Crank/rod bearing clearance questions

[Hallen]

I had rebuilt the engine because it was burning tons of oil at the track. I completed it, dropped it back in the car, started it, and it made an amazingly loud banging noise. I shut it off.

Pulled the engine and in checking out the bottom end, I found that the rod end cap bolts were loose. I hadn't done the angle torque on them. Lesson learned.

I ordered new bearings and new end cap bolts.

After assembling the first two, I noticed a ton of play even after the initial torque was done. Hmmmm.

I got some plastigage to check things out. It looks like the clearance between the bearing and the crank journal is way off. I might be being stupid here, again, and doing something wrong, but it looks to me like there is something seriously wrong here.



You can see the blue plastigage there. That is after having new bearings in and torquing to spec.

It measures out to the highest value on the scale. The problem is, the blue is .102mm to .229mm. The bearing clearance should be .03mm to .07mm.

So WTF? It's the same crank that I took out of the engine when I did the rebuild. I doubt they make bearings thick enough to compensate do they?
Am I misunderstanding something here? Did I do something wrong?

[ikkitousen]

could you have gotten the wrong bearings? I mean could they have given you the wrong ones? Is that even possible?

Only dumb question is a question unasked...

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DAMN MY NOOB QUESTIONS!!!

[nando]

bearings for another motor that happen to fit?

you sure nothing happened to the rods or crank?

[jlevie]

The possibilities are; the journals are undersized (worn or ground in the past), or the bearings are wrong. A check of the journal diameter would be the next step.

[Hallen]

I ordered from Pelican parts, 11-24-1-284-849, and that's what I got. That is the standard size bearing. They are the same size as what I got before so unless there is another 6 cylinder motor bearing set for an E30, I got the right bearings.

I don't have a clue what has happened in the past to these components. It seems somebody rebuilt it before, but what they did, I don't know. They certainly were not very careful and I doubt they knew what they were doing. They might have gotten the crank turned.

Pelican has an oversize bearing that is .25 over (which seems like a lot given the allowable gap) 11-24-1-284-851 that could work for me...

I'll see about getting the journals measured. If they are way off, I'll have to pop the main bearings and check them too. If somebody turned the crank, it's pretty much crap now and I'll need a new one.

Thanks guys!

[Hallen]

A friend brought over a set of dial calipers and I measured out one journal.

It was between 1.750 and 1.751 inches or about between 44.45 and 44.4754 mm. That's the lowest side of the undersize 2 spec in the Bentley. I don't know how accurate the dial calipers are (it reads to .001in), but that pretty much has to confirm that somebody turned this crank, right?

Standard size is 44.775-44.991mm. (1.7706-1.7713 in)

That's about .33-.34mm off of that standard size for the journals. (give or take)

Time to find a new crank? Or would oversize bearings be possible?

[nando]

get a new crank. you should be able to find a good M20 bottom end for $~100.

I don't like the idea of fucking around with oversized bearings on M20s. it's more work to machine a BMW crank than just turning it down, it also needs to be nitride hardended. Given the uncertainty, a good used crank seems like a much better idea.

[Hallen]

get a new crank. you should be able to find a good M20 bottom end for $~100.

I don't like the idea of fucking around with oversized bearings on M20s. it's more work to machine a BMW crank than just turning it down, it also needs to be nitride hardended. Given the uncertainty, a good used crank seems like a much better idea.

OK, I would agree with that. Just the crank? You say "a good bottom end". Does that include anything other than the crank? Sorry for the possibly obvious questions. This is new territory for me.

[nando]

well, you'd probably want to find a whole block with pistons/crank, rather than a bare crank by itself. rather than a crank that's been laying around exposed to the elements.

[Hallen]

Ah, I see.

Thanks!