Converting a 750 to an 850 engine, what does it take?

Dynamic balancing is often done at time of complete rebuild. Some argue that static balancing is all that is needed, others swear by Dynamic balancing. I'm not certain, but given the relatively low extra cost, I would always opt for dynamic balancing.
The dynamic balancing corrects any rocking couple that may exist in a perfectly( statically) balanced crank.
Glen

worntorn said:

Dynamic balancing is often done at time of complete rebuild. Some argue that static balancing is all that is needed, others swear by Dynamic balancing. I'm not certain, but given the relatively low extra cost, I would always opt for dynamic balancing.
The dynamic balancing corrects any rocking couple that may exist in a perfectly( statically) balanced crank.
Glen

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Is this type of dynamic balancing done on a rotating crank/rod/piston set to a specific rpm (like how vehicle tires are "Dynamically balanced" using weights placed at fixed locations or are we talking about some kind of mechanism built into the assembly that adjusts balance throughout rpm range automatically? The beauty of bead balanced tires is they are always adjusting to any out of balance condition across all rpm points. Just wondering if anything similar has been tried in engines.

Steve Maney was adamant that dynamic balancing of one of HIS cranks was a waste of time. His opinion was that the benefits come into play with longer cranks.

He statically balanced my 920 crank and it is fine. Really fine!

I had my 850 dynamically balanced, and it was fine too.

My uneducated thinking is that dynamic balancing may well be a waste of time, but I can’t see how it can do any harm.

What can do harm Is an poorly skilled operator! And that’s irrespective of static or dynamic.

So, I think that what really matters is using a trusted, reputable balancer, static or dynamic matters not, if it’s a trusted person, that’s all that really matters.

Tornado said:

Is this type of dynamic balancing done on a rotating crank/rod/piston set to a specific rpm (like how vehicle tires are "Dynamically balanced" using weights placed at fixed locations or are we talking about some kind of mechanism built into the assembly that adjusts balance throughout rpm range automatically? The beauty of bead balanced tires is they are always adjusting to any out of balance condition across all rpm points. Just wondering if anything similar has been tried in engines.

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It's very much like the dynamic wheel balancing. Generally speaking, the rods and pistons are removed and replaced with a made up bobweight for both static and dynamic balance.
Holes are drilled or , occasionally, weight is added to get the assembly to balance, that is to sit with mainshafts on very low friction surfaces and stay put at any stopping point in rotation. The dynamic part is seeing how that balanced assembly acts at speed.
The crank can appear to be perfectly statically balanced but still have a left to right imbalance. The Dynamic balance machine will find that and tell the operator exactly where and how much to drill in order to correct this imbalance.




Glen

worntorn said:

It's very much like the dynamic wheel balancing. Generally speaking, the rods and pistons are removed and replaced with a made up bobweight for both static and dynamic balance.
Holes are drilled or , occasionally, weight is added to get the assembly to balance, that is to sit with mainshafts on very low friction surfaces and stay put at any stopping point in rotation. The dynamic part is seeing how that balanced assembly acts at speed.
The crank can appear to be perfectly statically balanced but still have a left to right imbalance. The Dynamic balance machine will find that and tell the operator exactly where and how much to drill in order to correct this imbalance.




Glen

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OK, that makes sense. I still wonder if making a "live" balance mechanism would be even better. Perhaps a tubular ring concentric with the crank, welded in place, with ball bearings free to roll into counter-balance positions within?

Tornado said:

Has anyone heard of dynamic balance setups of cranks? I'm thinking along the lines of bead balancing like I have in my inner tubes on both my bikes.

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maybe this? anyone else try it? (looks like pix are gone)

I think it depends on the crank cheeks in a twin which were different in this engine so I used one 1/2" OD Mallory metal plug and one 5/8".
Only the crank being spun up will tell you. The cheeks mass on a Commando look to be similar if not the same besides the drilled passage which is steel plugged.



That guy must look in every now and again and shake his head.

Tornado said:

Is this type of dynamic balancing done on a rotating crank/rod/piston set to a specific rpm (like how vehicle tires are "Dynamically balanced" using weights placed at fixed locations or are we talking about some kind of mechanism built into the assembly that adjusts balance throughout rpm range automatically? The beauty of bead balanced tires is they are always adjusting to any out of balance condition across all rpm points. Just wondering if anything similar has been tried in engines.

