Broken Main Bearings (Outer Race)

johnfitzw

johnfitzw

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Gentlemen and Ladies... I've come across a problem with a relatively low mileage (30,000 miles) pair of NJ306E C3 main bearings in my round-the-world Mk2A 850 engine that I've not experienced before. I'd be grateful for any suggestions / insight..

I've just shipped the bike back to San Jose, CA from Lima, Peru after my 2021-Oct 2022 trip from here to Ushuaia then back to Lima. In Lima the drive side piston skirt cracked and detached itself, and as I was out of time anyway, I decided to ship The Norton back to San Francisco.

Upon stripping the engine yesterday, I was surprised to see that both drive and timing side NJ306E C3 mains outer races were cracked (see the images), tho intact. There had been no indication of main bearing trouble (noise /roughness) while riding in Central & South America. The bearings were new, sourced from RGM, and fitted to my SBR billet crankcases, with dynamically balanced Ro-Dy one piece crank in May 2021. End float was set at 10 thou. The interference fit between mains and case bearing bore before assembly was on the tight side, at 0.0029, but the inner bearing sleeve still slid easily, nicely and smoothly inside the race when assembled cold.

Attn Mike in NZ - did you have any main bearing assembly issues with your SBS cases?

On the entire trip the engine was never spun above 6000 rpm, cruising rpms were typically around 3500 to 4000.

I've owned this bike for nearly 50 years now. I come from a background of motorcycle engineering, and have rebuilt many Commando engines. But I've not seen a cracked low mileage "Superblend" before..

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Many thanks for any input / suggestions.

Many thanks in advance

John
 
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Glad it didn't further soil the sheets.🏁👍🏻

There are clear signs of contact there. Discoloration. And it should NOT be contacting there.
And, that is where the cracks are. NOT fatigue, but impact.

The thrust load is SUPPOSED to be born on the rollers.


Geometry error is the reason. I'm being as polite as possible there.

Pictures of the crankshaft journals please.


If the shoulder diameter is too great, that would cause this problem. File photo & example below.

1691791380518.png


Magnetic drain plugs and ROUTINE scheduled oil samples are useful items for being alerted as this begins.
 
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Yeah true.. I agree there should be no load on that surface - that's why I included the original end float figure.
I'll post some pics of the crank journals, but will make some comments..
1. Because of the custom nature of many of the components in this engine (one piece crank, SBR cases, Jim Comstock's reverse entry steel rods, a precision engineer friend and I carefully checked for alignments, end floats and rod centralisation in bore during assembly with specialist precision measuring tools. SBR cases are designed for the wider Mk3 length crank, and my one piece crank is the pre-Mk 3, which necessitated a special precision made spacer to reduce end float, AND centralise the crank vis a vis the bores.

But I did make a mistake once... It was a Tuesday, as I recall 😁

2. The rubbing /blueing could have occurred AFTER / as a result of cracking..

3. Re your image... Yes, I agree in concept... But surely if the crank has the required endfloat AND is centralised to the bores, then in my application the dimension of that shoulder is of little consequence (remembering that I'm running a pre-Mk3 crank in wider Mk 3 dimension crankcases...)
 
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johnfitzw said:
Yeah true.. I agree there should be no load on that surface - that's why I included the original end float figure.
I'll post some pics of the crank journals, but will make some comments..
1. Because of the custom nature of many of the components in this engine (one piece crank, SBR cases, Jim Comstock's reverse entry steel rods, a precision engineer friend and I carefully checked for alignments, end floats and rod centralisation in bore during assembly with specialist precision measuring tools...
But I did make a mistake once... It was a Tuesday, as I recall 😁

2. The rubbing /blueing could have occurred AFTER / as a result of cracking..
Click to expand...
Nope. The contact/bluing is prior. See it more pronounced on the raised/broken part, yet still there on the lower height.

No discredit nor disrespect implied. Just nuts & bolts, no feelings. 🍻
Show the crankshaft, and the shoulder will be equally scored.
That crankshaft raised shoulder should be LESS than the inner race shoulder O.D.
Probably not a full-on "mistake", rather, taking a leap of faith the crankshaft was machined properly.
For a long time, I've said: "show me a person who's never made a mistake, I'll show you a person who's never DONE anything"
 
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johnfitzw said:
I edited my reply with additional information...
My apologies.. Many thanks for the response.. 👍
Click to expand...
"3. Re your image... Yes, I agree in concept... But surely if the crank has the required endfloat AND is centralised to the bores, then in my application the dimension of that shoulder is of little consequence (remembering that I'm running a pre-Mk3 crank in wider Mk 3 dimension crankcases...)"

Read on, only if you're still fact finding, not on defense.🍻🤝🏻

The mistake occurred, when you checked end float, it was contacting the machined shoulder on the crank, rather than BEARING shoulder to roller, as designed.
Again, it's easy to see how it was missed.

Please measure the outside diameter of the raised shoulder on the crankshaft. Here is the bearing O.D. for reference.
Broken Main Bearings (Outer Race)
 
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johnfitzw said:
Pics of the bearing inner races in both sides..
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Click to expand...
 
concours said:
Read on, only if you're still fact finding, not on defense.🍻🤝🏻

Of course.. 👍 In the assumption of course that same spirit of problem resolution applies on both sides.. 😇😁
Click to expand...
concours said:
The mistake occurred, when you checked end float, it was contacting the machined shoulder on the crank, rather than BEARING shoulder to roller, as designed.
Again, it's easy to see how it was missed.
Click to expand...
Sorry, I don't follow.
If the bearings are assembled in the case and on the crank, the crank inserted into the cases and the cases bolted up, and a DTI fixed to the end of the crankshaft, you ARE measuring bearing shoulder to bearing shoulder end float. The crank shoulder can't influence the reading with this method.

concours said:
Please measure the outside diameter of the raised shoulder on the crankshaft. Here is the bearing O.D....
Click to expand...
Yes, see pic uploaded - within a few thou of the race in your pic..
But... Why, Mr Anderson.. Why? 🙄
 
If the bearing race is buried inside the bearing bore cavity then it is necessary to have the crank shoulder OD slightly smaller than the bearing OD. It seems that this might be the case shown in the pics, alas the crank shoulder OD seems a bit large. Any pics of the cases?
 
