M20B25 had slight rod knock, found a rod bearing fractured in two!
Posted: Wed Apr 29, 2026 10:40 pm
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.
The bearings all looked very acceptable for 210K miles with one very obvious exception...
This guy, cylinder #5.
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.
1991 325i manual, 210,000~ miles.
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.
The bearings all looked very acceptable for 210K miles with one very obvious exception...
This guy, cylinder #5.
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.
1991 325i manual, 210,000~ miles.