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How hard is it to convert my 750cc Commando engine up to an 850cc? I would really like to do this but do not know what is involved and what new parts are needed. Thanks in advance for any help!
Upgrading a 750cc Commando engine to an 850cc
- Thread starter Peter Waller
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Not really that easy at all: The barrels fasten to the crankcase in a different pattern due to through bolts from the top deck, and the head bolt pattern changes as a consequence as well.
This is a 30,000' summary - I am sure others will fill in more details.
Whether you could resize a set of 750 sleeves/barrels to take a set of 850 pistons is maybe a better approach, but I am not sure of the feasibility of that.
Bigger question would be why would you want to? A good working 750 is no slouch...
This is a 30,000' summary - I am sure others will fill in more details.
Whether you could resize a set of 750 sleeves/barrels to take a set of 850 pistons is maybe a better approach, but I am not sure of the feasibility of that.
Bigger question would be why would you want to? A good working 750 is no slouch...
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Very true, that was a very popular approach back in the day. For the price of a good running 850 motor, you could do a lot of work on your 750 motor... unless your 750 motor is junk of course.
Richard Tool
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Have you ridden an 850 ? If you haven’t then that would seem the prudent thing to do to see if you like it better .
I much prefer my 750 Combat - I love the way it spins up - but this is very subjective of course.
I agree with the others in that selling the 750 and buying an 850 will be easier and likely cost less as well.
Fast Eddie
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At last, a sensible answer !
Tornado
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Oh Man, now you've done it.....do I need to start another thread on how to "upgrade" my 850 to a 750?
Richard Tool
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Now why didn’t I think of that !
Except when loaded two up and passing a string of traffic on a mountain grade with headwind. Then get the 750 out of the way please, I have some traffic to get past!
That's been my real-world experience, not seat of the pants
56 ft lbs seems to work a lot better than 46 ft lbs.
maylar
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Do a 45 mph roll-on in top gear and watch the 750 disappear in your mirrors. The 750 will want to downshift, and the 850 will just pull.
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I ride both regularly. Riding styles are different for both. My Combat definitely requires more transmission effort to keep it in the power zone. I would consider both bikes equal in a hard pull on the road.
Compared to the amount of shifting that it takes to keep a modern 600 multi on the boil, both Norton's are torque monsters.
I don't have a whole lot of experience with a standard 750 - been a while since I have ridden one.
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What if the compression ratio was raised on the 850. 10.5 :1 850 would be pretty peppy I would think.
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I have not done it myself - but I remember my uncle Dick putting a 2S cam in an 850 - really woke it up from what he said.
I am sure some here has done that... ??
If you get into hot conditions in the mountains, varying fuel on the road, 9.5 to 1 might be about it. That on its own gives about 1bhp according to the calculators.
There are some cams that give more top end but according to the fellow with the dyno in his shop, the stock cam gives the strongest midrange.
So it depends on what you like and how you use the bike.
It seems the trans is the weakest link so you can't go far before needing to deal with that. The solution for that costs more than most will have paid for their old bike!
Glen
There are some cams that give more top end but according to the fellow with the dyno in his shop, the stock cam gives the strongest midrange.
So it depends on what you like and how you use the bike.
It seems the trans is the weakest link so you can't go far before needing to deal with that. The solution for that costs more than most will have paid for their old bike!
Glen
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Sell the 850 and buy a 750….Ha, Ha.
My 850 MK2 has a 2S cam, ham-can air filter, straight through pea-shooters and a skimmed head
due to there being a deep score across the gasket face. I have a composite head gasket. I've never
ridden a 750 so cannot compare, but I think it goes better than when it had std cam, plastic air box,
black cap silencers (I think ?), non-skimmed head and a copper head gasket.
It's been as it is now for 35 years or so. I thought most people did these changes ?
An 850 Combat ?
The 2S cam was installed on the advice of Carl Rosner who said it would "pep it up a bit" .
due to there being a deep score across the gasket face. I have a composite head gasket. I've never
ridden a 750 so cannot compare, but I think it goes better than when it had std cam, plastic air box,
black cap silencers (I think ?), non-skimmed head and a copper head gasket.
It's been as it is now for 35 years or so. I thought most people did these changes ?
An 850 Combat ?
The 2S cam was installed on the advice of Carl Rosner who said it would "pep it up a bit" .
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