Uneven crank wear?

[MAG58]

I know most of you haven't played with MB motors, but the bottom end isn't anything special.

 

So, to the jist of the argument, I was bolting the bottom end of the M104 together, since RTz has been ridiculing me for some time to finish it, and noticed some abnormalities when plasti-gauging it.

 

I gauged the seven mains and came out with these clearances front to back:

 

Main

1: .002

2: .0015

3: .0015

4: .001

5: .0015

6: .0015

7: .002

 

 

If you'll notice, it has the most clearance at the nose and tail with the center still being almost factory clearances. I checked them for roundness, and all of them were well within tolerances for roundness and trueness along the bearing face. I was interested in this phenomenon so I drug out my other two cranks, mic'd them and came up with very similar numbers. This crank in question has 202k and the max allowable clearance on the crank is .003 on the mains. It is also well within tolerances on runout, i.e. it's not bent.

 

My questions to you guys are what would cause this to happen? And how can I rectify it? I'd like to nip this in the bud and I'm not sure whether to address this as oil starvation to the front and rear mains or a harmonic issue. MB used three different types of balancer designs over the years and even with the different designs on each of the cranks, they still exhibit this. There was no scoring or gouging of any of the journals or bearings that came off of this motor either... It also may be nothing, but the L28 cranks didn't display this (IIRC) when I built it, hence my intrigue.

[MONGO510]

Bob, plastigauge is not the most accurate means of measuring a crank journal. Any oil, dirt, or contact of any kind in any other journals is going to give you a false reading. Don't get me wrong, I use it and rely on it for approximations. For real accuracy you need a good Mic.

With the #'s you gave I would have no qualms at all by using the crank as is. But hey, that,s just me.

FWIW, $.02?

 

Mongo

[MAG58]

I knew plati-gague was a ball-park kind of thing, and promptly after I got the numbers I drug out my trusty starret mic and mic'd them, getting the same numbers.

 

I plan on running the crank, I'd just like figure out why it does this, since it is not just one engine, but three separate cranks, of three separate strokes, out of three engines.

 

You're probably right though mongo, it's probably nothing.

Edited October 11, 2010 by MAG58

[jt1]

It could be the harmonics. Imagine the firing impulses on the crank, then imagine the crank trying to assume a sine wave shape along it's length as it rotates, with a node at the front, rear, and center. High end V8's are sometimes built with larger clearances on 2 & 4 to allow this.

 

jt

[johnc]

So, an OEM production crank, turned out by the tens of thousands, with 202K miles on it, is within tolerance (by 50%) in all measurements and you're wondering what might have caused it to not be perfect? You must spend time every week carefully sorting your sock and underwear drawers.

[BRAAP]

Bob, plastigauge is not the most accurate means of measuring a crank journal. Any oil, dirt, or contact of any kind in any other journals is going to give you a false reading. Don't get me wrong, I use it and rely on it for approximations. For real accuracy you need a good Mic.

With the #'s you gave I would have no qualms at all by using the crank as is. But hey, that,s just me.

FWIW, $.02?

 

Mongo

I'm with this guy... errr... I mean, I agree with him...

[MONGO510]

I'm with this guy... errr... I mean, I agree with him...

 

 

Hey,you better clarify that!! LOL!

 

Mongo

Edited October 11, 2010 by MONGO510

[MAG58]

I wasn't worried about this crank, I'm going to run it. But if you noticed, I said all three of my cranks, mic'd, have this same pattern with the #1 & #7 mains having considerably more wear than the middle journal. I've got a crank that shows more wear, and it appears that it becomes amplified with wear. 

 

 

 

 

FWIW, the thrust bearing on this motor is located on the #5 journal. 

 

 

 

 

And John, I'm a college student, I don't have a 'sock drawer'. I do have a drawer where clothing exists to be dug through when I decide that going to class in my birthday suit is a poor decision though. 

 

 

[cygnusx1]

I would be even more surprised if they were all exactly the same.

[Tennesseejed]

MAG58, here is my theory.

 

The only journals that are not supported on both side are the outermost (#1 and #7). They have, respectively, a pulley on one and a fly wheel on the other. And even if the pulley and flywheel are themselves perfectly balanced, the pulley still has one or more belts asserting lateral forces on it and the flywheel undergoes torsional (twising) pressures every time the clucth is operated. For these reasons, it would make sense for the #1 and #7 journals to develop wear first and before the inner journals.

 

The interior journals, #2 & #3 and #5 & #6 show half as much wear as between the outer journals and the center journal (#4). My theory, which is admittedly not based on actual knowledge or experience, is that the journals on your engine wore from the outside to the inside. In other words, as #1 and #7 began to wear, there was less (albiet a minsecule amount) support to the next interior journal causing your journals to wear from the outside to the inside.

 

Just my undeducated opinion, so that it for what it's worth.