I found an internal tensioning device in my Mk2 chaincase.

Just starting disassembly of my Mk2 (314219) and discovered this device in my primary chain case. Have you ever seen this before? Is this an aftermarket item (it looks nicely done)?

This Mk2 arrived with a Mk3 cradle, bolted to a Mk3 gearbox (shell, anyway via serial number). and converted to right hand shift. Here's more: the rear cradle's hex support pin is nicely aligned with the backside of the factory Mk2 chain case, and there is no 1" cutout for the right hand shift as I might expect if this is a factory Mk3 rear cradle.

Depending on opinions from this group I will look for an original inside chain case, adjustable rear cradle with Mk3 cotters (aftermarket upgrade) and avoid potential damage to the bike.

That’s an interesting mod!

My comments would be:

1. Chains are tensioned in that way in the vast majority of automotive applications, so I do not believe it should be viewed as a botch job.

2. It looks like proprietary parts have been used for the mod. Again, not a botch job.

3. The bennefit to the mk111 cradle is that the gearbox is fixed and cannot slip in its slots.

All in all, if I were in your shoes, I’d leave it be.

Only IMHO of course.

Ditto.

The only downside to the tensioner as installed is that it has a static fixed adjustment, and will require manual adjustment as opposed to the MK3's automatic hydraulic tensioner.
But a definite improvement to the standard pre-MK3.

A few more points - the nuts are a perfect Whitworth fit. There are grooves on the surface of the tongue against the chain, mostly consistent but most at the end of the tongue - wonder if it was flapping around a bit from loose adjustment.

I may bend and bond a medium Derlin layer on it unless everyone is fine with metal to metal. I looked for metal shards but nothing of the sort in the oil (motor oil I am pretty sure) that came out.

As long as you are keeping oil in there it shouldn't be a problem.

A most interesting find indeed.
Ideally it would have one on the top pushing up and the bottom one pushing down. This would insure the bottom links would be in the oil bath.
That is how the OEM hydraulic tensioner works for the MKiii.
The oil level in the primary is not very deep and this set-up IMHO would leave the chain out of the oil bath.
But what do I know? Wish I would've thought of this.
Ride On
Dave

It does look like there's a bit of rust on the chain. Maybe the installer didn't increase the amount of oil enough to get the chain to run in the oil bath.

The Buckeye Rider said:

A most interesting find indeed.
Ideally it would have one on the top pushing up and the bottom one pushing down. This would insure the bottom links would be in the oil bath.
That is how the OEM hydraulic tensioner works for the MKiii.
The oil level in the primary is not very deep and this set-up IMHO would leave the chain out of the oil bath.
But what do I know? Wish I would've thought of this.
Ride On
Dave

Click to expand...

So the oil level is below the chain at the clutch basket?

swooshdave said:

So the oil level is below the chain at the clutch basket?

Click to expand...

Yes I believe so. If the bottom of the clutch basket was submerged in oil the "dry" clutch would me swiming in it.
From my observation on the MKIII the chain is scimming the oil when at proper level.
But I have been proven wrong on more important things then this!
Ride On
Dave

looks like there are a few screws(?) securing the base to the bottom of the primary? bonded? looks like an angle corner piece going up the side..

wouldn't want that or any part to break free

dunno if the screw adjustment part is robust enuff, how is it secured to the base?

the metal on metal part i'd think is problematic
https://www.classicbritishspares.co...nsioner-blade-70-8310-68-0244-1962-72-a50-a65

Jdub said:

I may bend and bond a medium Derlin layer on it unless everyone is fine with metal to metal. I looked for metal shards but nothing of the sort in the oil (motor oil I am pretty sure) that came out.

Click to expand...

Metal to metal is not a problem, when first run the side plates will wear grooves in the tensioner plate, once these grooves are deep enough the rollers will start running on the tensioner plate and roll over the unworn sections and the sideplates will stop deepening the grooves.

Jdub
It,s very similar to what I did on my Mk3 in 2012 but using a Harley Sportster manual tensioner with a blue nylon pad contacting the chain and single bolt adjuster underneath like yours which is plenty strong enough for the job.
Manual adjustments are minimal with a triplex and probably even less when I fit the Chainman Duplex chain.
I did a short video of it idling with the outer cover off and running really smooth. Now with 10,000+ trouble free miles to date.

Don,t worry about lubrication, it picks up plenty with the usual 200cc just like the e start assembly, so no need to increase the level.
Only thing you need to do is tie wrap the alternator wires to the mounting pillars to keep them away from the chain.

