Mk2a front adjustable iso’s

Knackered isolastic rubbers won't help. More depends on whether they've stiffened up, which seems likely since they've set in the distorted position. A bent frame or engine plates can also stop the isolastic moving freely. You'll have to rebuild with new rubber - the Andover Norton ones get good reviews here for rubber grade. And check the frame alignment while the engine's out. Fit the sprung head steady while you're at it.

What did you have the crank balanced to?

Yes, the Mk3 type that you are using is a one-piece, bonded item: https://andover-norton.co.uk/en/shop-drawing/136/engine-mountings-spring-head-steady-

Balance factor for a Commando is 52%.

From the Mk3 workshop manual: 63% dry, 52% wet.

Ok Bennie, but I have some questions about that.

1. Pre-Mk3 the workshop manual specifies 52% dry (and the OP has an 850 Mk2A). Is there something about the Mk3 that's different?
2. Why is the balance factor dry greater than wet? Surely the oil in the crank is adding to weight at the big end so there's a greater percentage of the rods and pistons to be balanced out.
3. Does the modest quantity of oil in the crank really amount to 20% of the weight of rods and pistons?

On the occasions I have had cranks balanced they have been done to 52% dry and I've had no complaints. But I will happily stand corrected if someone can enlighten me here (Grant Tiller maybe).

The Isolastic system calls for a lower balance factor than a rigidly mounted engine. The idea being that the engine moves up and down in the plane of the cylinders, constrained by the relatively soft front mounting and only rotating around the rear mounting. And, since Domis are supposedly done to 65% - 70%, 52% on a Commando seems consistent.

[edit] Ok scratch #2. I've thought about it some more and it's a 100% minus b.f. relationship.

Hi  Stan, Maybe I can explain my vision on the effect of oil by means of a calculation I did back in 2012 while re-balancing a Mk2 crank after fitting heavier EMGO pistons. I posted it on accessnorton at the time, attached a straight copy of the text 20240527-Mk2-balancing-wet-or-dry.docx You are correct, I also just found out that the pre-Mk3 manual mentions 52% dry. My guess would be that it is an error. In practice, with the wonderfull isolastics being so forgiving, non-perfect balance factors are masked. 

But which manual has the "error"? The pre-Mk3 workshop manual was in print for far longer than the Mk3 one but I can find no reference to either being incorrect. I've searched the Commando service notes here to no avail. The 52% figure is quoted pretty much everywhere and I can't see why (or even how) a crankshaft would be balanced "wet". I can't think of any reason why a Mk3 should be balanced to a different factor either. I was hoping someone would come along with a definitive answer,

Also, it would be worth going down a grade on the fork oil, too heavy an oil transmits more of the road imperfection to the handlebars and that makes a big difference in the comfort.  The one many owners miss with the vernier isolastic is that when it mentions 1 and 1/2 holes.  We get many queries where owners have tightened the adjuster, this is not correct. The adjuster turns with fingers until it stops, no need to go any further then adjust back 1 and 1/2 hole positions.   

Perhaps Andover can shed a light. What is the BF of the new 850 crankshafts they offer?

Stan, I am not an expert on this. Let me try to repeat in a few words the background for the BF value as I have seen it explained by Jim Comstock. With a BF of 100%, crankshaft shakes only horizontally. No shakes vertically. With a BF of 0%, only vertical shakes. None horizontal. Any other value will give a movement combining vertical and horizontal, more or less ellipsoïdal.  The BF is to be chosen to match the frame (shape, mass, etc) that holds the engine. Norton engineers went for flexible engine mounts with a circular housing and dito rubber dampers inside around a mounting tube: the type we recognise as "isolastics". (My guess: who knows it was cost driven, it is fairly easy to fabricate..) Naturally, the movement that suits this housing best is also a circular movement. This yields  maximum efficiency out of the rubber dampers. Theoretically this would mean a BF factor of 50%. It is actualy slightly higher (52%) because change of piston speed is higher around TDC than around BDC. The BF has to relate to the actual circumstances, so taking in account all the individual weights contributing to the forces on the crankshaft. This includes the weight of the oil in the sludge trap as part of the rotating weight.

I suppose we should apologise to the OP for hijacking his thread - sorry.

Anyway, Bennie: Yes, I understand what crank balancing is for and how it works. And I'd refer you to my earlier comments about favouring vertical motion at the front mount with rotation at the rear mount. I agree that there is enough oil in a Commando crank to make a significant difference between 'wet', i.e. running, and 'dry', or bench, balance factor.  And, making a quick extrapolation from your calculation, 52% dry would approximate to 40% wet.

The question is - what's the best value to use? Conventional wisdom has been that it says 52% dry in the Norton workshop manual and, since they designed and built the bike, that must be right. Then, for an as yet unexplained reason, the MK3 850 manual says something different.

Now, in 1977, I rode a new MK3 straight after it's 500 mile service and it was the nicest, smoothest Commando I've ever ridden. I put this down to it being new and the sprung head-steady. That and my reference being my 105k mile, ex-police Mk2A Interpol. I've never been able to get a Commando that smooth since, although my references have changed and recollections can be rose-tinted.

It would, as you say, be interesting to know what Andover Norton are balancing their one-piece cranks to; these will not have the large void in the centre to fill with oil so the wet and dry balance factors will be very similar. And also whether anyone has published experiments with balance factor to determine the optimum value for an Isolastic frame.  

The above is about right, but was Norton way out with their figures? Starting with the 500 twin they most probably had a figure which was accurate and worked very well but as things changed and the crank and capacity of the engine changed they most probably had to compromise on the BF.  I'll try and keep this simple.  Considering Norton were forging one piece forgings for crank manufacture around 1970, it suggests that they spotted something that would preclude a one piece crank being made and cut into chunks, even then one piece crank manufacture was the norm and would have been cheaper than a 3 piece crank. Fast forward and several race engine builders and crank manufacturers have had to add weight to the new cranks being made since the 70's to get them to balance. I knew of this prior to getting our one piece crank made but on purpose did not tell the manufacture of the cranks. They soon spotted the issue with the working envelope as it is does not allow it to get rotating mass far enough out on the periphery of the crank with simple design, this maybe why the original cranks were 3 piece as it allowed the flywheel to be machined to get the weight where it was needed. This was also found using the simulation software, the crank would not be heavy enough and to reduce weight in the area needed to make the weight further out on the crank effective they needed to a make a tool to machine the one piece crank.  Simulation software is very good, but recently in the past couple of years I know of cases where it has not got it quite right, the front hub and the crank. When the cranks were made they were actually slightly heavier than predicted and that allowed material to be removed and not added as we thought. They manufacturer was given the conrods and pistons we sell and used these to work out the balance factor needed along with the rev range of the engine as the crank has to achieve many things but two of them are to be able to tickover and accelerate, too light and it will not tickover and too heavy and will not accelerate efficiently. If you are racing you can as always sacrifice these parameters to suit the race engine being built.  The balance factor on the drawing was used as a guide for the manufacturer as the pistons and conrod weights change slightly, however the figure was very close. The drawing states 52% using 23.55Oz on the crankpin. So to answer the question above, and some of you have found, it seems that Norton were actually very close as to what was needed for BF over the life and use of the various cranks. 

Thank you, Ashley, for chiming in. The BF factor on the factory drawing, did it specify dry or wet? Do the new Andover cranks still have a sludge trap? If so, is the weight of the oil in there taken in account by the manufacturer for establishing the BF?