88 Cp. D'ville with 167,000 miles. I've read many of the posts on the "main Thump", Mine has a thump but it doesn't act the way I would expect a Main Brg. It knocks 3 times on cold startup after sitting over a day. Runs silently until normal temp, then starts the rythmic thud. The thud is 1/2 crankshaft speed, and only occurs if there is the slightest load on the engine. In neutral it ceases. Steering makes it thump, the A/C compressor makes it thump, putting it in gear makes it thump if sitting still; if the car is allowed to move the thump stops as soon as it begins to move. I tested by shorting ignition one cylinderat a time. Shorting #2 stops the thud, nothing else affects it except speed. The thud quiets on acceleration and is inaudible above about 1200 rpm. This is not what I would expect of a main brg. It should rattle at full crankshaft speed. The oil pressure starts at 55+ psi but drops to around 25 psi at 30 mph and idles at about 16 psi.
The rod side clearance in 1-2 is .015", the others .010" or less. Manually sliding the rod(s) side to side makes a noise that sounds exactly like the thud I am hearing. My plan was new rod brgs and oil pump. The clearance is .002" and they look great, the oil pump appears to have excessive end clearance but now I'm finding a new oil pump wont fit because of the pickup. It requires a new pan which is no longer available. I found the new blue main brgs, but I have a slight problem with installing one new bearing for a symptom that doesn't fit. Anyone else been here? Any recomendations?