Oops, the cases are shown in the first pics, I should have looked up there too! It does not look like the crank rubbed on the case, the dirty contact surfaces clearly indicate relative movement between bearing and crank. I would recommend careful measurements of the bearing contact surfaces, also the bearing, with real micrometers, not vernier calipers, calipers can easily be off by more than required bearing fit tolerances.
 
kanenas said:
If the bearing race is buried inside the bearing bore cavity then it is necessary to have the crank shoulder OD slightly smaller than the bearing OD. It seems that this might be the case shown in the pics, alas the crank shoulder OD seems a bit large. Any pics of the cases?
Click to expand...
But, it is not contacting the cavity.
 
Perhaps, this cutaway view of a crankshaft bearing will help clarify the design of this type of bearing. The shoulder is where the end float should be determined.
Because your crankshaft has an oversize (diameter) shoulder, it has no clearance

1691799062112.png


I work with this stuff everyday, far more complex and precise. 😎

Broken Main Bearings (Outer Race)
 
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Pour yourself a drink, and wrap your mind around the geometry.
Reference the diameter of the shoulder on my '74 (OEM) crank.
1691800870394.png



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Being in manufacturing my whole life, seeing design, engineering, manufacturing fuggups is not at all shocking. 🤡
 

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I agree with you that something has either not been manufactured correctly...but cases or crank is still to be determined.

I'll check your suggestion of the shoulder.. Thanks for the pic.

"Because your crankshaft has an oversize (diameter) shoulder, it has no clearance"

Sorry.. I can't follow this logic.
How can the DTI indicate 10 thou end float and the crank spin freely if the shoulder is the first point of contact on the bearing outer race? If there is indeed 10 thou free float and the crank shoulder contacts BEFORE the race shoulder, then the inner race has MORE than 10 thou or it would be (likely) preloaded and zero end float would be detected on the DTI..

For context.. I was beaten to sleep daily with scientific measuring instruments for 47 years by a scientific instrumentmaker (my father) who handmade gyros, barometers, avionics guidance equipment back in the electromechanical days before computers and GPS. I had to make gear pinions and Vincent selector forks from stock on a tiny Myford lathe before I was allowed to eat dinner..
I know how a bearing works, and I know how to measure clearances...

But if it is indeed an oversize crank shoulder...and I agree that it's plausible, even likely... I don't understand how the crank had a measured 10 thou assembled end float, and the crank spun with an effortless flick of the fingers...

I'll report back..

Many thanks all.. 👍
 
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johnfitzw said:
I agree with you that something has either not been manufactured correctly...but cases or crank is still to be determined.

I'll check your suggestion of the shoulder.. Thanks for the pic.

For context.. I was beaten to sleep daily with scientific measuring instruments for 47 years by a scientific instrumentmaker (my father) who handmade gyros, barometers, avionics guidance equipment back in the electromechanical days before computers and GPS. I had to make gear pinions and Vincent selector forks from stock on a tiny Myford lathe before I was allowed to eat dinner..
I know how a bearing works, and I know how to measure clearances...

But if it is indeed an oversize crank shoulder...and I agree that it's plausible, even likely... I don't understand how the crank had a measured 10 thou assembled end float, and the crank spun with an effortless flick of the fingers...

I'll report back..

Many thanks all.. 👍
Click to expand...
🍻
 
johnfitzw said:
Gentlemen and Ladies... I've come across a problem with a relatively low mileage (30,000 miles) pair of NJ306E C3 main bearings in my round-the-world Mk2A 850 engine that I've not experienced before. I'd be grateful for any suggestions / insight..

I've just shipped the bike back to San Jose, CA from Lima, Peru after my 2021-Oct 2022 trip from here to Ushuaia then back to Lima. In Lima the drive side piston skirt cracked and detached itself, and as I was out of time anyway, I decided to ship The Norton back to San Francisco.

Upon stripping the engine yesterday, I was surprised to see that both drive and timing side NJ306E C3 mains outer races were cracked (see the images), tho intact. There had been no indication of main bearing trouble (noise /roughness) while riding in Central & South America. The bearings were new, sourced from RGM, and fitted to my SBR billet crankcases, with dynamically balanced Ro-Dy one piece crank in May 2021. End float was set at 10 thou. The interference fit between mains and case bearing bore before assembly was on the tight side, at 0.0029, but the inner bearing sleeve still slid easily, nicely and smoothly inside the race when assembled cold.

Attn Mike in NZ - did you have any main bearing assembly issues with your SBS cases?

On the entire trip the engine was never spun above 6000 rpm, cruising rpms were typically around 3500 to 4000.

I've owned this bike for nearly 50 years now. I come from a background of motorcycle engineering, and have rebuilt many Commando engines. But I've not seen a cracked low mileage "Superblend" before..

PXL_20230810_235655552~2.jpg

PXL_20230810_235655552~2.jpg

  • Media owner johnfitzw
  • Date added

PXL_20230810_235705669~2.jpg

PXL_20230810_235705669~2.jpg

  • Media owner johnfitzw
  • Date added


Many thanks for any input / suggestions.

Many thanks in advance

John
Click to expand...
Hi John,
Who are (were) SBR?

Thanks,
Ed
 
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