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

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I'm confused... why does nobody answer the actual question, regardlless if the guy comes back or not. Isn't this site about technical information? Can it be done? Adding a 850 barrel to a 750 bottom? Different size studs? Different spacing of base studs? Can it be done? We know anything can be done, but what would it actually take? What about the difference between Commando and Atlas bottoms? Can a 850 barrel be fitted to both an Atlas and a Commando 750? What would it take? The discussion goes way off the original topic. Then posts of links to stupid non Norton videos.
Why does nobody answer the question in a technical manner? I'm confused.
I don't see any mention of "atlas" in the 1 & only OP 1st post, that would be another topic or thread

"I am considering converting my 1972 750 Commando to an 850 /920 and 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? "

The above can mean a swap or an engine conversion.
 
Personally I think the question was answered well, and could be summarised thus:

Yes it’s possible (anything is possible).

But it is NOT just a barrel and piston swap. Whatever you do will involve a lot of work and / or other changes (to head bolt pattern, crank balance, etc).

And given the fact that Norton made perfectly good 750 engines and perfectly good 850 engines, many (who appear to know what’s involved in the change) simply question the purpose / benefit in trying to make one into the other !
 
As we are on the subject year's ago I fitted a combat top end on an 850 bottom end with a 1mm spacer under the barrel on a mates commando and I have to say that was a superb bike to ride
I did pretty much the same assembling the best parts from 2 motors to get one last race in the season.....no 1mm spacer.....went very well...

But it wasn't the question.
 
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I did pretty much the same assembling the best parts from 2 motors to get one last race in the season.....no 1mm spacer.....went very well...

But it wasn't the question.
His 850 engine had a 4s cam 33mm inlets and a skimmed head
But it'd been a lot of trouble mainly with the cylinder head the inlet guides coming loose
One side had a top hat fitted the other was cracked
And one valve spring was broken
It had also broken the compression rings ,so can't have been gapped properly
So he wanted a more reliable bike but what surprised me was how well it went as a 750 low compression 4s cammed combat
 
Personally I think the question was answered well, and could be summarised thus:

Yes it’s possible (anything is possible).

But it is NOT just a barrel and piston swap. Whatever you do will involve a lot of work and / or other changes (to head bolt pattern, crank balance, etc).
Then the new guy sez ~ hey hey, what about the atlas engine:D
 
... it makes sense to just provide a correct technical answer for sharing knowledge. I typically find off-topic replies to indicate they don't know the answer. Comments will be visible for as long as the site is accessible, and will be available for others in the future to learn, That's it, no suppositions, just technical based replies.
You are correct in the assertion that off topic replies can be annoying, however this site is better than most in that regard. Way better in fact.

But I disagree with you about doing a ‘pendulum swing’ so far in the opposite direction as to ‘just provide a technical answer’.

Because… that’s not how the best problem solving works, you need more spontaneity than that, brain storming if you prefer, to bennefit most from the guys on here who know their stuff, they need to understand what you’re trying to achieve, and importantly, why.

Because when they know this they are likely to have a dozen or more suggestions of how to achieve that, or better, with ideas hitherto not considered.

In my opinion, a more important factor in reducing off topic chat and getting better answers is to POST BETTER QUESTIONS!

Its often the case that a posted question actually raises more questions, and that triggers much off topic chat. If posters posted clear questions about what they want to do, why, and what their defining limitations or preferences are, etc, they’d shift the odds in favour of better answers.
 
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Also it can be a bit tiring sometimes answering a question that can easily be found on the net
Personally I'd do a search first before asking a question
And as Eddie says sometimes a suggestion or an alternative can be beneficial
 
Personally I think the question was answered well, and could be summarised thus:

Yes it’s possible (anything is possible).

But it is NOT just a barrel and piston swap. Whatever you do will involve a lot of work and / or other changes (to head bolt pattern, crank balance, etc).

And given the fact that Norton made perfectly good 750 engines and perfectly good 850 engines, many (who appear to know what’s involved in the change) simply question the purpose / benefit in trying to make one into the other !
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.

Then unless you can find someone local

this diy would be interesting

If wondering about suitable atlas engine parts, looooooooooong story
 
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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.

Then unless you can find someone local

this diy would be interesting

If wondering about suitable atlas engine parts, looooooooooong story
"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 "
I wouldn't bother with re balancing it made no difference to my friends when she fitted an 850 top end
And when I fitted a 750 top end to an 850 bottom end it made no difference to vibration
 
If the pistons are lighter in the 750 motor, it will probably spin up a bit easier. But there would not be much in it. A smaller capacity motor of the same stroke can be faster than a bigger one. An 850 motor might be slightly better at very high speed in a head wind, if you were passing a truck on a highway. But faster passing can more easily had by using closer ratios in the gear box.. When you only change down half a gear, the motor has less trouble spinning up. So the bike accelerates faster. When you change down with wide ratios, you become more dependent on throttle response. A commando has a very heavy crank. Inertia stops it from accelerating fast With an 850 motor you might sometimes be able to avoid having to change down. But as I said, there is probably not much in it.
 
"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 "
I wouldn't bother with re balancing it made no difference to my friends when she fitted an 850 top end
And when I fitted a 750 top end to an 850 bottom end it made no difference to vibration
If the vibration moved up or down 1000 revs, would you notice the difference ?
 

hmmmm


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 think so. But 2500 and below is a pretty good point to avoid it affecting the bike on a ride.
 
It took 50 posts, but I believe these are finally the responses the OP was looking for. Once the group gets together the knowledge starts to flow and things get very interesting.
If he had asked about a 850 to 920 conversion the meat and potatoes would have begun by the third post.:)
 
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