Deets55 said:
Check to see if there is a gasket between parts #29 and #27. It does not show up on some (maybe all) parts diagrams. It would be part # 06-6570. Also make sure part # 29 is flat.
http://www.oldbritts.com/1975_g10.html
Pete
Double ditto on Pete's opinion. No gasket or a warped plate will only exaggerate the problem. I'm not sure how the plate gets warped but I found mine not flat. Also pay close attention to the proper torque values for reassembly.
Here is a article I wrote a few years ago on the MKIII primary chain tensioner.
Please keep in mind this only applies to MKII's
Tuesday, April 29, 2008
The Continuing Adventures of The Buckeye Rider
The Hydraulic Primary Chain Tensioner
As Fitted to the MKIII Commando
‘Pa’ my trusty Norton is a MKIII model from 1975, this makes him different from all earlier Commando models because the engine and transmission are fixed in place and the slack in the primary chain is not adjusted by moving the units away from each other. This is taken care of by a two piston vertically opposed self-filling hydraulic tensioner. The tensioner expands automatically as the chain wears keeping the chain tight and the power transmitted from the engine to the gearbox constant. Or that’s the way it’s supposed to work!
When spring arrives in South West Ohio that means its time to go to the shed and perform the annual inspection and major service. While I’m inside the primary chain case taking care of business I notice that the chain is rather slack and decide to have a go at the tensioner and see what it’s about. The only mention in the service book is that the tensioner requires no maintenance. The book should be amended to read “Service Every Thirty-Three Years”.
The tensioner pistons were not filled with oil as they were supposed to be so the only force they had to tension the chain was from the small internal springs. The tensioner was removed from its mounting studs as a unit and the top and bottom portions were marked so as to install them later into there original bores.
Inside the piston is the spring and a white plastic mushroom headed spacer. The spacer is inserted into the piston small end first so the big end is toward the spring. This spacer is used as a damper to slow the passage of oil into and out of the cavity created by the shape of the spacer inside the piston. There is a very small bleeder hole through the piston that leaks oil into the cylinder bore and lubricates the outside of the piston and the bore.
Connecting the two vertically opposed cylinders is a small orifice so the two units can share the oil from a common reservoir.
The shape of the top half of the tensioner and a dual-purpose cover plate forms the reservoir. At the lower end of the reservoir is a simple ball and spring one way check valve that lets oil pass into the tensioner.
When all is working as planned the tensioner is self- adjusting, self- bleeding and maintenance free. Except when the pistons accumulate sludge and the tiny orifices get clogged up and small pieces of I don’t know what fall into the oil reservoir after 35 k miles.
The tensioner was carefully disassembled, cleaned, inspected then studied to understand how it all works. The inside of both parts of the case were also cleaned and the primary chain inspected and replaced.
To assemble I turned the empty pistons upside down and filled them with oil then inserted the white plastic spacer (small end first) then the spring. Then the top bore was filled and the piston inserted then the dripping mess was inverted and the bottom bore filled and the piston installed. The tensioner is now filled and there should be no air inside but the unit cannot be depressed enough to install it on the mounting studs and make it fit between the two rows of chain. While holding the tensioner unit with the fingers of one hand and applying slight pressure to compress the pistons take a small pick and depress the one way check valve just enough to let the tensioner slip in between the chain snuggly. Install a new gasket ( p/n 06-6570 not illustrated in the parts book) and the dual purpose cover plate and tighten as directed. Fill the reservoir with oil.
The tensioner should now be very tight on the chain, allowing the two pistons to move up and down as a unit but be unable to be compressed.
The cover has two purposes. One is to form the oil reservoir for the tensioner the other is to catches the oil that is flung by the chain and directing it into the reservoir, keeping it full.
Fill the primary case with 7oz. (200cc) of the oil of your choice; we won’t get into an oil thread here. I have used both ATF and several weights of motor oil with the differing results. But the “correct” oil would be whatever you have in the engine. (IMHO)
The corrected primary chain tensioner was noticed instantly with a smoother and quieter idle and smoother operation at cruising speed and a noticeable increase in performance when torque was demanded.
If, after many years of service, the primary chain is in need of replacement this is the perfect opportunity to replace the crankshaft main seal. After all the work involved in removing the primary chain the main seal is only another fifteen minutes deeper behind the alternator.
Is there anything as fulfilling as riding a British motorcycle at speed when you have done all the work yourself, and everything is working as it should in harmony with all the other parts?
Ride On
Dave
© Dave Mathers
The Buckeye Rider
Cincinnati, Ohio
4/29/2008
