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MG MGB Technical - Replacing Bearings


I was reading a post by John Twist in the North American MBG Register newsletter. He states that replacing the bearings in an MGB around 70 thousand miles is a good idea.

Wondering what the group thinks of that?

If you do this, do you plastigauge the crank/bearings to see if you need to go with a larger bearing?
Bruce Cunha

Bruce, the bearings come in 10 thou increments (or decrements), -.010, -.020 etc undersize. 10 thou is way too much wear, so you cannot swap to the next smallest if the plastigauge reading is too much. Instead you need a regrind to fit the smaller shells. Having said that, renewing the shells will obviously put the clearances back closer to where they should be, and will extend the crank life if the old ones have lost the babbit layer.
Art Pearse

Bruce. John wrote that tech article a number of years ago. Things can change a great deal over time and I would suggest that you contact John to see if he still makes such a recommendation.

In general, I have not seen a great deal of wear on the more modern MGBs used with modern oils. My first rebuild was a 61 Sprite with 65K miles on it. The bearing shells were badly worn and the cylinders had a significant lip above the piston rings. Sufficiently so that a ridge reamer had to be used before the pistons could be removed. Fairly common "back in the day". In fact, Ocee Rich, in his book on sports cars, recommended replacing the bearings and piston rings about every 10K miles. That began to change in the late 60s and things are considerably different now. When I rebuilt the engine on my daughter's 77B, before she went off to college, the crank journals did not have to be turned and the wear in the cylinder bores was minimal. Much better oils today, and since the mid 70s, than back in the 50s and 60s.

If you do decide to do a mini rebuild, the bearings should be marked if they are designed for use on an undersized journal. If they are so marked, you replace them with a new set of the same size. If you want to use Plasti-Gage, that is fine. But, if you find that the bearing to journal clearances are excessive, you will need to remove the crankshaft, have the journals turned to the next undersize, and purchase another set of bearings of the proper size to install. By the time you do that, you might as well consider checking out the cylinder bores, honing the bores (if they are still within specification) and installing new rings. Quite quickly, it can turn into a full engine rebuild rather than a quick bearing replacement.

Thus, it might be well to discuss with John what his current thoughts are before making any decision. His tech hour time and his telephone number are available off of his University Motors Ltd website.

Les Bengtson

Personally I wouldnt even think about doing this if you are getting good oil pressure with a warm engine and you can believe the readings, starting high and coming down as the oil warms up. I agree with above comments, modern oils are just so good that engines, even the venerable B series, can now run for indefinite mileages. I have never seen the factory design life for the B series but read once that the A series in the Morris Minor was designed to achieve 30/40 K miles between overhauls.
Stan Best

Haynes recommends changing the big-ends at 30k intervals and the mains at 50k for maximum service life of the crankshaft before regrinding. Fair enough, but who is going to have two different intervals that aren't a multiple? 30k and 60k I could understand. Last year I plastigauged all the bearings on my V8 and found them at or just inside the tolerances for *new* bearings, and they have done 100k. The journals are mirror finish, and that has done 200k without a regrind as the bearings are standards.
PaulH Solihull

I would consider that John's current thinking. The reference to replacing rod bearings and the three mains that are accessible with the engine in situ, along with the oil pump guts, at 75,000 miles is in the Tech Talk column John currently writes for MGB Driver. It's not a re-print of one of John's existing articles. It is an answer to a member submitted question in the current Nov/Dec 2010 issue..
Kim Tonry

I am at a loss as to why anyone considers replacement of bearings at a Mileage. They should be only "On Condition" If an engine is out for any reason other than a bearing failure or history of LOP and they are not disturbed, then leave them alone. If they are disturbed, then do not refit but replace, The pins wear oval due to loadings so forget plastigauge as this only gives minimum clearance rather than maximum which is what you require. Whether to go undersize is a judgement depending on measured ovality and scoring/damage together with your engine shop rework prices.
Geoff F.
Geoff Farthing

Les. I used to work in my uncles machine shop back in the 60's and remember that 90 to 100 thousand was just about it for most of these engines before they needed a rebuild. I saw a lot of rebuilds brought in around this mileage.

