MK3 strengthened areas

Fast Eddie · Jan 24, 2017

Chaps,

Rather than hijack the thread where this was recently brought up, I'd thought I'd start afresh.

Glen recently posted that "The 75 MK3 850 had a lot of things strengthened from the earlier bikes, (cases, crank, swing arm and more)".

I can't be alone in being interested in understanding this further me thinks.

I'm pretty aware of the actual swinging arm strengthening that was done. And whilst it seems sensible, I am not aware of what actual problem it solved, was there a known issue of failures here?

I'm kinda aware of the strengthening to the crank cases (primary drive side main bearing area mainly?), but would like to understand the details better.

I'm not really aware how the crank itself was strengthened, so would like to understand this.

And similar for other strengthened areas.

Can anyone can show photos of pre MK3 vs MK3 showing the actual strengthened items and explain the failure mode that was addressed ?

worntorn · Jan 24, 2017

Nigel, the crank was strengthened with larger fasteners holding it together.
The swing arm started out quite light and flexy on the 750s, got some additional heft with the 850 and more yet with the MK3 850.
I suspect this was not because of failure but just to reduce flex and improve handling.
Along with the added strength, the change was made to lifetime lubricated swingarm bushings for the MK3.

Glen

gortnipper · Jan 25, 2017

mk3-changes-from-models-t21935.html

Fast Eddie · Jan 25, 2017

Yes, I understand the larger crank fasteners Glen, again, its something that makes sense, even to a numpty like me. Yet I'm not actually aware of the earlier ones being a known failure mode?

The 'Mk3 changes' thread is a bit generic for my queries in this thread though Gortnipper, I'm particularly interested in strengthening mods, and what they were, and what they solved.

For example, the MK3 crank is wider and the main bearings slightly further apart to accommodate. I cannot see how this strengthened anything though, or what it was solving. Input into things like this would be great.

And specific 'before and after' photos or explanations would be even greater'er !

gortnipper · Jan 25, 2017

lcrken said:
Crankcase and crankshaft related changes:

The crankcase is thicker in some areas, in particular at the upper rear area where the earlier cases are pretty thin, and tend to crack.

The crankshaft is wider than earlier models, and the main bearing bores in the cases are further apart to accommodate it.

As you mentioned, the crankshaft fasteners were increased from 5/16" to 3/8" diameter.

The main seal in the crankcase is held in place by a wire circlip, as opposed to the earlier cases where the seal is pressed in against a lip in the crankcase.

The alternator is mounted further out, on a separate mounting plate, which sites on four mounting studs from the crankcase, as opposed to three on earlier models.

Ken

worntorn said:
From some reading this am, it appears that for the MK3, the gearbox shell was made thicker in some crack prone areas.

Glen

worntorn said:
From recent posts on the forum, it appears the MK3 cam bushings are an uprated design vs earlier engines.

Glen

Fast Eddie · Jan 25, 2017

Well yes, I've already read all of that !

Actual verification of what's true, and explanations, and pictures is what I'm hoping for now.

For example, the gearbox shell, is it true? If so where / how was it strengthened? etc...

Danno · Jan 25, 2017

And were any of these upgrades used in the earlier 850s or just the MkIII?

Dances with Shrapnel · Jan 25, 2017

I recall that the wider crank on the Mk3 is due to the crank cheeks being made a bit thicker. Intuition would seem to lead one to believe that this would maybe be better for durability but I broke a couple of Mk3 cranks in my sleeved down 750 road racer. The one failure seemed to start in the oil feed hole on the drive side journal but maybe that's where it ended up at.

The Mk3 cases were thicker in and around the drive side main bearing, a known source of poor durability in earlier Commando models.

Madnorton · Jan 25, 2017

Gearbox shell - yes, put a MK3 one and a earlier one together and look closely, the MK3 appears to have thicker walls.
The others, not so sure, like most things Commando, changes made - we know what they are but why, just seems to be myth in many cases - even those with living memory of making the things don't even know.

Crankcases were most probably made thicker in places to compensate the gassing needed to get it to cast - look at a failed LH case on a machined surface and you will see the Aero bar consistency which is very common.

gortnipper · Jan 25, 2017

Fast Eddie said:
Well yes, I've already read all of that !

Actual verification of what's true, and explanations, and pictures is what I'm hoping for now.

For example, the gearbox shell, is it true? If so where / how was it strengthened? etc...

