Main bearing failure

Not likely lack of lubrication
Not likely flawed product or manufacture.

Likely too much heat to the bearing surface upon install.

The metal is brinelling and spalling. This exact looking failure happened to me once many years ago (1970's) as a result of too much heat on the inner race during install. A knowledgeable bearing engineer once told me that you could easily damage the bearing surface (especially if free of oil) with very little direct torch flame exposure; just a brief flash of a torch could ruin the hardening locally. In my case I used an oven but too hot and a few thousand miles later I had a rumbling down below.

Not likely lack of lubrication
Not likely flawed product or manufacture.

Likely too much heat to the bearing surface upon install.

The metal is brinelling and spalling. This exact looking failure happened to me once many years ago (1970's) as a result of too much heat on the inner race during install. A knowledgeable bearing engineer once told me that you could easily damage the bearing surface (especially if free of oil) with very little direct torch flame exposure; just a brief flash of a torch could ruin the hardening locally. In my case I used an oven but too hot and a few thousand miles later I had a rumbling down below.

If upon applying a micrometer to the rollers you cannot see the diffference in diameters either
1 The bearing was an earlier version of the bearing produced before FAG changed the roller profiling...One dealer in London bought a large pile of FAG brass caged Nj306E bearings .....VERY CHEAPLY.... I went to put them into stock but 'wondered' and opened one up, popped out a roller and applied my micrometer to it.....it had the old style rollers. The dealer flogged them all to Commando owners ...... Now that was MANY years ago but I wonder if some are still floating around at jumbles etc???
Actually it only needs a good pair of eyes and a light on the rollers to spot the bearing has the correct rollers.
2 Either you need your eyes testing and new glasses OR a micrometer that reads to 1/10 of a thou..... Mind you I doubt there are many in this World who know how to correctly read a micrometer and even fewer who can use a slide rule and as for log tables.......My monety is on most people only having el cheapo digital calipers.
OOPS, slide rules and log tables....showing my very old age again!!

Oddly enough JM, if you go looking for main bearings for Commandos, you find them in the Cylindrical Rollers section.
Emphasis on CYLINDRICAL.



The bearing I measured all those years ago came out of a factory built 850.
"Popping a roller out" isn't that simple, you have to destroy the cage !

Refer too to Jims posting on roller shape.
Mythbusters has taken care of that one for sure, for all time.
The lightsource under the straight edge is a quick-n-dirty test of the same thing.
Those FAG rollers are cylindrical, plain and simple....

This roller flat vs. roller crowned is starting to take on the similarities of a political discussion. Some people will not accept proof of something opposite what they believe.

It's easier to fool someone than to convince them they've been fooled.

Mark Twain.

JimC said:

This roller flat vs. roller crowned is starting to take on the similarities of a political discussion. Some people will not accept proof of something opposite what they believe.

It's easier to fool someone than to convince them they've been fooled.

Mark Twain.

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Indeedy Indeed - or a religious discussion, depending on what ye believe.

FAG Bearings say their bearings for Commandos are a heavy duty plain cylindrical roller bearing.
And always have been.
Maybe they know what they are taking about...

Don't be mistaken in that 'crowned' word though, that is entirely what happens at the very ends of the rollers.
The former fairly sharp edge has been modified to be ground/machined to a curved surface, so it can't dig into the bearing track if the crank flexes.

Rohan said:

FAG Bearings say their bearings for Commandos are a heavy duty plain cylindrical roller bearing.
And always have been.
Maybe they know what they are taking about...
.

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Who do these tin pot little companies think they are? And with do they know? Just because they make the bearing does not mean they make it correctly. We have experts that know better an can use a log table but not it seems a keyboard as am sure there is no "t" in money. :roll:

hobot will have to reply now, thats not what he said at all.....

