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Commando Mk lll Questions #784495 09/16/19 6:18 pm
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Samsmc1 Offline OP
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I'm curious about the timing cover on a Mk lll. Does it really stop the crankcase from wet sumping when it's not running?
Is the Mk lll timing cover interchangeable with the earlier (70-74) covers as long as you use the later oil seal on the pump?
Last - why is the timing idler sprocket different between the Mk lll and the earlier models? Are they interchangeable?
Thanks
Sam

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Re: Commando Mk lll Questions [Re: Samsmc1] #784503 09/16/19 7:33 pm
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The MKIII timing cover has a non-return valve built into it. This valve is machined into the inside of the timing cover, behind the small bulge you can see in the surface of the cover.

The valve does work as long as the bike is ridden regularly ... so that the valve plunger does not seize. If the plunger seizes, it will usually fail safe and stick in the open position, meaning that it will not stop oil flow (but neither will it prevent wet sumping in such case). I've seen lots of the plungers stuck in the open position, which is considerably better than being stuck in the closed position and causing an engine failure due to oil starvation.

The MKIII covers are interchangeable with the older covers with a few mods (like the flat pump seal to allow the plunger to seat). I even updated a very early 20M3 Commando engine with a MKIII cover years back, though on that one I had to machine back the boss in the timing cover that supports the idler gear shaft.
.. Gregg


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Re: Commando Mk lll Questions [Re: gREgg-K] #784509 09/16/19 8:04 pm
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Lannis Offline
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The non-return valve on the MKIII does work unless, as Gregg says, it sticks.

For a surprisingly small amount of money, an outfit called AMR-Tucson will take your timing cover, replace the "bucket"-shaped piston with a ball, machine the surfaces, and otherwise modify it so that it always works and never sticks. I've done it to mine and it's great; no blocking the pump inlet with a check valve and risking your engine, no putting a valve in the inlet line and forgetting it or the interlock failing.

Lannis


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Re: Commando Mk lll Questions [Re: Lannis] #784520 09/16/19 11:33 pm
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Jon W. Whitley Offline
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Originally Posted by Lannis


For a surprisingly small amount of money, an outfit called AMR-Tucson will take your timing cover, replace the "bucket"-shaped piston with a ball, machine the surfaces, and otherwise modify it so that it always works and never sticks. I've done it to mine and it's great; no blocking the pump inlet with a check valve and risking your engine, no putting a valve in the inlet line and forgetting it or the interlock failing.

Lannis



Wow, excellent advice. $80 + shipping for just that mod alone is quite fairand a no brainer. I know what I am having done over the winter...thanks Lannis !


Jon W.


1957 6T Thunderbird 650
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1971 TR6R Tiger 650
1970 BSA A65F 650
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Re: Commando Mk lll Questions [Re: Samsmc1] #784703 09/18/19 6:35 pm
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HawaiianTiger Offline
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Jon, while you're at it, send your tach drive to them for a proper lipped seal mod. Those buggers always seem to leak otherwise.
Cheers,
Bill


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Re: Commando Mk lll Questions [Re: Samsmc1] #784704 09/18/19 6:47 pm
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gREgg-K Offline
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Fortunately, the good people at Andover Norton seem to have "read the memo", and the new tach drive couplings they sell now come with a proper lip seal fitted.

.. Gregg


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Re: Commando Mk lll Questions [Re: gREgg-K] #784705 09/18/19 6:53 pm
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Samsmc1 Offline OP
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I had forgotten about that mod to the tack drive.
Can anyone tell me what size seal they use - I can modify the housing easily enough if I have the seal.
Thanks
Sam

Re: Commando Mk lll Questions [Re: Samsmc1] #784706 09/18/19 7:19 pm
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Originally Posted by Samsmc1
Can anyone tell me what size seal they use - I can modify the housing easily enough if I have the seal.


The AN seal is 6 x 15 x 4mm although 6mm seems a little tight on the spindle.

https://andover-norton.co.uk/en/shop-details/16978/oil-seal-tacho-housing

Another seal often used for this purpose is Honda part 91207-222-000 (7 x 14.5 x 5mm).
https://www.oldbritts.com/11_065200a.html


The seal I used was 7 x 14 x 4mm.

Re: Commando Mk lll Questions [Re: Samsmc1] #784707 09/18/19 7:19 pm
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gREgg-K Offline
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Sam, I can give you the generic part number of the seal.

The last seals I brought in were made by SKF, and came under SKF part number 2450. I'm pretty sure National or some of the other big names do it as well.

... Gregg

Last edited by gREgg-K; 09/18/19 7:20 pm.

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Re: Commando Mk lll Questions [Re: Samsmc1] #785035 09/22/19 2:40 am
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Samsmc1 Offline OP
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Gregg
Thanks - I have 3 of those puppy's coming.
Sam


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