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MG MGA - Tight rear main bearing

Guys,
This is an MGB 5 main problem but someone may have seen it. A friend is building an MGB 5 main motor. Had the crank turned, block bored, etc, etc. When he laid the crank in and tightened the caps the crank would not turn. When he checked it appears that the rear main bearing tight. The journal sizes on the crank measure OK. The shell does not appear to fit properly in the block. All the other journals are fine. He tried a different shell in that location with the same result. He said the cap side feels OK. Has anyone seen this issue or know what would cause it? Haven't yet mic'd the block to see where the problem is.
Thanks,
Gerry
G T Foster

Gerry, it sounds like the block has been knocked somewhere near the rear bearing housing. Is there any sign of damage, a burr or anything? It wouldn't take a lot to bind the bearing, just a couple of thou. In what way does the shell "not appear to fit properly in the block"?
Lindsay Sampford

Normally you would suspect a bent crank or a distorted block.
These can be checked by putting the crank in a lathe and using a dial gauge on all the journals, and then checking the block with a line borer (without taking a cut in the first test).
However, as Lindsay said, what do you mean by "the shell does not appear to fit properly in the block"?
Could you describe that in detail?
It has been known for people to grind metal off the mating face of the bearing cap to compensate for worn bearings. A new bearing will not then fit properly and will lock the crank.

Mick
M F Anderson

Thanks guys.
By "not appear to fit properly in the block" the shell does not seat all the way down. There seems to be clearance at the bottom. It seems to be pinched somehow. But Mick's suggestion about the cap being filed is a good one. Still, this should force the bearing down, I would think. One last piece is that the old bearing, in this location, looks smeared, like it was tight already.
Cheers,
Gerry
G T Foster

If you do not have easy access to line boring equipment you should be able to find the problem using only a dial gauge.
You can take a series of measurements of the block where the bearing sits and compare those measurements with ones from the other bearing seats in the block.
It does sound like line boring is going to be necessary.
A hint if you line bore. Always coat the surfaces with a stain such as Prussian Blue, which is used by machinists, to show that the cut has been correctly done.

Mick
M F Anderson

If you line bore the block, the bearing OD is bigger. Can you get oversize shells?
Art Pearse

OK, stupid question of mine. The rebore is the same dia, bored through shaved cradles.
Art Pearse

If you line bore, the crank axis is raised about 5 thou. Does this affect the gearbox lineup?
Art Pearse

GTF -
Even if the bearing saddle were undersized a bit, the bearing would fit down to the bottom (top!), though the bearing would be tight after assembly. If the shell does not snap in, the usual reason is that the retaining/locating tab is not fitting into the little notch correctly. A jeweler's file on the sides of the notch in the block is the usual fix, if there is no obvious fault of the shell tab.
Check that, then assemble the caps w/o shells to the block and torque, then measure the bores. Then install the shells and repeat. Should show you where the problem is.
Close visual (magnifying glass) examination of the cap parting faces will show if they have been filed, and the end caps will be flush with the pan deck if it hasn't been screwed with; the deck is machined with caps torqued in place.

Art: Yes, also timing drive issues which chains can usually accommodate but gears do not care for, and you will have that much more compression. The fix for the gearbox is to move the back plate up and recenter, then ream the dowel pin holes for OS pins, or make offset dowels.

I often hear people say that "you should line bore on any rebuild" etc, but this is very bad advice. Expensive and difficult to do right, and if wrong a disaster. The correct thing is to measure everything and then linebore only if necessary and with minimum centerline movement.

Centering the plate can be a difficult proposition on engines where there is no "concentric to crank" feature on the mounting plate. 5 bearing MGB no problem because of the seal bore, but 3 main engines require measuring/setup off a bare gearbox case, measured off the front bearing bore.

,005 centerline offset is a lot, should only be necessary if there has been a catastrophic failure. A good shop will minimise this, frequently to .001/.002 or so, which you can usually live with without correction. In a case of a single US bore with no other problems, it would not be necessary to move the centerline at all.

I once had this situation on my big truck - Cummins NTC400. On rebuild, the thing was linebored and the professional shop did not measure the crank to bellhousing alignment as specified in the WSM - I wasn't there when they put the flywheel/bell housing on. 3 on-highway clutch failures later at 30,000 mile intervals, I measured it and found that between the linebore and wear on the bell housing, the crank/gearbox centerline was out by about 010/.020 in several directions including parallelism. I remachined all parallel surfaces and made offset dowels to compensate for the centerline error.

FRM
FR Millmore

In my original post I mistated that it was the rear main. Turns out, it't actually #4. Further information includes the fact that the old bearing (side in the block) was "smoked". My friend did not notice this at first, he's new at this. The shell half in the cap looks fine. The shell half in the block is badly worn (smeared) and the oil hole was full of worn material. There is a black ring (see photo) around the block that appears to be the result of extreme heat.


G T Foster

I am also including a shot of the bad bearing shell..that spot at the right center is what should be the oil hole.


G T Foster

Gt -
Odd indeed! You say the cap side shell was good? A filed cap would tend to damage the cap side shell first and worst.

The check is the same, install the cap and torque to spec, then carefully measure the ID in several positions. You can also check saddle alignment with a precision straightedge, but this is very fussy.

The nature of the machining means all the bores will be inline and the same size, but a hard spot in the casting could deflect the tool at a single point. That would give a "bump" in the bore there. The question becomes how did the thing live this long; such generally result in early death under warranty, if they even make it out of the factory.

FRM
FR Millmore

That is bizarre GT! FRM suggests a precision straight edge across the saddles, I wouldn't be at all surprised if No.4 stops the edge from touching the other 4!
Lindsay Sampford

Hi Gerry. I'm as puzzled as most of the others are at this. Its hard to imagine the engine ran for many miles before the tear down/rebuild. I suggest you send the info and photos to John Twist at University Motors, he's seen a lot of unusual things, and usually tries to offer his help.

George
G Goeppner

Wild guess with no supporting information: Someone previously mixed up the bearing caps or substituted one from a different engine.

Once the block is line bored the bearing caps must remain forever mated with the same cradle in the block, because the split lines are never exactly on center. swapping two bearing caps inevitably results in one loose and one tight bearing.
Barney Gaylord

Barney, sounds like a possibility, but I can't get my head round how it would wear the cradle shell but not the cap shell. The cradles are all in their correct positions because they can't move, but the caps can move, so if it were a case of a cap being transposed wouldn't it be the cap shell that would be out of centre and therefore mashed?
Lindsay Sampford

Hello all - I am the friend that G T Foster is referring to with the MGB crank. Took my block to the machine shop today and confirmed that I do need a line bore. The #4 position was definitely overheated and caused the cap and block to warp. The cap does not fit tightly in the block and it is possible that I will need a new cap. I bought this car 2 years ago and it had been off the road for 17 years. Don't know much about the history, but this could explain why it hadn't been driven for so long.

Tom
Thomas Dimock

Tom-
When I had a Cummins diesel rebuilt with caps refitted and line bored as I said above, the very professional shop brazed the caps up and reground them to fit tightly again, then bored them. I say this because it is possible and routine in good shops, and spare caps are not a common item, not being considered a replaceable part.
Best of luck!

FRM
FR Millmore

This thread was discussed between 09/05/2011 and 14/05/2011

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