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

to make a 750 into a 850 put VW bug 77 mm pistons in

the first 850 was build by a north american customer who put VW pistons in his 750 .

Eldert

74ok said:

So for new folks, a 750 engine conversion to 850 involves a complete engine stripdown, including splitting the cases to pull the crankshaft, to deal with a new balance factor.

Balancing of a Commando Crankshaft

I am in the engine overhaul stage of restoration of my Norton. I will soon be ready to have the Commando crankshaft that I am using balanced. I need reference to somebody that does this work.

www.accessnorton.com

Then unless you can find someone local

Balancing of a Commando Crankshaft

I am in the engine overhaul stage of restoration of my Norton. I will soon be ready to have the Commando crankshaft that I am using balanced. I need reference to somebody that does this work.

www.accessnorton.com

this diy would be interesting

DIY dynamic crank balancing

For do it yourselfers. Its an extra step but it should help bring your crank to a more accurate dynamic balance. Remove the flywheel. Rotate one side so it is like a 180 degree crank (journals 180 degrees from each other). Bolt it up without the flywheel and drill the heavy cheek until its...

www.accessnorton.com

If wondering about suitable atlas engine parts, looooooooooong story

https://www.accessnorton.com/search/41759/?q=atlas+commando&c[title_only]=1&o=relevance

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My friend has an Atlas which he road-raced in the 1960s. THe crank is balanced to 79 per cent. It is probably balanced to be smooth above 7000 RPM. Norton valve gear should cop 8000 RPM without tangling.. When the Commando was built, it was designed to be almost as smooth running as a CB750 Honda. But smooth running at commuter speeds is not what many people need in their Commando. Isolastics do not protect your crankcases from being damaged by internal forces at high revs.
The crank in my Seeley 850 is balanced to 72 per cent. THe motor runs extremely smoothly to over 7000 RPM. I don't think that crank will ever kill my crank cases. If I had a road-going Commando, I would raise the balance factor of the crank and live with the shake at low revs.

The balance factor is changed to produce smooth running. But it cannot be balanced to be smooth over the whole rane of possibilies. So you choose how you are going to ride your bike. If you do a lot of high speed. I would raise the BF.
I do not believe Atlas crankcases would be stronger than Commando crankcases. From memory, the failure in the Atlas was the barrel flange, dur to the bigger bore size.
A 650ss was an excellent bike. When they were made bigger, the problems started. The 850 is reliable, but revs will kill the crankcases.
My spare motor came from a bike a young guy used to ride on the Great Ocean Road on Sundays. His mates had Japanese bikes, so he used to rev his Commando to 8000 RPM. The drive side crankcase gave up.
EVen though my 850 motor's crank is balanced to 72%, I still do not rev it to 8,000 RPM. 7,500 is the absolute limit.

Eldert said:

to make a 750 into a 850 put VW bug 77 mm pistons in

the first 850 was build by a north american customer who put VW pistons in his 750 .

Eldert

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Porsches usually have forged pistons by Mahle. I don't know about Volkswagens.

74ok said:

Converting from 750 to 850

I've got a 72 Combat that I'm considering converting to an 850. I know I need 850 barrels, pistons and head. Can I bolt the 850 barrels to the 750 crankcase? Anything else I need to know?

www.accessnorton.com

hmmmm

Converting from 750 to 850

I've got a 72 Combat that I'm considering converting to an 850. I know I need 850 barrels, pistons and head. Can I bolt the 850 barrels to the 750 crankcase? Anything else I need to know?

www.accessnorton.com

https://www.leschroniquesdegoliath.com/wp-content/uploads/2017/12/Norton-Commando-750-Manual-dentretien-GB.pdf

pg 7
"The silver and black 750 barrels are identical dimensionally and incidentally, the studs all round the base
are exactly the same positions on the 850, so you can build a 750 using 850 crankcases, but not the
other way round, because the holes for the downward extensions of the barrel aren't big enough"

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I'm understanding the above "you can build a 750 using 850 crankcases, but not the
other way round" as you can't build an 850 from 750 cases

"because the holes for the downward extensions of the barrel aren't big enough"

Lotsa folks have said they have gone with 850 jugs & heads, on 750 cases, that would make the NOC Commando service notes incorrect, or i'm missing something.

I also don't see much on whether the balance factor is a real issue or not, that would have a big impact on going with a 750 to 850 engine conversion.

Lotsa folks have said balance factor is relevant, others not.

74ok said:

I'm understanding the above "you can build a 750 using 850 crankcases, but not the
other way round" as you can't build an 850 from 750 cases

"because the holes for the downward extensions of the barrel aren't big enough"

Lotsa folks have said they have gone with 850 jugs & heads, on 750 cases, that would make the NOC Commando service notes incorrect, or i'm missing something.

