850 Commando Timing Chain Installation
Article by Mark Trotta
After the pinion gear is on, it's time to install the timing chain, cam sprocket, intermediate gear, and tensioner.
Tensioner and Plates
For proper adjustment and stability, it is important that the backing plates are installed correctly.
The inner bracket is thinner and installs long end facing down.
The tensioner goes on next. If the rubber face is worn, replace it.
Pictured: New tensioner on left, old one on right.
The outer bracket is thicker and installs long end facing up.
Studs and nuts are 5/16 UNF with internal (fan disc) washers.
Install Woodruff Key
Pictured below: camshaft sprocket woodruff key not yet fully tapped in.
The flat end of woodruff key faces engine.
Camshaft and crankshaft keys are the same.
Valve Timing Marks/Lining Up Sprockets
When the chain assembly is ready to go on, the painted factory mark on the intermediate gear should line up with the pinion gear punch mark.
There are also punch marks on both the intermediate gear and cam sprocket. These are the valve timing marks.
The punch marks will face up during install, but they're difficult to see once you wrap the chain around them.
10 Pins Between The Dots
Position chain on the gears so that there are 10 pins between the two marks.
Timing chain, intermediate gear, and cam sprocket are installed as an assembly.
Cam Nut Socket Size
The cam nut measures 1.01" across the flats. The factory manual calls for a 9/16 Whitworth socket, but a standard 1" socket works just as well.
To prevent the intermediate shaft from flexing while tightening cam nut, a support plate is necessary.
Pictured: Norton 06-7325 Intermediate Shaft Support Plate Tool
Proper Chain Tension
The factory manual calls for "a maximum of 3/16" up and down movement at the tightest spot of the chain".
With the two nuts snug but not fully tightened, slowly rotate crankshaft assembly and check for tight spots. Here, I took a small piece of tape to temporarily mark the tight spot in the chain.
Measurement was done with a 6" machinist's rule.
With 3/16" chain movement at the tightest spot, there was 1/4" chain movement at the slackest spot.
Chain tensioner nuts were then final-torqued to 180 inch/pounds, which is equivalent to 15 foot/pounds.
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Notes and Misc
Before installing, the chain was allowed to soak in engine oil for several days.
Because of it's high zinc content, Valvoline VR1 racing oil is an excellent choice for classic British motorcycles. The silver container pictured is conventional oil, and has about 1400 ppm ZDDP. Also available is full synthetic in a black container, which has about 1200 ppm ZDDP.
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