I just rebuilt an engine that had a timing side main that looked just like the one that you have. But this one was in a Combat engine that had over 40K on it's original bearings. I think the one I replaced was shot due to age, normal wear and tear, and lack of a filter for many years. I swear the oil tank was a couple of pounds lighter after being cleaned out! An oil filter was added to the bike as part of the job.
But I also have the original mains in my MKIII that have well over 50K miles and are just fine. I am surprised that your bearing failed so soon and unless you used a torch to heat it up during assembly, hopefully just a bad one slipped through. I have found that a heat gun on the lower setting works just fine for the inner race, it really doesn't need too much heat to slip into place.

milfordite said:

But this one was in a Combat engine that had over 40K on it's original bearings.

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When you say 'original bearings', just which bearings were these though ?
The uprated FAG 306e which were fitted as the cure for all the combat bearing woes,
or the earlier variety which were the "problem". ?

Some folks, who didn't thrash their Combats, have reported good bearing life,
or maybe there is some issue with individual bearing quality here ??

And someone on the NOC reports racing his 650SS with ball bearings fitted,
with no problems,
so maybe there really is/was a problem with individual bearings here...

They were the earlier bearing type. I know there was a problem with them, but not ALL the bikes blew up! Seems this one wasn't thrashed, just ridden.

Firstly....
The bearing that was used as PART of the cure for the 'ooops my motor has done less than 4,000 miles and the mains are already failing' problem was a SPECIAL bearing manufactured by Ransom and Marles of Newark England. The Atlas roller main and early Commando bearing was a 8MRJA30 Ransom and Marles bearing and it had 11 rollers of approx 11.12mm diameter and 11.12mm totaL width with the working parallel portion of the rollers being approx 10mm with only a very small radius at each end. The Ransom and Marles bearing employed to help solve the main bearing problem was a SPECIAL and had the MRJA30 stamped on the inner raceway but with a 6 etched in front of it. This bearing had 13 rollers of approx 9.53mm diameter and 9.53mm total width with a 6mm parallel working width and logarithmic profiling (crowning) that reduced the diameter down to approx 9.46mm before the very small radius at each end. Yee gods it aint hard to see the profiling / crowning under a decent light assuming one is not blind....even I can see it!! I can also use a micrometer that is calibrated to show tenths of a thou but I did serve an Engineering apprenticeship where we was taught such things!! If my memory is working correctly I believe the bearing was a C2 internal clearance bearing but dont quote me on that.Unfortunately a friend only a week ago borrowed my last remaining new one so I cannot measure its internal clearance on the jig a certain tool room made for me9for free)after I failed to relieve our local Hoffman bearing works works of a jig when they closed...... Did get a few cheap suitable for Nortons bearings mind you......
For info the dynamic and static load values of the 8MRJA30 and 6MRJA30 bearings were...in Newtons....and according to a Gentleman in Newark who dragged out the drawings for me......
8MRJA30 Dynamic 41,900 and static 35,200.
6MRJA30 Dynamic 35,800 and static 31,000.
The FAG bearing has logarithmic profiled rollers but according to my olde eyes even 30 years plus ago when I was investigating these bearings and talking to the manufacturers my measuring of a FAG bearing showed each of the 12 rollers was 12mm wide and 11mm O.D with an 8mm wide paral section reducing to 10.97mm before the very small radius at each end. Load values Dynamic 51,000N Static 48,000N
An RHP NUP306ETN bearing employed on Triumph twins had identical rollers except that the 'crowning' reduced to 10.96mm......but that was with my even then olde eyes and micrometer......
I did measure several other manufacturers bearings and found that crowning of the rolling elements was widespread but that does not mean all roller bearings are..... Especially cheapo bearings made elsewhere in this World after all straight parallel rollered bearings will work perfectly happily in the greater majority of industrial usage applications and have done so for a great many decades..hell I fitted enough of them to motors and generators etcin my youth and they employ shafts that DO NOT FLEX like a Commando crank!!. We even used to scrape the outer housings to obtain the required internal clearance in the bearing......bet that doesnt happen often these days.....
When I asked why they had changed bearing manufacturer I was told by two NVT friends it was because FAG were cheaper and were just down the road.......

