Rod Bearing Replacement

[The Dark Side of Will]

My M20 has an incipient rod knock. The car is an 11/87 325iX. This makes pulling the pan to replace the rod bearings a particular pain in my ass.

Is there a way to determine ahead of time which color code bearings I have?

Or do I have to pull them and look at the dot, then order new bearings?

If I can get the bearings ahead of time, it may be a three day weekend's worth of work... otherwise the job will take up two weekends because I'll have to wait for the bearings to be delivered.

[digger]

I though colour for Rods was only for next oversize +0.25 or + 0.50mm unlike mains. This would mean most likely it's standard size unless it was reground unlikely I'd think

[ForcedFirebird]

The colors are for large/small cranks/block/rods at the factory. Aftermarket bearings don't have a color or size, yet every singe rod bearing I have replaced was perfect .001" clearance with the SK aftermarket.

On another note, I have seen some pretty gnarly m20 failures over the years, and I can say that if you have rod knock, it's likely you will have to have the rod reconditioned and not be able to get away just slipping a new bearing in that rod (hopefully not the case for you).

I would just suggest get a set of on size. I have only ever seen one under size crank of all the m20's I've broken (there's at least 10 m20's floating around the shop here now). If you want to go OEM, I use part #'s 11241284850 and 11241284849 upper and lower halves, but they cost as much as a whole set of SK, and I haven't been able to tell the difference in quality.

[The Dark Side of Will]

Ahh... ok. Thanks.

That's why I said "incipient" rod knock. It doesn't bang around at idle yet, but I can hear a slight clatter at the right engine speed/load combinations.

Also, there was a problem with the throttle resulting in it sticking open smidge when the engine was shut off. The engine would then flare and clatter for a split second on cold start. I was a lazy asshole and didn't fix it soon enough.
Also, it has 200k on it and, if the condition of the rest of the car is an indicator, previous owners did NOT treat it nicely.

I'm not wedded to OE by any means. I don't need to pay amortized overhead for a part to sit in BMW's warehouse for 30 years.

[ForcedFirebird]

Have you checked your oil pressure? That was the giveaway on the few I've seen develop a real rod knock. One time was a 14hr race at Daytona. We lost oil pressure right at the beginning of the race. Figured it was the gauge since the spray bar in the head was moving oil. Well, about 3 hours in the car started losing power rapidly (I was actually driving), then the knock set in. The gauge was perfectly accurate when we replace the motor. DOH! The car went 3 hours in wheel to wheel racing conditions, at Daytona no less...

Reason I ask is because there's a few other harmonics that sound like a light rod knock in an m20, typically in the top end. I've put these poor motors through the ringer, and have bought cars with double/triple that mileage. Usually they will start to burn oil well before the rotating assembly goes out. The rings get gunked up and stick in the pistons letting blow by, increasing the gunk, causing a runaway issue. How black is the inside of the head when you pull the oil cap?

[The Dark Side of Will]

I've been driving it for a while and am familiar with the noises it normally makes. This is new. It also gets worse as the engine warms up, which is another rod knock telltale. I also don't have a racetrack exhaust to interfere with diagnosing a subtle noise.

I haven't checked pressure with a gauge. The characteristic of oil pressure over a range of temperatures and RPM's goes more toward diagnosis of rod bearing wear than a single gauge reading. Yes, I have seen other manufacturers' engines develop oil pressure light at heat soaked idle with worn rod bearings.

[zaq123]

. Yes, I have seen other manufacturers' engines develop oil pressure light at heat soaked idle with worn rod bearings.

The problem is that oil pressure light has such a low threshold, it's irrelevant unless something is terribly terribly wrong. I would definitely get the actual pressure value.

[ForcedFirebird]

Correct. Light doesn't trip until about 5psi.

[The Dark Side of Will]

I think the old GM switches were 3 psi...

[mrsleeve]

Keeps the oil idiot light from coming on cold winter temp start ups, and there by keeps the idiot owners from bringing them into the dealer every other week stating low oil pressure is a an issue

[ForcedFirebird]

I think the old GM switches were 3 psi...

Horrible.



Anyways, posting to see if you had time to pull the pan. I now feel your pain as we lost another m20 due to rod failure, and at Daytona again, no less - and - an ix just came in the shop for oil pan reseal, axle rebuilds and a bunch of little nick knacks. So, your post came to mind.


Symptoms on the race motor was strange. We changed the clutch TWICE in the middle of a race in Sept due to driver error (he actually managed to sheer the rivets on both discs from engine braking), and the second time one of the guys used a bolt a little too long on the PP and it pushed the ring gear off the FW stop just a couple of threads. When we got the car home, we noticed a light rattle on start up. 68psi cold OP, 16psi at 200f idling. I sensed a rod clearance issue, but my partner insisted it was the starter. Well, changed the clutch and repaired the ring gear, noise went away and wasn't there when push starting the car prior to pulling the trans, so we thought we were good.


Anyways, two laps in the race and the block decided it needed an inspection hole right behind the alternator bracket. I have in car video and it held 40psi, even with a broken rod!!!


So, apparently the m20 will hold oil pressure even with a rod completely missing off the crank. The last time it happened, we lost oil pressure far before the rod let go. :/

[The Dark Side of Will]

D-oh.

I have not yet. I have not been driving the car. I'm designing/building the mounts for the Chevy small block into AMC Eagle swap that is occupying my dad's garage now. I changed jobs when I came back from the 'Stan and bought a house with no garage (1.2 acres though!), so I can only get things done at my dad's place.

After I do the rod bearings, I'll temporarily fit an E53 oil pan and use it to check my developmental iX 24V swap crossmember.

I did *NOT* expect the car to be off the road this long.