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Main bearing clearance tapered

Posted by Beauii60 
Tony Cox
Beaumaris, Australia   aus

Hi All,
I'm rebuilding the engine out of my 70B and have a concern about the rear main bearing clearance which is showing a taper of about 8 tenths of a thou'.

The other 4 bearings are fine a consistent clearance.

The crankshaft journal is the same diameter along its length, the block and cap mating faces are clean and divot free and checking the clearance with different shells fitted gives the same result.

Therefore, I'm thinking that it is the bearing cap itself, that the block mating surface is not parallel with the bearing recess centreline.

It could also be the block though I think this more unlikely as I expect that all of the bearing cap faces would be final machined in a single sweep.

The clearance ranges from 1 1/2 to 2 and a bit thou.

I'm calling onthe collective brains trust out there to advise if I have a concern or I can finish my rebuild.

BTW, the engine is standard apart from +0.030" bore and a mild road cam.

Many thanks for any posts.

Cheers,
Tony (Beauii60)
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Basil Adams Avatar
About 12 miles from Sears Point, CA, USA   usa

That's the only thing that makes sense. Do you have bolts or studs? If you have bolts, try different washers in case one is side-loading the cap (you might try that with studs too but less likely).. Best of luck. Basil 707.762.0974 basiladams@yahoo.com



Basil C. Adams
1956 MGA Coupe (Show Car)
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1959 MGA Coupe (unrestored)
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Speedracer Avatar
Hap Waldrop
Greenville, SC, USA   usa
1967 MG MGB "The Biscuit"

I guess my question would be is how did you come to that conclusion, how did you measure?



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Dick Moritz
Philly 'burbs, PA, USA   usa

With the four other main caps torqued to spec, suggest you check the rear main journal with a dial indicator for a possible bent condition. This is unlikely, but possible. And if no bending is present, then suggest you have the block with main caps align-bored. Once this process is complete, you will still be able to use the same bearing shells, as align-boring restores all dimensions to OE...

Dick



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Basil Adams Avatar
About 12 miles from Sears Point, CA, USA   usa

In reply to # 2167070 by Speedracer I guess my question would be is how did you come to that conclusion, how did you measure?

I assumed he used a dial bore gauge???



Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains
Tony Cox
Beaumaris, Australia   aus

Dear All,
Thanks for your comments.

The possibility of a bent crankshaft did not occur to me.

Although I measured the bearing clearances at least twice, the crankshaft was always in the same position.

I do not have a dial indicator so fitted the engine backplate and flywheel to check for flywheel axial runout.

With my trusty micrometer I measured a total runout of 0.007" at a radius of 4.9".

This gives a deviation of 0.0035 from nominal (assuming zero axial runout on the flywheel).

Using similar triangles, this gives exactly 0.0008" on the crankshaft journal face.

Coincidence? Hard to tell.

One thing I didn't mention but now seems relevant is that the crankshaft must have been dropped during shipment (from the US - long story) as there is a significant tell-tale ding on the outer edge of the flywheel mounting face).

This lines up perfectly with the "low" runout region of the flywheel.

I will now check the bearing clearance with the crankshaft in 2 positions at 180 degrees to each other to confirm.

My question now is - is this a problem?

I discussed this with Andrew McDowell from the MG Workshop here in Melbourne who advised that on a 5 bearing motor this amount of run-out will not be a problem but now I have flywheel runout as well that may lead to imbalance problems.

Squaring up the flywheel mounting face is easy enough, not sure about straightening the crankshaft.

Looks like I'm not finished yet.

If you'll excuse the language, here is Australia we say "bugger".

Cheers,
Tony
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Speedracer Avatar
Hap Waldrop
Greenville, SC, USA   usa
1967 MG MGB "The Biscuit"

I always straight check my cranks, and I hardly ever find one bent, but they alway get checked.



Hap Waldrop
Acme Speed Shop
864-370-3000
Performance Street/Race engines- modified heads, and DIY engine rebuilt kits
New alloy wheels options for MGBs, see vendors forum for details.
http://www.mgexp.com/phorum/read.php?68,2657584
Be sure to check my engine rebuild kit thread in the Vendors forum for weekly tips. http://www.mgexp.com/phorum/read.php?68,1828263



Edited 1 time(s). Last edit at 09/14/2012 11:44AM by Speedracer.

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Dave Headley
Cortez, 4 corners, Colorado, USA   usa

Tony, I think that's a problem. Also check radial runout on the crank pilot for the flywheel. The flywheel needs to run true, both radially and axially. The crank could be reground to bring the journal runout back in spec. The FW pilot may be more of a challenge. You might also check FW axial runout with FW repositioned 180*, see how much is crank flange and how much is FW.confused smiley

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Dave Headley
Cortez, 4 corners, Colorado, USA   usa

I just measured a crank rear flange axial runout in the lathe, less than .001".

Member Services:
Dave Headley, dba FAB-TEK offers full service race car parts and preperation for MGB & MGA race cars, SCCA and Vintage. Dave is a mechanical engineer and has raced MGBs since 1963.
Tony Cox
Beaumaris, Australia   aus

Thanks All,
Next step is to borrow a dial indicator from work and start measuring.
I'll post the results.
Cheers,
Tony
Tony Cox
Beaumaris, Australia   aus

Dear All,
Thanks again for your input.

I have taken many measurement and can report the following.

The axial run-out on the flywheel did line up with the ding in the crankshaft but this was purely coincidental and was proven by measuring the axial runout on the crankshaft mounting face.

The axial runout on the crankshaft mounting face did not align with the ding so the flywheel runout must have been due to a lack of parallelism of the flywheel mounting face and its rear face.

After removing the motor end plate and sump, the radial runout of the crankshaft main seal face was measured and was about half a thou.

The rear journal radial run-out was also measured and was about 2 tenths of a thou at the rear end and zero at the front.

Using plastigage twice (with the crankshaft turned 180 degrees to the first measurement) confirmed that the crankshaft was not bent but that the taper was in fact 2 thou (so much for my memory).

Therefore I had confirmed that the crankshaft was not bent and that the rear bearing cap looked to be the culprit.

Measuring this on a surface table confirmed that there was in fact a 2 thou difference between the front and rear of the cap that aligned with the plastigage results.

Now, how to take 2 thou of the bearing cap?

A quick measure of the other side proved that it was not flat so could not be used to mount it on a grinding table.

My approach was to use extrememly fine wet and dry sandpaper (1200 grit) with a piece of paper under the "good" side of the bearing cap and rub-rub-check-check for about an hour.

A quick check with bearing blue confirmed a flat mounting surface.

This worked a treat and the photos attached show the end result.

The engine is now running with excellent oil pressure and no noises.

Next month the MG is going on a 700 mile round trip to the seaside.

Cheers,
Tony
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