Bush bearing question

[fbenach]

Hello everyone!!

I´m afraid the low pressure issue I have on my bike, is getting to ugly places in the last few days...
After checking the infamous $2000 o-ring, changing the oil pump and a few other changes, we ended up measuring the oil pressure on cold start at 6psi idle and 35psi at 4000rpm...
Short story: we will have to open the engine and search for the damage...

Anyways, we were checking the bush bearings at maxbmw and noticed there are different specs for the same bearing, for example:
0.25 RED, 0.50 RED, 0.25 BLUE, 0.50 BLUE, etc, etc.
Same happens to the bearing shell, there is +.25, +.50, +.75

My question for the gurus is: how can we decide which is the correct one?
Again, although my mechanic probably knows the answer, it´s always good to hear it somewhere else, in order to learn how this whole thing works...
Would it be too dangerous (in case I need it) to buy a crankshaft from ebay??

Thank you all!!
Regards, Fernando

[Barry]

Hi Fernando

Sorry to hear it's turning into a big job.

Which bush bearing are we talking about - sounds like crankshaft bearings  ? If it is they have to be chosen after careful and expert measurement of the crank shaft. I'm no expert but it's a case of choosing the correct running clearance which will be specified somewhere. I'd say something in the range of .001 to .002""  If your mechanic has to ask what the clearance should I'd be worried. This is a job for an experienced BMW airhead mechanic. The Guru's say that the crankshaft should not be re-ground. If he's suggesting that I would be looking for a good 2nd hand engine and a new mechanic.

Getting back to the root cause of your oil pressure. Is your mechanic suggesting it's due to worn crankshaft bearings ? They would have to be seriously worn to be the sole cause of oil pressure that low and that sounds unlikely to me. Assuming the oil pump and filter installation are correct I'd be looking at other possibilities like the pressure relief valves.

 

[Bob_Roller]

There's a filter bypass valve inside the oil filter cavity check it out to make sure it's not open and releasing oil pressure .

[Barry]

There are two oil pressure relief valves.   The one that governs maximum oil pressure is the mounted on the front crankshaft housing and the pressure release vents at 5 bar to lubricate the timing chain. You need to remove the timing chain cover to check it's function.

[fbenach]

Thanks Barry and Bob...
When I said "we" I meant my brother and I!!! thankfully, my mechanic actually knows what he is doing!!!

We will check on those relief valves for sure... this bike actually had an issue with the crankshaft before I bought it, and  it seems the mechanic that took care of that, didn't do a good job somewhere...

Is not hard to believe either he didn´t, for in my country there must be 2 or 3 qualified persons to check on old airheads... being my mechanic one of them!!!

I´ve been looking in the books for some more info on those special "clearance bush bearings", but it seems there is nothing about the subject...

Take care!!

[fbenach]

Well, in order to finish this story, we opened the engine and took out the crank... it was a little more "wasted" by millimeters on one of the surfaces, so we had to decide either regrinding the surfaces and buying +0.25 bearings or trying to get another crank.
By luck, I went to the BMW agency down here and they had a brand new crank on their "non selling at all stock" and I was able to buy it for only $100!!!!
Bought new standard bearing and put all together and now the engine develops the oil pressure as it should!!
Bottom line was around $500 and had the change to repaint the emblems and ribs on the engine!!

Happy again!!!
Thanks for all the tips!!!

[Julio A.]

Wow! $100 for a brand new crankshaft? That should take the best bargain of the year award.