The Type 4 P60/P66 complex (as used on the Server 95 machines) is probably the most troublesome complex for upgrading thus far. There are two known candidate upgrade parts to increase the performance of the stock Intel P60/P66. The first is Intel's own P120/P133 Pentium Overdrive part, and the second is the more sophisticated PowerLeap PL54CMMX interposer. There are no known successful transplants using these parts to date.
The (Failed) Solution
The stock P60/P66 Type 4 complex will look like one of the following two:
The type of processor socket on the P60/P66 complex is known as a low-insertion-force "Socket 4". The socket voltage is 5V and has a grid of 273 pins:
Intel's Pentium 120/133 Overdrive part BOXPODP5V133 fits nicely on there:
but doesn't allow the machine to power up to POST. There are reports from fellow MADman Jim Shorney that he occasionally does see the POST start though, so maybe there's some hope.
This next part is PowerLeap's upgrade adapter for socket 4 platforms:
The part name is PL54C-MMX and can be bought with or without a processor. This is quite a sophisticated adapter that theoretically allows one to upgrade a Pentium 66 to a 233 MHz Pentium MMX. Unfortunately, I've had no luck whatsoever with it on any of my complexes. The plan now is to send it to Jim Shorney and let him have a go at it. Maybe a combination of interposers and adapters will yield better results.
Here are some excellent pics of Intel's interposer courtesy of Jim Shorney: