Engine knocking after high revs

[jego]

Hey guys,

Today on my way back from work it was snowing/sleeting so I found an empty parking lot and thought I'd have some fun. Tried drifting for a bit and then some autoX-ish driving. The revs went high. I wasn't paying attention to how high they went but I don't think it went all the way to redline. I'd say 5 or 6k. But I would hold it there for a few seconds at a time. Anyway, when I'd had enough I continued on my way home. After a minute or so some knocking started. Here are some symptoms:

-knocking frequency increases as rpm increases
-knocking ONLY occurs when the gas is pressed, regardless of rpm
-knocking is a very regular rhythm, perfectly in time with the engine

I don't know anything about knocking, but I am going to read up on it. Just want to post this here to see what you guys have to say because I need to fix this ASAP.

Tomorrow I'll see if it knocks with the engine cold, and I'll get a video of it.

[epmedia]

Listen for the noise first, and figure out where it's coming from.
Hopefully it's just a bad valve eccentric nut

[jego]

I listened for it and it just seemed like it came from the very center of the engine, but I didn't really know what to listen for or where the listen.

I forgot to mention that after the knocking started the car still ran great. Same power and no misfire as far as I could tell.

I'm going to take a shot at adjusting the valves tomorrow. Read over the instructions in the FAQ and it seems simple enough. Just one question, what size socket do I need to rotate the motor?

[e21Terry]

How many miles on the engine? Were you turning left? What's your oil level?

[Tom D]

^ these are important questions, so think about it very carefully.

[jego]

The engine has less than 2000 miles on it since being rebuilt (if it was actually rebuilt when the PO said it was). The knocking occurred while turning left or right, going straight, or idling. Just checked the oil level and it's good. Also I changed it two days ago. The oil pressure light has been coming on for the past week but it comes and goes without much of a pattern. Engine temp always stayed right at 12:00 and I didn't hear any grinding so I didn't think much of it.

[e21Terry]

IF it was rebuilt properly. The question about which way were you turning was meant at the time you were "drifting" at high rev's. Because of the 30 degree tilt to the right of the engine, the oil can climb up the right side in a hard left turn and uncover the oil pump pick up causing a momentary loss of oil pressure which is not a good thing. It's especially a problem if you're oil level is low to begin with.

Given that you don't know what quality of rebuild the engine got, right now I'm going with a spun rod bearing.

[epmedia]

...I'm going to take a shot at adjusting the valves tomorrow. Read over the instructions in the FAQ and it seems simple enough. Just one question, what size socket do I need to rotate the motor?

If you have a manual transmission and several feet of space, just put it in 2nd gear and roll the car by hand, from the tire.

[e21Terry]

Personally, I use a remote start switch & bump it around with the starter.

[Tom D]

i usually push down on the alternator belt and turn the fan.

[sbcrockett]

i usually push down on the alternator belt and turn the fan.

works for me too. I also take the plugs out (good time to check them anyways) and the engine turns quite easily.

[gsumzzz]

crank nut = 30mm

[jego]

Thanks for all the help and suggestions fellas.

I'll report back later this evening or tomorrow.

UPDATE:
I just finished adjusting the valves. They all seemed a bit tight. All the ones I checked before readjusting were tighter than 0.008 (as in, the feeler gauge didn't even fit at all). 0.006 fit pretty easily so I'm guessing they were set to 0.007 or 0.006. The knocking is not totally gone, but definitely quieter. I'm going to reset the timing and mix. Probably a good idea anyway since I've replaced and cleaned several things since I last set the timing.
If that doesn't get rid of the knock can I assume it's a rod bearing?
Also, how serious is a spun rod bearing? Can I continue to drive it daily (about 35 miles on the freeway one way).

[e21Terry]

Without knowing how bad it's knocking it's hard to tell. I would say driving it with a bad rod bearing would be like playing russian roulette. You might get that loaded chamber first time, maybe not, but eventually you will. You might also be doing further damage to the crankshaft. I wouldn't drive it unless I were prepared to accept the consequenses. If you absolutely must drive it, keep the rev's as low as possible and start looking for a replacement engine. You'll piss off everyone behind you on the freeway, but you don't have to run the posted speed limit. You might want to add a towing rider to your insurance as well and add a towing service to your cell phone contact list.

[gsumzzz]

i am gonna pass on this drifting stuff. good luck with the fix.

[jego]

Ok you scared me. I'm gearing up to replace the bearings. After some research it looks like all I need to do is drop the oil pan, open the rods, and replace the bearings. If I do it in the next few days and I drive the car carefully, could I get away with just replacing the bearings or should I also have the crankshaft machined down? (I don't know what the proper terminology is but my understanding is that when the bearings go out, the crankshaft often has to be resized to make it smooth again).

