your motherboard officially supports 1066fsb, and you know what pins you need to modify to get to 1066 fsb, then it likely would work. I did this on my e4300 cpu. I am unsure if pentium 4 socket 775 cpus use the same bsel configuration, but I'd imagine they would.

Edit: You might want to check out the intel datasheet "Intel® Pentium® 4 Processor 6x1∆ Sequence Datasheet
On 65 nm Process in the 775-land LGA Package and supporting Intel® Extended Memory 64 Technology."
Read especially pages 30, 39, and 42 as they reference the bsel config for the 6X1 series pentium 4 cpus. On page 30, the only listed bsel config is L, H, L for 200mhz (800fsb), this is the same bsel config as the newer core2 cpus. It looks like the only pentium 4 cpus with a 266mhz (1066fsb) were a few pentium 4 extreme edition cpus. It appears these p4 extreme 1066fsb cpus used the same L,L,L bsel config as the newer core2 cpus as well. So it looks to me like bsel signaling didn't change between pentium 4 and core2 775 lines. So it appears that you'd do the same 800->1066 fsb mod I did with my e4300.

thanks for your reply vnf4ultra, I might try doing the mod on my processor, the only problem is I don't know what auto defogger repair paint means in spanish, being that I live in a spanish speaking country I wont be able to get the materials for my mod.

The defogger paint is basically a highly conductive paint that hardens in a way that makes it somewhat difficult to remove. Some people have had success using "circuit writer" pens which are a little bit like ink pens, but instead use conductive ink for drawing circuits. I've also seen a guy use a small piece of metal foil cut into a "U" shape and glued in place so that it shorted out the two pins.

thanks a lot, I did some research and found that the mod also works with copper tape and aluminium tape, so I'll get some of that. I will try it on a e4500 im getting to replace this pentium 4. No use in modding this If the performance gains will be marginal.