I'd had a slight ticking noise at higher RPM for a while which I thought was top end noise, or a noisy injector, or a slight manifold leak, but I addressed all of these things and the noise persisted.
It then became a distinctive rod bearing 'tap', at which point I stopped driving the car and tore into the bottom end to have a look.
It didn't knock at idle, or when driving gently, or when free revving. Only at moderate RPM with moderate to heavy throttle.
(It is difficult to hear because of the lower tones from the exhaust etc, its much clearer if you listen through a "tinny" speaker like on a phone.
https://www.youtube.com/watch?v=jS0Yy8N_zN8
The bearings all looked very acceptable for 210K miles with one very obvious exception...
I've personally never seen a bearing fail in this way before, and I am curious what might have caused it and how long this has been brewing!
Fortunately there was only light scoring to the crank, which I was able to fix with a light abrasive, and it all measures in spec. Lucky escape.
This is the before picture, it is now a perfect mirror finish again.
Just thought I would post this to document it in case it helps anyone in future. A post like this might have helped me diagnose my issue sooner!
I am absolutely shocked at how much the car completely shrugged this off, and didn't even get the crank scored up too badly.
1991 325i manual Touring, 210,000~ miles.
Run on Helix HX7 10W40 semi synthetic oil for the past 10 years, often not driven with the respect it deserves. 😉
Put back together with King Racing tri-metals and new genuine rod bolts.
Rod bolts are now E12 torx, originals are 10mm 12 point.
Rod cap torque specification is 20nm + 70 degrees, I see the 70 degrees omitted in a lot of places. It is the difference between hand tight and f-tight, its very important...