Inside Big Red - 120,000 mile 850 Commando

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My friend Ian has disassembled his high mileage 850. This is the first time the bottom end has been apart so all of those parts are original and have done the full 120,000 miles.

Here's his note to me

Hi Glen,


Feel free to share with your blog (please send the link). This is after 120k miles. I got the bike at 13k miles (40 years ago this June), new pistons (+0.020”) and rebuilt head at 55k or so miles. I used to change the oil and filter every 1,000 or 1,500 miles so I suspect that made the difference. I think the deposits in the timing cover are from the Castrol straight 40W I ran for years.





 
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Its showing wear down the bottom end but its not to bad for the miles its done, changing the oil at lower miles do help in long life, good on him.

Ashley
 
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Really good condition for the miles.

Those are trimetallic big end shells, the top shell has suffered from losing oil film between it and the journal, so the top plating layer has been removed with contact with the journal. This top layer serves two purposes, embedibility ie the ability to absorb hard particles and also protection to stop the acid build up in the oil attacking the lead in the copper/lead layer. The lower layer shows little if any damage from particles so the oil filter did its job so the loss of embedibility was not an issue. If you look at the dark areas on the upper shell they show as 2 circles, this could be the wet sumping oil slowly leeching the lead from the bearing at this point, when you turn the engine off the crank will stop mainly in one position as it comes up against compression so the hole in the journal will be in one spot. These lead free areas would be the first area's to fail if the engine did more miles.

So, oil filter definitely doing good, oil change frequency has kept the acid levels low or the whole top bearing would be dark.

The oil needs to have a higher oil film strength to reduce the top shell contact with the journal so the plating lasts longer, or is the oil not getting to the big ends immediately on startup so there is contact until the oil is present, maybe from the straight 40W being too thick until warm or the bearing to journal clearances have grown allowing too much oil to exit.

Be interesting to have the journal sizes.
 

baz

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Looks better than some 10,000 mile commandos!!
Especially the cam and followers
 
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Top half of rod shell bearings appear to have all the babbit worm away down to the copper substrate. If that is in fact the case, I would say it was long past due a rebuild - yikes!
 
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Looks better than some 10,000 mile commandos!!
Especially the cam and followers

I've seen 1500 mile rebuilds gone wrong posted here that looked a lot rougher than this.
The cam and followers look good enough to continue on for another 120,000.



Glen
 
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The other half of the story besides the mileage and the frequency of oil changes would be how the owner rode this bike... ?? As has been said, the big end shells are showing significant wear, but probably reasonable wear for a bike that shakes up and down for 120,000 miles... Pretty cool to see a bike with that much original mileage apart and in reasonably good shape.

I also am an oil change fanatic, but my bike has been completely apart once (superblend change) and the barrels have been off twice since then for a lost follower pad and a piston circlip which popped out. I hope to make it to 50,000. without a total rebuild and am at 28,000 now.... those pictures give me hope!
 

grandpaul

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I believe there are actually quite a few more of these long mile bikes, just that their owners aren't on the forums...
 
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Piston skirt thrust surfaces look pretty damn good. Would be good to hear about the clearance between the ring and groove for the compression and second ring to see how far things have opened up.

I have to agree, other than the bearing shells, looks good.
 
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I'll see Ian this morning at breakfast and will get some additional info.
I wonder what sort of lifespan the makers were designing for?
I know with that Phil Irving commented in the 70s that he and Phil Vincent never expected those bikes to be still in use at that late date. They figured ten years at ten thousand miles per year then throw it away and get something new.
And that was considered to be a long life machine compared to its contemporaries.

I did cheapie top end rebuilds on a dozen or so local Hondas in the early 70s. Those were all 2-5 year old 10-15,000 mile bikes and they were in sad shape already. With a cylinder hone job, new rings and valve lap the owners would squeeze another 8-10,000 miles out and then they were truly junk.

Glen
 
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That layer of plating that disappeared off the top bearing is microns thick, it was an Indium/Lead plate and does not act as a bearing but a soft overlay for grit absorption, acid protection and the Indium acts for the first few miles as an anti seize agent. Yes it was wiped off but dimensionally the bearing would not be too much smaller and the bearing material is the leaded bronze that was revealed but now unprotected. It is recommended that as soon as any bare bronze is seen the shell is changed but that is due to the acid attacking the lead in the bronze, not because the bearing is worn.

Current Tri Metallic bearings can have a hard Tin overlay, a softer Tin lead overlay, softest Indium lead or the Ferrari requirement of silver which is unique to them. All attempt to achieve the same requirements. Fitting a filter means you can get away with the harder overlays or just go bimetallic Aluminium Tin which does not need protection from acids but does not have the higher fatigue strength of bronze which is needed for racing.
 
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In the bearing industry Babbit is used for the Whitemetal shell bearing alloys made from Tin and Lead, they do not use the term for the Aluminium/Tin and the Leaded Bronze shell bearings and they are not included in the Wiki page so confirming this.

So the bearings in the engine and all of the replacement bearings currently made for Norton twins are not Babbit.
 
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From wiki:

"In more modern practice, the crankshaft and connecting rod big end bearings in a modern automobile engine are made of a replaceable steel shell, keyed to the bearing caps. The inner surface of the steel shell is plated with a coating of bronze which is in turn coated with a thin layer of Babbitt metal as the bearing surface."

"it was an Indium/Lead plate and does not act as a bearing but a soft overlay for grit absorption,"

So it sounds like technology has moved past the nomenclature of the past. Though Wiki is not an in depth treatise it was offered up as providing some context. After reading the Wiki I now appreciate the distinctions of the shock load durability of the different types babbitt in plain bearing applications.

Bottom line for me is if the bearing shows the copper looking substrate it has lost much if not most of its debris embedding protective ability - it is in need of replacement.
 
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Its like Hoover, its a vacuum cleaner, but because it was so dominant all vacuum cleaners became hoovers, but they are not all hoovers.

Babbit was the first alloy shell bearing but its now defunct except for old engines that need it poured molten into the big end, but they are a silvery bearing alloy. 95% of all shell bearings nowadays are Aluminium/Tin, a duller silvery bearing material than is mistaken for Whitemetal/Babbit so the confusion continues. Worst bit is there are NOS Whitemetal BSA 250 and the 500/650 Twin bearings around which are an accident waiting to happen, the material is wrong for the application courtesy of Vandervell. There are also perfectly adequate Al/Tin versions around so the confusion can be the root cause of a blown engine when the Whitemetal is picked over the Al/Tin. There were also 500 Triumph twin whitemetal bearing around but the higher turnover means they are too scarce to be a problem.
 
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Many of the the heavy industrial applications of poured and hand scraped babbitt bearings have now been eclipsed with large roller element bearings.
 
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