J. M. Leadbeater said:

The FAG bearing has logarithmic profiled rollers but according to my olde eyes even 30 years plus ago when I was investigating these bearings and talking to the manufacturers my measuring of a FAG bearing showed each of the 12 rollers was 12mm wide and 11mm O.D with an 8mm wide paral section reducing to 10.97mm before the very small radius at each end. Load values Dynamic 51,000N Static 48,000N
.

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JM, did you read and view Jims video of measuring a roller. ?

The problem here is that FAG know nothing of this 'superblend' or 'logarithmic profiling' bizzo.
They just supply a heavy duty E i.e. NJ306E bearing, mit cylindrical rollers....

If I may be so bold, can we get this thread back on track? The OP was talking about premature failure of a new Commando main bearing.

Perhaps debates about roller types and part numbers could take place in a dedicated new thread?

That might get the OP back interested again! And I would like to ask: Reggie, did you get chance to ask the supplier or someone for a professional opinion of your failure yet?

Reggie said:

I fitted this FAG NJ306E.M1.C3 main bearing (without India stamped on)about four years ago and have done about 8,000 miles on it. I've had the crankcases apart for a reason unrelated to this bearing, but just prior to what was going to be re-assembly time, I noticed this :shock: :shock: :shock:

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A late reply on the original post but my engine was apart about year ago and a similar story, the main bearings had been in for probably 6 years but for a number of reasons with not a lot of miles, and one showed a similar failure. We just put it down to bad luck (and a lucky save). Two cases is hardly a trend but does make you wonder whether something else is going on.

Nigel wrote;

Reggie, did you get chance to ask the supplier or someone for a professional opinion of your failure yet?

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No I didn't bother. As ntst8 wrote;

We just put it down to bad luck (and a lucky save).

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It cost me £60 so not too bad. If I have another failure, I may be more concerned and take the above action. I have honed my bearing fitting knowledge after reading this thread and a link or two, so there have been some pluses.

The Indian made NJ306E.M1 FAG bearing inner races on my build were so tight on the journals that I needed to heat the hell out of them to get them on, so after reading and rereading this thread I decided to pull them off and scrap them, putting it down to a learning experience. If I thought they were hard to get on, they were a real bitch to get off. It took a lot of heat and almost destroyed the bearing splitter.
Whats the fix, relieve the journals with emery tape?
Scratching my head on this one.
jug

jug said:

The Indian made NJ306E.M1 FAG bearing inner races on my build were so tight on the journals that I needed to heat the hell out of them to get them on, so after reading and rereading this thread I decided to pull them off and scrap them, putting it down to a learning experience. If I thought they were hard to get on, they were a real bitch to get off. It took a lot of heat and almost destroyed the bearing splitter.
Whats the fix, relieve the journals with emery tape?
Scratching my head on this one.
jug

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]If I had to hone something to get a correct fit it would be the ID of the bearing. Putting metal back on the crank journal is tough. Jim [Of course they should fit pretty damn snug}

If the bearing inner ID is undersize then use emery on the ID, you do not want to reduce the journal. If the next bearing is correctly sized then it will slip on the journal. On the driveside with the sprocket held on by a taper the crank nut does not bear down on the bearing inner so you would need to get the journal back to size.

comnoz said:

jug said:

The Indian made NJ306E.M1 FAG bearing inner races on my build were so tight on the journals that I needed to heat the hell out of them to get them on, so after reading and rereading this thread I decided to pull them off and scrap them, putting it down to a learning experience. If I thought they were hard to get on, they were a real bitch to get off. It took a lot of heat and almost destroyed the bearing splitter.
Whats the fix, relieve the journals with emery tape?
Scratching my head on this one.
jug

Click to expand...

]If I had to hone something to get a correct fit it would be the ID of the bearing. Putting metal back on the crank journal is tough. Jim [Of course they should fit pretty damn snug}

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Jim, these were not snug. These were ridiculously tight. I even contacted the supplier to see if they had had any other customers have the same problem, if they did they weren't admitting it.