Should I replace the crank Bearings on my 850 Commando??

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Mar 1, 2007
I am in the process of restoring a 1974 850 Commando. The bike only has 2000. original miles on it but the previous owner left it in a humid leaky shed for about 12 years and never started it. It is a cosmetic mess but the cylinders and heads look great as well as the inner primary. I am restoring the bike to original and I was wondering if there is any reason to replace the crankshaft bearings if the existing ones look good. If you think I should replace them what type and from who do I get them??? Should I replace the rods while I have it apart??? I plan to replace all engine seals. I want it to be a nice looking rider and not a show bike. I don't plan on hot rodding it around or doing any special engine mods except for maybe a Mukini and Boyer. Thanks fo any suggestions. :D :D :D :D :D
2000 miles , Oh No don't change the bearings.Unless there are rust pits. But do clean the oil tank and the sludge trap in the crank. Replace all the rubber bits. If you have been lucky the oil will have left the tank and filled the case protecting the bearings. The trans now that will need a new lay shaft bearing for sure and a good look for pitting.
I would not replace the roller main bearings, but if you have the rods apart, I would polish any nicks out of the rods and replace the rod bearings using new rod nuts. The rod bearings may be OK, but I don't like to tear an engine down that far and take a chance for $50 worth of bearings.

My question would be, why was it parked with only 2000 miles? I'd be very careful about making any assumptions with such a bike. I'd be checking all rotating parts with a bright light and magnification.
Mileage doesn't mean anything. Rust never sleeps.

My 850 only had 5K miles on the clock but the right cylinder had heavy rust etching and the sludge trap was full of rust. You just never know unless you pull it apart and check everything.


I would definetly replace the bearings as I doubt the motor has been turned over regularly and sitting in one position for that amount of time is no good.
I think Your best bet would be to buy a bottom end rebuild kit which includes 2 x superblend main bearings,a crankshaft fixing kit, big end shells + big end nuts & bolts.
RGM sell these kits for a reasonable price but I dare say you could purchase in the states as well.
It could be false economy to use the old bearings as you could end up doing the rebuild again. :)
As regards carbs I run 3 commando motors on single 34mm MK2 amal carb kits with great results also available from RGM as a complete kit.
Good luck with your rebuild.
If your main bearings are original then bin them. Get FAG NJ 306 C3's (Superblends). They are the heart of the motor, Dont try and save money there coz the originals were crap.
OBO in town said:
If your main bearings are original then bin them. Get FAG NJ 306 C3's (Superblends).

As the bike in question is a 74 model, then it should originally have had the the FAG Superblend bearings?
You are right, Les. And the Norton affinity to wet sump should have covered the bearings in oil. Assuming the PO didn't drain the oil before he put it up.

I would definitely pull the entire engine down and clean the sludge trap and replace the rod bearings, but if there are no pits on the main bearings I would have no hesitation to re-use them. Of course, if there is any doubt when inspecting, by all means replace them.
OBO in town said:
FAG NJ 306 C3's (Superblends).

Also worth remembering that the correct bearing to use is the brass cage FAG NJ306*E* C3.
Main Bearings

Thanks for the advise guys. She did wet sump and when I split the cases yesterday the bearings looked perfect. It is a 1974 with only 2000 original miles on her. I am a poorboy so I guess I will pass on the mains but definatley split the crank and put in new rod bearings. Thanks again===Mark C
When did they begin using the superblends in stock 850s? Mine is one of the first 73s - #427 to be precise. Thanks - BrianK
BrianK said:
When did they begin using the superblends in stock 850s? Mine is one of the first 73s - #427 to be precise.

All 850s should have originally been fitted with Superblends.
Mark C, Don't forget to clean the oil tank. The crud inside will tell the story of the engine care. Look on the bottom of the tank real well.
Oil Tank

Thanks for the info. I looked inside after draining it of oil and it looks good with minimal sludge. I am going to put some drywall screws and kerosene in it to clean it real good with a long rattle shake and multiple wash as suggested this weekend when it is warmer. This method worked wonders on my slightly rusted tank. It made it shine inside---Thanks for the advise---Mark C.
Les, thanks very much for your reply. One less job to undertake, especially one of that magnitude, is welcome news indeed! - BrianK
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