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My mate is building the fast 750cc Triumph and had a major vibration problem. He made weights to clamp around the crack journals so he could have the crank dynamically balanced. When the crank came back, it was scrap. Some moron had taken an angle grinder to one of the bob weights. Secondary balance is not a major factor in most 360 degree cranks. A 180 degree crank might need dynamic balancing, and three and four cylinder cranks might benefit much more from it. With a 360 degree twin, it isc almost impossible to get smooth running right across the whole of it's usable rev range, andc still have performance. My 850 motor idles perfectly, but when it idles at about 1000 RPM, the whole bike moves backwards and forwards. It is dead smooth at 7000 RPM. When you fill the hole in the Commando crank with a steel plug, you probably have an Atlas crank.
Beware of experts, particularly car guys. Some of them should have been strangled at birth.

Another post indicating that for 1, a 750 crank is lighter ("the lighter crank in the 750 spins up faster"), then balance factor considerations when mixing parts

the above thread has a ton of solid related mixing parts info, including going big issues like the drive side half being the weak point, other case options out there like dreer or maney, Gus Kuhn stamped, modern light weight Jim Schmidt pistons and rods etc .... including

ICRKEN has addressed this in his "Short Stroke 750 Builld" Thread here: (brocken link but fixed here)

related crank balancing thread /info & what you may have

With the known stress riser in some DS combat cases I would have thought dynamic balancing to reduce harmonics at rpm would be recommended.

Fast Eddie said:

Steve Maney was adamant that dynamic balancing of one of HIS cranks was a waste of time. His opinion was that the benefits come into play with longer cranks.

He statically balanced my 920 crank and it is fine. Really fine!

I had my 850 dynamically balanced, and it was fine too.

My uneducated thinking is that dynamic balancing may well be a waste of time, but I can’t see how it can do any harm.

What can do harm Is an poorly skilled operator! And that’s irrespective of static or dynamic.

So, I think that what really matters is using a trusted, reputable balancer, static or dynamic matters not, if it’s a trusted person, that’s all that really matters.

Click to expand...

I have had two Norton cranks balanced for me. Both statically, one by the late Owen Greenwood, one by Steve Maney.

By contrast I had an MGB crank dynamically balanced by Bassetdown engineering, who had the best reputation in the UK at the time. And I would not have had that crank done statically, based on input from the top MG racers in the UK at the time.

Owen was the first guy in the UK to start balancing cranks for solo and sidecar race use, and he started in the '60s, maybe even earlier. A very empirical approach and a lot of trial and error, all under the bridge by the time he did mine.

In both cases, albeit many years apart, I have ended up with motors just as I had hoped at race rpm! But in both cases the balancer stated clearly up front that if I had any concerns to note the rpm range I was not happy with, and they would sort it for me, no charge. I think they were both confident for good reason. I also think Steve's comment about crank length is correct.

(as far as cases go, one set of MK3 cases one set of Steve Maney cases, no observed issues over many rebuilds each.

Yes I think Dynamic balancing should offer an advantage, but I honestly don't think you would be able to tell the difference compared to a static balance if performed by the most skilled people. (If you get a monkey to do it, you are on your own).

And certainly you are not going to tell the difference at the seat of your pants and particularly the other side of isolastics, for that you will need some form of test rig measuring vibration and deflection that way exceed any cost benefit analysis! And even then, you are going to have to decide what you need it balanced for, choosing a factor that works at 7000rpm will disappoint you if you rumble around at 3000 to 4000 all day!

BTW, how come this is now a balancing thread?

Someone suggested rebalancing is needed for the 750/850 change and away we went. OP long gone anyway!


This is one of my favourite videos.
The Dynamic balance for my 920 was $175 cdn, less than the cost of a couple of lightweight wristpins.

And , for contrast this is Jim's 880 crank which had rec'd a Dynamic balance done by others years earlier.

SteveA said:

BTW, how come this is now a balancing thread?

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With an absent OP, asking about a 72 750 engine conversion to 850 - 920, which of course can be wideranging, some folks said a rebalance was involved, others said pffft, can't tell the diff.

Prob with a basic upgrade, not a huge deal but not much details or info from that yet that i'm aware of, or can find

Big diff in work & cost involved with the fundamentals, depending on a rebalance or not.

74ok said:

With an absent OP, asking about a 72 750 engine conversion to 850 - 920, which of course can be wideranging, some folks said a rebalance was involved, others said pffft, can't tell the diff.

Prob with a basic upgrade, not a huge deal but not much details or info from that yet that i'm aware of, or can find

Big diff in work & cost involved with the fundamentals, depending on a rebalance or not.

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When I converted my mates 850 to a 750 I just did it
Didn't even think about asking on here I knew the parts would fit
Didn't worry about crank balance and as it turns out it didn't matter
When my friend converted her 750 to an 850 that was 40 years ago and that made no difference to vibration
And that commando pulled a sidecar for many years no problems
Sometimes it's better just to get the tools out and do it

Ya sure, everyone has tools & knows as much about Nortons as you do, just do it.