I know that, but was doing it also for the benefit of others... :mrgreen:

o0norton0o · Jan 25, 2017

So,.... since I have a 70 commando, does my thinner left crankcase half have a known life expectancy? Will it inevitably fail or is it just less able to stand up to a continuous flogging?

I also wonder if my earlier steering yoke geometry with less rake and less offset is also considered "problematic"? *(even though I get a little more rake because of my 18" rear tire's slightly smaller diameter)

... all the rest of the differences, I'm not worried about.... thanks

Torontonian · Jan 25, 2017

Don't worry. My 70 got Superblends , then I did a ton of miles.

Fast Eddie · Jan 26, 2017

Madnorton said:
Gearbox shell - yes, put a MK3 one and a earlier one together and look closely, the MK3 appears to have thicker walls.
The others, not so sure, like most things Commando, changes made - we know what they are but why, just seems to be myth in many cases - even those with living memory of making the things don't even know.

Crankcases were most probably made thicker in places to compensate the gassing needed to get it to cast - look at a failed LH case on a machined surface and you will see the Aero bar consistency which is very common.

The pre MK3 ones sold by AN today are labeled as being stronger than stock, does that mean they're as per MK3 pattern or different again?

Fast Eddie · Jan 26, 2017

Dances with Shrapnel said:
I recall that the wider crank on the Mk3 is due to the crank cheeks being made a bit thicker. Intuition would seem to lead one to believe that this would maybe be better for durability but I broke a couple of Mk3 cranks in my sleeved down 750 road racer. The one failure seemed to start in the oil feed hole on the drive side journal but maybe that's where it ended up at.

Interesting, I thought the weak point of the stock crack was supposed to be the flywheel, which oft disintegrates in high revving motors?

Deets55 · Jan 26, 2017

I don't know if this only applies to the MKIII but the crank seal is held in by a "C" clip.
Also with respects to the two piece cam bush. When I staked the blanking plugs in the timing case the inner bush fell out. I could then see it was loose and it had spun. At Comstocks suggestion I used a one piece scrolled bush and a hardened spacer behind the cam gear.
Pete

EDIT: scrolled cam and NON-scrolled bush

Danno · Jan 26, 2017

The MK III is the only engine I've seen with the circlip-secured mainseal. 750s and early 850s all have the press-in seal.

L.A.B. · Jan 26, 2017

Fast Eddie said:
Dances with Shrapnel said:

I recall that the wider crank on the Mk3 is due to the crank cheeks being made a bit thicker. Intuition would seem to lead one to believe that this would maybe be better for durability but I broke a couple of Mk3 cranks in my sleeved down 750 road racer. The one failure seemed to start in the oil feed hole on the drive side journal but maybe that's where it ended up at.

Click to expand...

Interesting, I thought the weak point of the stock crack was supposed to be the flywheel, which oft disintegrates in high revving motors?

Surely you must remember J. M. Leadbeater continually banging on about that "drive-side stress raiser". :roll: :? :wink:

sludge-trap-t22451.html#p294354

Fast Eddie · Jan 26, 2017

L.A.B. said:
Surely you must remember J. M. Leadbeater continually banging on about that "drive-side stress raiser". :roll: :? :wink:

sludge-trap-t22451.html#p294354

How could I fail to remember that!?

I did manage to resist the urge to strip my engine, split the crank, and fettle it though.

Maybe now I've said that... the crank will break...!

toppy · Jan 26, 2017

Fast Eddie said:
L.A.B. said:

Surely you must remember J. M. Leadbeater continually banging on about that "drive-side stress raiser". :roll: :? :wink:

sludge-trap-t22451.html#p294354

Click to expand...

How could I fail to remember that!?

I did manage to resist the urge to strip my engine, split the crank, and fettle it though.

Maybe now I've said that... the crank will break...!

But of course the clutch would slip so badly you will not be able to leave your house. So when the crank explodes you won't have far to walk.

Madnorton · Jan 27, 2017

Failed left hand cases usually show signs of being gassed to inch of their life - looks like Aero bar, often confused with porosity. Foundry's still use it if need be.

MK3 strengthened areas

Fast Eddie

worntorn

gortnipper

Fast Eddie

gortnipper

Fast Eddie

Danno

Dances with Shrapnel

Madnorton

gortnipper

o0norton0o

Torontonian

Fast Eddie

Fast Eddie

Deets55

Danno

L.A.B.

Moderator

Fast Eddie

toppy

Madnorton

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