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MG MGB Technical - Center main bearing access

I have a simple question.

Is it possible to remove, and replace, the centre main bearing cap of the crankshaft with the engine in the car and with the sump off?

I ask as I want to inspect and replace the thrust bearings as there seems to be excessive crank end float. I am aware of what is involved in removing the sump in the car by releasing the engine mounts described in many other posts here.

Any advice would be gratefully received.

Richard.
Richard Thompson

Richard. Perfectly possible to access and remove the center main bearing cap. The problem may lie with replacing the uppermost half of the thrust washer. I do not know because I have only replaced thrust washers and main bearings as a set, with the engine out, during a general rebuild. But, perfectly possible to replace the lower halves of the bearings in situ.

Les
Les Bengtson

Thank you Les for your reply - this is reassuring and I note your caution with regard to the upper half of the bearing.

Richard
Richard Thompson

Richard / Les
Yes it's possible-
The bearings mounted on the sides of the bearing cap have the locators to stop the bearings spinning that fit into cutouts in the cap---The bearings up in the block don't have locators and are half round shape----With the bearing cap off it's a simple task with a small screwdriver or similar to push on the end of the bearings and slide them round to the bottom where you can extract them-
'BEFORE' you pull it apart, accurately measure how much crankshaft endfloat you have, only then you will be able to determine which bearings to get.
Bearings are available in Std-.005"oversize and King bearings do a .010"set
A std thrust bearing new is .093" thick
Be aware that a .005" oversize set is .005" overall the actual bearings are .0025" thicker each making up the .005"
The ideal is to get down to .001"-.0025" clearance
Also
Working upside down looking up there it's easy to get yourself mixed up a bit--so take note of the centre cap, it will have front cast into it and needs to go back exactly as it comes off, and when you get to the thrusts, the thrust bearings have a steel back, usually flat, and a bearing side which usually has a couple of oil grooves----make sure you get the bearings round the right way when you fit them up, The backs of them go towards the block / cap and the bearing surface goes towards the crankshaft surface------if you get them round the wrong way it'll wreck the crank
Hope this helps
willy
William Revit

Thanks Willi. It has been a while since I last assembled an engine and could not remember if both the upper and lower halve had the tab on them or not.

Les
Les Bengtson

Thank you Willy.

This is very helpful.

Now I've to dig out my dial gauge to measure the end float.

R

Richard Thompson

As a second thought are there any recommended suppliers/manufacturers of thrust bearings? or ones to void?

I'm aware of the usual parts suppliers have product.

R
Richard Thompson

Richard-
Thrust bearings are fairly straight forward, I'd think whatever you can get the correct size to suit will be ok
I mentioned the King bearings earlier because they do the 10 thou oversize---early on here when king bearings started being about engine builders turned their nose up at them but several people use them now without any issues at all
ACL are good if you can get them over there

willy
William Revit

Thanks Willy,

A quick search has revealed a few suppliers for both brands, two reasonably near by.

R
Richard Thompson

A question--how much end float did you get with the dial indicator--just interested as it usually feels a lot worse than it actually is.
William Revit

@ Willy,

Excellent question!

I have not yet measured it as the day job has got in the way.

I did look at how I can get the magnetic base and gauge onto, and straight, on the crank. Looks a tight fit but I think I can get the dial gauge onto the lower side of the pulley. I won't know until I get the whole thing mounted up; the engine is still in the car.

What I measure I'll report.

Best
Richard.
Richard Thompson

Measure the end float that you have and then put in new stock thrists and remeasure and see if it is now OK.

There is no problem changing the centre main bearings - use a small screwdriver to push the upper half around the crank until it falls off and slide the new upper bearing in the same way.

FWIW, I had one race engine that had excessive end float and no oversize thrusts were available. I cut shim stock with scissors and superglued it to the back of new standard thrusts (where it would be trapped between the block and caps and couldn't fall out) and it worked beautifully.
Bill Spohn

Richard-
Note that some, if not all, of the new crankshaft thrust washers that are currently available are not flat. Instead, they have a bit of a 'Belleville' shape. Measurement by means of a micrometer will only reveal a thickness measurement. When set on a good flat surface and measured by means of a depth micrometer, not a digital caliper, the dimension that controls the end-play/end-float will then be revealed. The metal side of such crankshaft thrust washers should be flattened by means of a surface grinder.
Stephen Strange

they sound a bit dodgey Stephen, i'd be sending them back and ask for real ones----
William Revit

William-
I just now checked King Bearings website and couldn't find a listing for crankshaft thrust washers. By any chance do you have their part numbers?
Stephen Strange

Stephen-

KING-
TW216AM --- std or 0.25mm(.010")o.s.

ACL-
2T2252 ---std or .005"o.s.

Cheers
willy
William Revit

This thread was discussed between 27/02/2022 and 01/05/2022

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