Technical Today’s Ford 260 question. Main bearing clearance

Dropped crank in today. Main bearing clearance is supposed to be .0006-.0026”. 4 of them look good, my number 3 is showing.0030”. Checked twice with Plastiguage, then pulled crank and used my telescoping gauge and a micrometer. All 3 times it was showing.0030”. Wear limit is .0034” on a used bearing BTW
My crank is .010 undersized from an older rebuild. So I looked to see if they make a bearing.001” thicker, and they do make a .011” main bearing, but they are on back order for 2 months, and won’t break up a set. I only need 1.
So….. any ideas besides cutting the crank .020 and starting over? Just run it? It’s gonna blow?
Bone Stock engine rebuild on a weekend cruiser, if it makes a difference.
Thanks

 

I would use it and not worry. New oil pump? No problem.

 

I would use it. Although...the 327 I put together is looser than that on all the mains, and really needs them to be a bit tighter, as it loses a little more oil pressure when hot than it maybe should. Still holding together fine after 10k miles. I can find 011 bearings for it pretty easily, for next time I have it apart.

 

Yes, new oil pump going in

 

Yes, SBC I saw multiple options for a .011”bearing.

 

The U.S. Army’s freight depot on 39th St. in Chicago, it occupied a building in the Central Manufacturing District. This 90 acre complex housed all types of manufacturing facilities and wartime goods made there were routed to the Army warehouse for shipment to the war effort.

 

I wouldn’t be afraid to run it. And this may raise some eyebrows but it’s a standard procedure in some circles but take the bearing out of the cap and on a flat surface with some320 sand paper make a figure eight with the cap. You’re only going to take off a couple of tenths but it will tighten up the clearance. As you said it’s a weekend cruiser.

 

Check the mating surface of the cap for a raised lip right at the edge. I'd run it though.

 

I actually had already run each cap on a fine file to check for burrs. All was good.

Thanks guys!

 

Since you mention plastigauging AND miking, I'd guess that you checked each bearing and journal and that it's the crank journal that is slightly smaller, not something that swapping bearings around and select fitting can fix?

 

All the journals were in spec, though this problem one-barely.
The bearing that is causing the issue is the #3, which is also the thrust bearing on my 260, so there’s only 1 in the bearing set. So to try a different bearing, I’d have to buy more sets.
I put a call in to my machine shop today, and left a message. Figured I’d see if they have any bearings laying around I could swap in just to see if they’re a fuzz thicker, couldn’t hurt.

 

Anything wrong with a piece of .001 shim stock under the shell?

 

If I were a betting guy, I'd bet that you can put it together and run it with no prolems. However, if I had found this I'd either source the thicker bearing shell or I'd have that journal turned to get the proper clearance. Even if it ran fine, it would worry me to death. But, that's just me.

 

Do you plan on 6000 rpm runs ? Or is it a daily cruiser?

If it was just a means to push the car down the road I'd have no problem running it. So I say put it together and enjoy...

..

 

It'll run without one bit of worry.

 

Not as long as it is a tapered shim.

 

Do you have access to a dial bore gauge and a mic. Plastigauge is iffy. If you mic the crank and use a dial bore gauge in the bearing and do the math, you will find out with more accuracy than the plastic. That being said, a half of a thou on a street engine is not horrible. You need to find out if the crank is small or the main bearing bore is within spec before getting another bearing to try. I do mic the thickness of all bearings anymore. I found a set marked standard but they were 10 unders. These were Sealed Power parts. Nobody I know had ever seen that before.

 

0.0004" is 10.16-microns.

We are not talking about much here. That is half-way between new, and the wear limit. That's inside the spec.

I'd run it.

 

My default comment is always "a micrometer is only as good as the standard that came with it".

 

Could it be a slightly bent or twisted crank? Try turning the crank 180 degrees and check again or use a dial indicator to check while slowly turning the crank by hand, all bearing shells in place in the block, all caps off of course. I have had a broken crank due to a bend at the front journal. Twists are more difficult to find. These engines can put out more stock power than some of the original cranks are able to handle. A good automotive machine shop can check it and usually correct it. After all, you're only looking at a few ten thousands of an inch. I would run it as is.

 

What type of bearings are being installed? Bi-metal (aluminum-silicon) or tri-metal (steel-copper-babbit)? The tri-metal is my bearing of choice for old engines, and I have found them to be slightly thicker sometimes.

 

Bi metal bearings.
Machine Shop called me back and we discussed all the options for a good 15 minutes. He actually said in his Mahle book it shows thrust bearing allowing up to .0029”. My two books don’t mention this??
Regardless, I ran the cap over a piece of 220 sandpaper on glass as suggested above, and by the shop. I am showing close to .0028” now. If anything I can feel like I at least tried something.
Thanks again guys.
On to the pistons!