How tough/reliable is an M20 oil pump?

[E21meister]

Just curious, as the sound that I heard from my engine that was the spit of big-end bearing knock has now gone... until I take the sump off I won't know for sure, but I have a theory that there was an airlock in the pick-up pipe, ergo the knocking noise at the front of the sump I heard.
I'm probably gonna replace the pump anyway, but how tough are the oil pumps on these engines? There surely must be some damage to the cogs in the pump due to running without oil in them?

[Jeroen]

Well being oil starved must have its effect but other than that I can't recall any failing oil pumps on M20's. Airlocks are not that common on these engines afaik.

[E21meister]

I'm ordering a new pump (FEBI/Bilstein) anyway, seeing as the current one is at least 28 years and 131k old and I'm betting has chipped a couple of teeth on the cogs.
Peace of mind for about £80 and while I'm under there I'll probably fit new big-end bearings and re-torque all of the bolts.

[Madhatter]

ive seen them with 400,000 on them and they are pretty well fine. Im wondering how you think you got an airlock in the pickup though?

[E21meister]

Well I don't know for sure as I haven't taken the sump off and checked it, I'm just using deductive reasoning. To give the full situation,there was a knocking noise at the front of the sump, after filling up an cold empty engine with cold oil, in temperatures probably below 10 deg C. After warming the engine up and slowly driving down the road to the garage, the knock was still audible and the oil light was on. Even fully warmed up and idling for 10 minutes in the garage (so the total running time of the engine was about 30minutes) the knock was still audible and sped up under mild revs.
A week later, after the oil's disappeared out the bottom of the sump again, I refilled the engine with the same (fresh) oil, started the car - and the knock wasn't there. I let it warm up, gently revved it, revved the daylights out of it for 10 secs. I then took it out for a test drive for a couple of miles, including accelerating hard away from the lights and from low speed in a high gear. Nothing. No knocks at all. If a big-end bearing had gone due to oil starvation surely it would still be knocking.