front main bearing walking

We keep having a problem with the front main bearing moving around at the parting line, the bottem half walks forword and the top walks back and rubs on the crank throw. this is a new century block (bbc) blown alky with a roots and I do have a crank support, any thoughts?

 

bearing problem

It has done it since the block was new, but we have checked it and had it checked, it checks out ok,that is what we can't figure out

 

Block problem

What is happening to the tangs on the bearing when it walks? Are they breaking off or bending?

Corey

 

block

We have a KB olds with 2 studs on the front cap and run way less than normal crush on the bearings with no problems. 1471 roots with a crank support. I'm not sure what your problem could be. I thought we'd seen every main brg problem possible!!!lol.

 

bearing problems

it's the other side without the tangs

 

Bearing Issues

I'm just trying to picture what you are discribing. Those bearings must be twisting to the extreme if the tangs are staying engaged yet the other side is slipping out past the cap and hitting the crank. Something is thrusting the bearings in opposite directions, this has me wondering if the crank journal is tapered. It seems to me that to do what you discribe though, the taper would have to be so severe that you could damn near see it by eye.

What is your crank thrust clearance?
Just throwing out ideas but have you considered trying to run it once without the crank support? I have never run one, so I have no experience with them, but is it possible if the support isn't made correctly its pulling on the crank funny?

Corey

 

What does the bearing face look like when this happens?

 

the crank is ok, we check it out and had it checked.We thought about taking the suppot of to see what it does. thrust is .006 to.008. But the rear thrust looks great

 

The face of the bearing looks great, it just screws up the side of the bearing

 

front main

We had a similar problem with the front main. Tried all that has been suggested,swaped crankshafts,problem went away.Don't know which crank you run,just food for thought. hope you find the problem.......overthehill

 

Mine did the same with a New Century block and a lunati crank, dont know if it was the crank or block though. same with and without a support, roots blower.

 

To eliminate it as a possibility, I would check the crank endplay and then remove the front main cap, check endplay and than remove the upper main bearings and check the endplay. Could it be that the crank radius is touching the upper crank bearing starting all of this?

 

What does #2 main look like, is it squeezing it or moving babbit?
Almost sounds like your crank is flexing from the tug of the blower and pivoting on #1 main

 

bearing problem

Other bearings look perfect

 

Front bearing

Had the same problem with BBC Aluminum block. we pined the front lower cap. Same as rods.... Problem solved.

 

You know, just thinking out loud. I have a KB Olds block which is pretty much the same thing except it doesn't have the cross bolted front cap. Wondering if that could make a difference, like torque sequence on the four fasteners or the torque numbers itself? Maybe twisting the cap when torqued? The KB block does not have an issue like this that I know of. Was New Century called to see what they have to say?

 

We are having this problem . Replaced the bearing twice in 10 hits.. Drilled and pinned #1 main cap yesterday. SO well see what happens this weekend??

How many passes are expected on a crankshaft? KB Olds roots 40 over ??

thanks

 

Have you tried narrowed main shells yet?

Alan

 

Any strange marks or coloration on the backsides of the bearing shells?