Is there any way of knowing what size bearings to get or do I just have to open it up and measure the crankshaft?

One last thing, is the torque on the rod bolts really important or can it just be "good n tight"? If it is important, what's the spec? I looked in the manual but I couldn't make sense of the torque table.

Sorry about all the questions, but I've never done this before and it seems like something that needs to be done right.

[e21Terry]

If I do it in the next few days and I drive the car carefully, could I get away with just replacing the bearings or should I also have the crankshaft machined down? (I don't know what the proper terminology is but my understanding is that when the bearings go out, the crankshaft often has to be resized to make it smooth again).

There is no way to know if the crankshaft requires a regrind or not without measuring the journals (the part of the crank the bearings ride on), and a visual inspection of the journal surfaces for scoring, scuffing, etc. Additionally you don't know if the crank has already been previously ground.

Is there any way of knowing what size bearings to get or do I just have to open it up and measure the crankshaft?

See above. You have to open it up.

One last thing, is the torque on the rod bolts really important or can it just be "good n tight"? If it is important, what's the spec? I looked in the manual but I couldn't make sense of the torque table.

Yes, the torque on the rod (and main and head) bolts is CRITICAL. The torque specification for the rod bolts is 38 to 41 lb-ft. If you don't have a good torque wrench, buy one. What you're doing is stretching the bolt to near it's elastic limit, which develops maximun clamping force and maximum resistance to fatigue failure. Too tight or too loose will end in disaster. Actually, measuring bolt stretch is more accurate than torque, but torque will do fine for you.

That all being said, the good news is that BMW crankshafts have surface hardened bearing journals and will take a fair amount of abuse without damage. If upon teardown you find that the rod journals are free of damage, and you verify if the crank has been reground undersize or not (which can be done with a relatively inexpensive set of digital calipers) you can then purchase the proper bearings. When you install the bearings, you can check the clearance between the bearing and journal with a product called plastigauge available at any auto parts store. You place the plastigauge across the bearing in the rod cap, install the rod cap and torque it, then remove it. This flattens the plastigauge, and the plastigauge package has lines of different widths printed on it that you can compare the flattened plastigauge to and determine what the clearance between the bearing and journal is. If the clearance is within tolerance, then reinstall and torque the rod cap. Oh yeah, do remove the plastigauge first. If the clearance is too large or small, it's time for a regrind.

This is the minimalist and best way that you can do the job without removing the crank and without a proper micrometer (a precision measuring device capable of measuring to 0.0001 inch).

I'll be more than happy to pm you my phone number and be on call to answer questions and help you through this. Remember, the only dumb questions are the ones you don't ask. You are capable of doing this.

[Tom D]

^ you just kept me from having to type out the answer.

i would be more concerned with the big end of the rod before the journal of the crank, BUT if you're prepared to repair the engine, then STOP DRIVING IT NOW!!!!

[e21Terry]

^ you just kept me from having to type out the answer.

Tom D - is that a thank you or a complaint?

[Tom D]

long hand is my second language, so it was a thank you.

just the fact that you capitalize in all the right places is very impressive.

[e21Terry]

It's damn near unbelievable when you consider the fact that I'm an engineer. btw... I could tell you knew exactly where I was going with the "which way were you turning" question.

[jego]

Ok so I'll be getting a torque wrench and some plastigauge (I love good excuses to buy tools). I read about plastigauge, pretty cool stuff. What's the tolerance on the bearing clearance?

I'll be more than happy to pm you my phone number and be on call to answer questions and help you through this. Remember, the only dumb questions are the ones you don't ask

That would be great!

Thanks a lot for the support guys!

[epmedia]

Did I read my Haynes correctly on this, folks?
Rods = .0009" to .003" @ .003
Mains = .0012" to .0028"

The max allowable clearance seems excess to me

[gsumzzz]

that's why the 20w-50.

[e21Terry]

Old rule of thumb for performance engine building - 0.001in of clearance for each inch of journal diameter. M10 nominal journal diameter is 1.8898, so 0.0019 would be a good target clearance to begin with. Up to a point, bigger clearances equal less frictional loss (shear stress) and reduced chance of metal to metal contact. Also, as previously mentioned, oil viscosity plays a role in how large the clearance can be. The high end of the clearance tolerance does seem big, but I have worked with enough german engineers to know that they just don't pull these numbers out of their buts. On the other hand, God help you if you ever have to convince one they've made a mistake.