My 67 has 93,000 original miles. It had 75,000 when I got it and I know both the last two owners were experienced with MG's, so it most likely has been taken care of correctly.

I had a slight burn on a valve last year from running lean, so pulled the head and had it redone.(head had been converted to unleaded some time in the past) There was minimal ridge on the cylinder. (just clicked very slightly on a fingernail) My oil pressure is a pretty consistent 65 to 70. Even thought the head was redone in the past for no lead, I can;t say that the engine has ever been rebuilt. Pistons are standard size. Compression is around 140 for each cylinder.

So if you were to ask if I am having any signs that things really need to be redone, I would say no. But, this would not be a super big project either. Just wondered if anyone has done this.

Bruce Cunha

I've taken hundreds of engines apart at all stages, and all sorts of maintenance histories. A well maintained engine of this general era will typically show wear on bearings such that replacing them will extend the life significantly. Typically, the soft bearing overlay is just worn through, but the crank is as new; with the OE bearings this will mean that you will see copper colour in the worn areas, but the crank will be perfecto. Any more miles and the crank starts to wear, which means regrinds later. There is enough excess capacity in the oil pump that there will usually be no pressure drop at this point; once pressure drop shows it is probably too late. Engines that have had extraordinary maintenance will last longer (>100,000) without wear, and poor maintenance (<25,000) means it is already worn out complete, but 75,000 or so on an MGB is prime time. The bearing wear is typically in accord with the first valve cleanup, but before the valves are really bad, and if the air filters have been attended to, the rings and bores will be good for further service. Cams & lifters on all BMC engines are likely to be dead at this point though!

FR Millmore

On the older Cummins diesel truck engines, the maintenance schedule was rod bearings every 100k miles and main bearings every 300k miles. Later engines had the schedule changed to 300k for both. Crankshafts were very expensive for these engines, so you didn't want to risk ruining one. It helped that you could replace all the main bearings by just pulling the pan (sump) and leaving the engine in the truck.

I was always under the impression that this was a factory recommended schedule, but I don't absolutely know that for a fact.


C R Huff

Its interesting to note that several of the experinces of engine service lives posted here go back as far as the 60s. I have achieved 250K miles on a modest modern engine without putting a spanner on it other than routine maintenace (1.6 petrol Citroen BX FYI). We used to see that car around town for several years after we sold it as well. I think it would have run for some indeterminate mileage. I dont expect the less sophisticated B series could do that, but with modern oils they must do better than the less than 100K of the 60s. With the use our B gets, and after a rebuild in 1996 I dont expect to know in my lifetime. Arnt the cam followers more of a weakness?
Stan Best

Our engines, with modern formulations of the *correct* oil grade should certainly do better than with oil formulations of the 60s. I'd also expect modern engines with their closer tolerances and modern oils to do even better than that. 'Flat tappet' valve gear such as on the B is said to be the weak point with the latest formulations of oils, with their lower ZDDP content, but arguments rage back and forth. The lower ZDDP is to protect the environment but also to extend the life of catalysers on modern engines. 20W/50 grades are said to be exempt from that as no modern engine use it, but on the one hand 20W/50 is difficult to get from reputable suppliers in the UK, and in any case the concentrations in 20W/50 in markets that *can* still get it has also been shown to be reduced from what it was. For myself I use oil suitable for both petrol and Diesel engines, as the API and ACEA classifications are a couple of generations older for Diesel and have higher levels of ZDDP. This is because Diesel engines need more impact protection for the little-ends than petrol, however levels in these are also subject to reduction in future. 4-stroke motor-cycle oil is also supposed to have more ZDDP as the small volumes in these engines result in faster circulation and higher oil temperatures.
PaulH Solihull

This thread was discussed between 04/11/2010 and 07/11/2010

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