Excellent mod by the way, don,t know why Norton didn,t do it like that from Day 1 rather than moving the gearbox back and it also allowed me to junk the awful Mk3 auto adjuster, which did nothing of the sort as time went by...
Find the bloke who did all this and treat him to a few beers, alot of thought and work has gone into that.

It looks like an "original" piece of kit though I know it is not - at least for a Commando. It appears to be cast iron or something similar - certainly not something that could easily be made in a back yard machine shop. Though I certainly don't know this to be the case, I think it was either a standard part for SOME engine back in the day that was adapted for use on that bike or it was an aftermarket part available at the time for that purpose. If I had a MK3 setup that the OP described, I'd keep that tensioner as is!

Very similar to how Triumph and BSA tensioned their primary chain. I'd leave everything as is unless you want to put it back original for some reason. Good job by some unknown tinkerer.

Lots of explainations except for the unslotted MK3 carrier. How is that explained?

Jdub said:

Just starting disassembly of my Mk2 (314219) and discovered this device in my primary chain case. Have you ever seen this before? Is this an aftermarket item (it looks nicely done)?

This Mk2 arrived with a Mk3 cradle, bolted to a Mk3 gearbox (shell, anyway via serial number). and converted to right hand shift. Here's more: the rear cradle's hex support pin is nicely aligned with the backside of the factory Mk2 chain case, and there is no 1" cutout for the right hand shift as I might expect if this is a factory Mk3 rear cradle.

Depending on opinions from this group I will look for an original inside chain case, adjustable rear cradle with Mk3 cotters (aftermarket upgrade) and avoid potential damage to the bike.

Click to expand...

I'll bet that tensioning shoe was and perhaps still is available from good engineering factors. I would reckon the back mounting plate is the only bespoke part. A very good mod indeed.

Dave

Nicely done mod !
If i'd discovered it I think I'd do the following..
As you say a layer of Delrin. It wouldn't hurt and may make it quieter, but more importantly iron filings ain't going to the alternator much good.
Maybe look to use a strongish spring instead it the adjuster bolt. Then I'd also support the outer side of the pivot screw, as on the overrun there is probably quite a load on that single fixing point.

Cheers,

cliffa.

Ok, I admit maybe I am too slow to get it, but where in the chain case is the proper amount of chain slack that is taken up as the
baskets heat up? I don't see any sag in the upper chain. This seems at first glance like a good solution that leads to a larger damage problem?

Not related to this primary discussion but tension similar - years ago I installed
a spring loaded timing chain tensioner. Two years later when Jim Mosher of Performance Indian (Commando built record holder) rebuilt my motor he
pointed out the oblong damage to the intermediate spindle caused by too tight constant timing chain tension the spring caused. He said the factory had good reason to use a simple non loaded ramp and adjust it every 5000 miles, they could have installed a spring loaded like the after market and obviously did not.
Yes I know the replies I can get, my spring loaded timing chain is just fine.
Yeah? Is it? How do you know unless you take the spindles off and examine
the spindle holes for wear, it is too damn tight.

I have just finished disassembly and you can view additional pictures here.

The item is numbered "4797" but I have been unable to map this number to any slipper tensioning device, though it must come from some British car or motorcycle given the Whitworth nut and bolt.

The tensioner is held in place by two horizontal bolts, one a stud from the tensioner with a nut on the backside of the chain case, the other a bolt run in from behind the chain case into a complementary internally threaded piece that acts as both the thread for the bolt, and the pin on which the slipper tensioner rotates. The pictures are pretty clear.

Good question on the rear cradle and with the chain case out of the way I can now have a closer look at this thing.

Derlin will not bond to steel I am told, so that is not a possibility. Ultem might work but is not good with wear, though it bonds well. If I keep this I will probably end up running chain as it has been run. Reasons to keep it would only be a rigidly-mounted gearbox - is it worth it for the trouble this device might cause? Not sure, but clearly it has been running this way for a bit of time so there's that.

Rust you are seeing is the result of the bike sitting for a number of years. For example, the movable sealing "gate" behind the clutch basket is also rusty, which would never happen in a regularly used machine.

Thanks for all your thoughts on this!

eskasteve said:

It does look like there's a bit of rust on the chain. Maybe the installer didn't increase the amount of oil enough to get the chain to run in the oil bath.

Click to expand...

It looks like where the chain wheel runs would be deeper than the center of the chain run being larger than the crank sprocket. My guess is that maybe there wasn't enough oil in the primary? Is the height of the level plug really correct?
John in Texas