Mechanic trying to take no fault

What the mechanic did wrong was to give you a rough estimate on the cost of repairs before he tore into the engine to know what was needed. No one expects car owners to understand the workings of the expensive pieces of machinery they trust to get them back home, but if you did, or you were a mechanic, you would understand it is very likely they didn't do anything wrong, so start with an open mind. They know another expensive repair bill is hard to swallow, and the more reputable shops will usually try to do something to help ease the burden on their good customers.

First of all, it is true that many engines use camshaft gears with plastic teeth, (nylon, actually), but the pieces you were told that broke off and plugged something didn't all break off at once. Some of the teeth broke off weeks or months ago and you were driving it like that all the while.

Second, 20 pounds of oil pressure is more than you'll find in some engines but for yours it's nothing to be proud of. In fact, that was a clue a long time ago that something was wrong and it gave you a lot of warning before the disaster occurred.

Third, going back to my first paragraph, once the knocking started the damage was done, and as someone who has had that happen three times, there's no way the crankshaft is going to survive. The "journals" are the precisely machined and highly polished surfaces the connecting rod bearings and crankshaft bearings ride on. The moving parts are isolated by the pressurized oil to prevent wear. 40 pounds of oil pressure is normal for your engine. At the mileage you listed the bearings will be worn a little making it easier for the oil to seep out. The result is lower oil pressure, and lower oil pressure results in increased wear. The engine may have indeed run fine with only 20 pounds of pressure but you were on borrowed time. With the pressure that low, each time a cylinder fired it banged the connecting rod into the crankshaft journal. That caused a moment of increased friction and metal tearing against metal. The bearings are smooth metal inserts that are very soft and that hammering action flattens them out even more. That increases the clearance and results in a further drop in oil pressure.

My best guess is the catastrophic event was going to occur even if you didn't have the recent service work done, or to put it another way, if you had the work done 1000 miles sooner, you would have driven it 1000 miles before this happened. It's highly doubtful there would have been anything the mechanic could have done to prevent this by the time he got his hands on it.

$1100.00 is not a nice repair bill but it seems awfully low for what they're telling you is needed. I had students rebuild one of my engines so labor was real cheap, but I paid almost that much just for parts and machine work.