850 Commando Bottom End Assembly
Article by Mark Trotta
Norton Twins first appeared in 1948 with the 497cc Dominator, which grew into 596cc, then 646cc, 745cc, and finally 829cc, which we know as the 850.
Main Bearings
Like all Norton Twins, Commando engines have left and right main bearings. Standard issue on all 850's were 'Superblends', part # NJ306E and manufactured by F.A.G.
The factory-installed Superblends have a brass cage with a double-lipped outer race that retains the roller cage.
The inner races are single-lipped and are attached to the left and right crankshaft.
This 1974 Roadster was a very low mileage bike, and it shows. The bottom end was clean and main bearings were in very good shape.
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Crankshaft Assembly
With all needed parts rounded up and everything neat and clean, it's time to begin engine assembly.
Read: Sludge Trap Cleaning and Crankshaft Reassembly
Read: Crankshaft Journal Polishing By Hand
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Connecting Rods and Bearings
Original Commando connecting rods were manufactured from high quality aluminium. The bottom caps are cast iron.
Connecting Rod Polishing
Any rough edge on the rod (like a nick) is considered a 'stress riser' and should be removed as a safeguard against possible cracking.
One of the rods was found to have a few small nicks in it.
After sanding out the nicks with Emory cloth, they were wet-sanded with 240 grit, followed by 400 grit.
Any sanding should be done lengthways, up and down the beam and not across it. You really don't want to remove any metal, just make it smooth.
The rod caps are steel and do not get polished.
Final polishing was done on a buffing machine with a 30-ply buffing wheel.
The bottom end of your motor relies on oil for lubrication and reducing friction.
By holding rod up to light, you can check that oil holes are clean and clear.
Replace Con Rod Bolts?
There's always potential trouble when knocking the old bolts out--and getting correct quality replacements--and having them go in correctly.
Quoting the factory workshop manual: ...it is unnecessary to renew the connecting rod bolts though new nuts should always be used."
So, the big-end bolts were left alone, with new nuts installed.
On high mileage bikes, or if it looks like the bottom end has been gotten into before, replacing the bolts may be wise.
Con Rod Bearing Inserts
Norton Twin connecting rods have bearing shell inserts. There are two different ones--the upper has an oil hole which goes into the connecting rod. It has a tab which should mate with the rod.
The lower bearing shell has no hole, and goes into the rod cap. It also has a tab which should mate with the cap.
Con rod nuts torque to 25 lbs/ft. Correct socket size is 1/4 Whitworth.
When installing, oil holes on rods face outward.
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Assembly Lube
During initial engine fire-up, assembly lube protects internal parts for those crucial 10 seconds before the engine can establish oil pressure.
For those who don't have a favorite assembly lube, read this article.
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Install Crank Assembly In Case
A simple work stand was made of 2x4 wood pieces, and it helped make engine assembly a little easier.
Setting the drive-side case on the wooden box, the crank assembly was gently lowered into position, making sure the con rods clear the spigot.
CAUTION: The alloy rods can be easily damaged by knocking into the sharp crankcase mouth. Protect them with whatever you have handy--rubber hose, bubble wrap, etc.
Gasket Sealant
Crankcase halves receiving a coat of anaerobic gasket maker.
Permatex 51813 sealant works very well with split engine cases.
Allow sealant 3-5 minutes to get tacky.
Camshaft Install
Camshaft thrust washer is installed with taper to the tach worm drive.
NOTE: On earlier Commando engines (between 200000 and 300000), there is an additional flat thrust washer with spring on the other end of the camshaft.
The timing-side case was lined up and carefully set down over drive-side case. A little case wiggling was needed.
After checking that the camshaft and crank assembly were rotating smoothly, crankcase fasteners were installed and tightened.
Additional bolts were temporarily installed in front and rear mount bosses.
Sump Filter and Drain Plug
By removing the circlip, the gauze inside the crankcase sump filter may be cleaned or replaced.
A special spanner wrench removes and installs the sump filter. A 38mm socket also works.
The oil drain plug removes/installs with a 9/16" wrench or socket.
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