That covers a question what's involved in a 750 to 850-920 conversion for anyone.

Stuff From NOC, RGM & what, vibration ?? or whatever else ....pfffft

Good to hear tho, from someone who went there, with a great result over a long period and lotsa history on this site.

74ok said:

With an absent OP, asking about a 72 750 engine conversion to 850 - 920, which of course can be wideranging, some folks said a rebalance was involved, others said pffft, can't tell the diff.

Prob with a basic upgrade, not a huge deal but not much details or info from that yet that i'm aware of, or can find

Big diff in work & cost involved with the fundamentals, depending on a rebalance or not.

Click to expand...

Just to be clear, I said that you would be unlikely to tell the difference between an expert performed static balance and a dynamic balance!

I would need to have any crank I used rebalanced, because it will go into a rigid frame.

I have ridden a bike belonging to someone else that had a Commando engine in a rigid mounting....so possibly a balance factor of 50%, certainly not at the balance required for rigid mounting.

My hands were swollen and red after each track session, I am amazed more parts didn't fall off......

I have huge respect for Jim Comstock, however, something worries me a little from the videos that I would like to hear Jim's comments on.

Inside Norton crankcases and mounted in good roller bearings the crank is both constrained and able to run to full operating rpm. Like in Jim's spintron, which folds in the full forces of the pistons and rods, including big end and piston/small end/rings friction and the effects of the entire valve train.

In Jim's rig, one, I would not stand in front of it, or indeed within 20 feet, two, what actual rpm is achieved? Three, what is the effect on not constraining the crank axles and not including these other forces?

The last parts of absent OPs first post "can any one tell me what I would need and how to go about it and anyone who might be able to do it here in the uk?" prob mostly covered but i don't think the very end.

Don't imagine it is easy pickings finding someone qualified or perhaps having norton expertise, to take an engine job down to splitting the cases, but much more important, someone known to stand by their work and warranty.

Not much worse than paying for major work done that if issues arise, customer service ends up in the toilet.

I think the topic has been well covered fundamentally for anyone who may be interested in going there & by the question, doesn't necessarily know what all may be involved or perhaps even anything much about a particular model. Better to be an informed customer where major experienced work & money is involved, particularly for something you may know little about. I never hearda balance factor prior to hearing or reading about it from the pros here. Might as well still be a noob compared to folks or vets on this site & norton knowledge.

SteveA said:

Just to be clear, I said that you would be unlikely to tell the difference between an expert performed static balance and a dynamic balance!

I would need to have any crank I used rebalanced, because it will go into a rigid frame.

I have ridden a bike belonging to someone else that had a Commando engine in a rigid mounting....so possibly a balance factor of 50%, certainly not at the balance required for rigid mounting.

My hands were swollen and red after each track session, I am amazed more parts didn't fall off......

I have huge respect for Jim Comstock, however, something worries me a little from the videos that I would like to hear Jim's comments on.

Inside Norton crankcases and mounted in good roller bearings the crank is both constrained and able to run to full operating rpm. Like in Jim's spintron, which folds in the full forces of the pistons and rods, including big end and piston/small end/rings friction and the effects of the entire valve train.

In Jim's rig, one, I would not stand in front of it, or indeed within 20 feet, two, what actual rpm is achieved? Three, what is the effect on not constraining the crank axles and not including these other forces?

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Jim is standing well off to the side when the statically balanced crank is spinning fast and shaking about.
The fellow who dynamically balanced my 1360 crank and the 920 crank uses a Hines Dynamic Balancer which has the same open v blocks as in Jim's machine. I don't know what rpm he takes the crank up to, but my 1360 went from nearly un ridable to a very pleasant 8 hour bike just with the dynamic balance vs static balance.
My take on it is that if there is very little rocking couple, the static balance will get a good result. If there is a lot of rocking couple, the static balance is about useless.
The problem is, you don't know whether the rocking couple exists unless you put the crank on a Dynamic balancer.
As far as the shaking forces in the first video, my understanding is that the machine measures all of them including uplift and then tells you exactly where to drill or add weight in order to cancel those forces. The fellow who dynamically balanced my crank starts out with very low speed, maybe 25 or 30 rpm. He starts correcting the rocking couple while working toward or maintaining the desired BF. By the time he gets the crank up to high rpm it is running very smoothly, like the dynamically balanced crank in Jim's second video.
Jim's first video is to demonstrate where the problem is with static balancing, so it was necessary to get it up to speed to show how wonky it really is, even with a perfect static balance.

I recall when he first showed that video, the safety police came out in large numbers.

Glen