I recently had the 4.0 SOHC engine rebuilt in my 2000 Explorer XLT due to the lose of oil pressure. The vehicle had 118,000 on it when this happenned. Had it towed to the dealer and they wanted $4800 to swap engines with a refurbed engine. After recovering from the initial shock I had it towed to a local shop. I personally know the guys that run this shop. Knowing something broke in the top end because the cams weren't turning (at least the cam on the side with the oil refill wasn't) and checking the oil pump wich seemed to be fine, they recommended dropping the engine and tearing it down to find the root cause. They removed the heads and found a couple bent vavles and the cam on the left side was seized up (due to no oil) caused the bolt that holds the cam drive to shear off. OK, so it needed head work (both heads) and replacement of the timing chain tensioners and a few other things. Still what caused the oil pressure to all of a sudden disappear? Further investigation into the oil pump did not come up with anything. The oil pump was in perfect condition. They went back to their recources and found an intersting but well kept item that Ford had updated about 3 years ago. Even the local Ford dealer had no documentation on this. There is a plug that goes in where (going back) a distributor would go that has a gear assembly that drives the oil pump. It's really a simple unit, a shaft comes up from the oil pump into the bottom of the gear and the engages in another gear. This drives the oil pump. It's basically the bottom of a distrbutor. When they design the original instead of putting a roller bearing of some kind in this assembly they put what is essentially a heavy duty washer. Over time the washer ate it's way up into the housing allowing for about 3/4" of slop. The gear rode up into the housing disengaging from the shaft that drove the oil pump and rest is history. When my guys at the shop ordered a new part they found that Ford had updated them with a roller bearing. Apparently this update never made it out of the factory. My guys saved about $1000 compared to just swapping the engine out. In doing so there were several other updates that got done since the engine was out of the vehicle. I've put over 8,000 on the the engine since the rebuild. It's running like the day I drove it of the dealer lot (bought it brand new). Hope this might help somone with similar problem.