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The NOC notes aren't exactly incorrect as it can't be done unless the 750 cases are opened out to take the 850 barrels so it isn't a straightforward swap job plus the 750 cases aren't as thick in certain areas.

Then so far there is this for reference
which includes
"Roger at RGM saying it WOULD break"

but it has been done running with 920cc including the other info in the post.

74ok said:

Then so far there is this for reference

750 - 850 Cases Comparison or 850 Barrels in 750 Cases

I have fairly late model 750 cases which I thought should be as beefy as the 850 cases and what I've heard would be suitable to widen out for fitting 850 barrels. However comparing them side by side to actual 850 cases from a Mark 2a 1974 engine they don't seem as thick in several places. Not...

www.accessnorton.com

which includes
"Roger at RGM saying it WOULD break"

but it has been done running with 920cc including the other info in the post.

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Roger has been around Nortons a while, so I don’t think he is making it up, do you?

Most likely, the poster here was just incredibly lucky and had abnormally think cases or similar.

As Harry would say… "You've got to ask yourself a question: 'do I feel lucky? ' Well, do ya, punk?"

Just when we thought this thread was dead !

baz said:

Just when we thought this thread was dead !

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We're our own worst enemy.

baz said:

Just when we thought this thread was dead !

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Lotsa folks said balance factor comes into it, u say pfffft

This is when we miss Comnoz. Any moment he would show up with the technical data to support or prove the weaknesses in the theories.

maylar said:

We're our own worst enemy.

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Is balance factor involved? Is going with 750 cases clear cut?

Anyone looking it up will likely run across what NOC & RGM have said. Irrelevant?

Fast Eddie said:

Roger has been around Nortons a while, so I don’t think he is making it up, do you?

Most likely, the poster here was just incredibly lucky and had abnormally think cases or similar.

As Harry would say… "You've got to ask yourself a question: 'do I feel lucky? ' Well, do ya, punk?"

Click to expand...

I know RGM is big time, prob just 3 wtf letters for new folks

New folks looking into upgrading a 750 engine to 850+ (absent OP mentioned 920) would wanna know & explained in the simplest way, what all might be involved. Parts alone aren't cheap or worse, paying someone else or even finding folks these days who know Norts enuff to do the work involved

No point in making it tougher than what it is, when there are a ton of cheaper bike options out there., including reliability.

Ya just throw on the bigger head & barrel means practically nothing. I remember now coming across awhile ago, that fab info Lab put up, but had completely forgotten.

Then just lookin up solid info from the past can easily lead to dead ends. Can't say i'm aware of any good solid feedback (details) from folks who went with basics, head / barrel or how it worked out long term. Having to deal with balance factor puts the upgrade on a whole other level.

I have no knowledge in this area but would say balance factor should be considered for the rpm range you would normally run.
On the 750 / 850 I have the case is very thin at 7 o'clock. It looks ok at the case mouth @ .146 but the later 750 cases are undercut at the top rear case bolt. I don't have the cases apart to measure the case area at the yellow arrow but it's very thin. So in the end, the common thought of sticking with 850 cases should be considered.

Please correct me if I'm wrong, but the vernier in the post above shows the thickness of the crankcases at that point to be 0.146"

I've just measured an open set of 850cc crankcases I have in my garage at the same point as being 0.454"

I cannot remember if mine are MK II or MK III cases, but they were fitted with a MK II crank by Norman White.

Staytite said:

Please correct me if I'm wrong, but the vernier in the post above shows the thickness of the crankcases at that point to be 0.146"

I've just measured an open set of 850cc crankcases I have in my garage at the same point as being 0.454"

I cannot remember if mine are MK II or MK III cases, but they were fitted with a MK II crank by Norman White.

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Staytite,
I was showing the case width of my 1972 750 case that was opened up to accept 850 cylinders.

From my reading, the 850 cases were strengthened vs the 750 and the MK3 are strengthened vs earlier 850.
I had some Combat cases here for awhile. They weighed just over 2 lbs less than the MK3 cases.
That's not a huge difference in total weight, however 2 lbs of aluminium spread out over the weak areas would make quite a difference to overall case strength.

Going with 850 cases then 750 up, some related & relevant balance factor considerations

"spare 750 barrel with .060" or .080" oversize forged pistons to compensate somewhat for the heavier 850 crank in terms of the balance factor. Potential combinations that would make sense for me:"

Staytite said:

I've just measured an open set of 850cc crankcases I have in my garage at the same point as being 0.454"

I cannot remember if mine are MK II or MK III cases, but they were fitted with a MK II crank by Norman White.

Click to expand...

The Mk3 case doesn't have the narrow section.

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.