Considering a 750 20M3S engine in an 850 frame/trans

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Sirwompus

Sirwompus

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I have a 850 frame/trans (302861) and found a 750 (145074) engine for sale a few hundred miles away that I plan on buying. Besides the obvious negatives of resale value etc I wondered if anyone could comment on compatibility issues. Could I use primary drive components from the 750 to mate the two? The cylinder/barrel looks rusty, would a .060 overbore be unsafe on that engine or are liners the only option? From what I've read it's generally doable but would like any relevant input you might have.

Thanks in advance, I'm new to British motorcycles and wouldn't blame any of you for thinking this is a fool's errand but I like learning.
 
Any primary parts difference is trivial so interchangeable.

I would hold out for an 850 but that engine will bolt in but would limit it to + 0.040".
It would want to be complete as possible, you could on sell it.

The rebuild cost on an 750 or 850 is all but the same.
The bolt patterns as far as fitting the frame are the same (30 mm carburetors and manifolds to suit on that bike / RH1 cylinder head)
If the engine was cheap, all but complete and not bodgerated, why not
 
It will bolt in. I don't know how much you've researched, but here's a few tips:
That 750 engine does have 30mm intake ports, but you should try to find '74 850 intake manifolds to mount 32mm carbs. They were made to mount 32mm carbs to a 30mm head. Good flow and more go.
Change the trans output sprocket to 19 tooth. Your 850 trans has a 20 tooth sprocket. That would be fine for highway cruising on a 750 (Interstate), but 19 tooth sprocket was the norm for 750's and better for all around fun. Depends on what you want for this project.
I doubt you'll need to bore the cylinders more than .020 over to clean it up, but that's up to your machinist's advice. The original pistons of the era s/n should be replaced with later design anyway.
The 750 motor has a crankcase breather on the drive side of the crankcase into the oil tank. No need to do anything special about that, just different than 850. Run the hose to the oil tank and all good.
The 750 motor s/n does pique my interest. It's an early '71 s/n and the area where the numbers were stamped on the crankcase moved from the curved area on the front
of the left drive side case to the flat area rearward. That change was somewhere in the 145XXX timeframe. Just wondering where the numbers are on this engine.
 
Be aware that as a 750 Barrel has a flange that attaches it to the crankcase and when the Barrel is over bored to 060 it can get extremely thin in places therefor you can run the risk of the barrel detaching from the crankcases especially with high compression, It has happened to me in a race motor years ago. I don't think the gearing thing is a big issue especially if you want to keep up on a modern freeway, I run 20 tooth gearbox sprockets on both my 650SS and Atlas and the gearing is not a problem as I'm past the age of wanting to win every Traffic light drag.
 
Welcome to the forum!

I expect to see photos of all this stuff before dark this evening, I mean the engine is "only a few hundred miles away".

My guess is that you won't read some of these replies until you ARRE back with it...
 
Got my interest up.... An 750 850.... very interesting. Only a Norton person would be suspicious looking it over, and many would have to be told what they were seeing without time to dig in.... Pics are a legal requirement at this stage of mental arousal.... Don't tease the animals.
 
jbruney said:
Got my interest up.... An 750 850.... very interesting. Only a Norton person would be suspicious looking it over, and many would have to be told what they were seeing without time to dig in.... Pics are a legal requirement at this stage of mental arousal.... Don't tease the animals.
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There are only a relative few that would know the frame is a 750... Only a few more that would know the engine is an 850.

It would be cool to NOT put the 750/850 decals on the sidecovers...
 
750 is 750cc but a 850 is only 828cc. That is only 10% more, so a normal 750 with a bit more compression than an 828 is only 2-4 hp less power... usually a little over 40 RWHP.
Wouldn't bother me a bit.
The barrel over bore is the biggest concern...
 
If the barrel is toast then fit a 750 barrel with the through bolts
Then it'd be really hard to see it's a 750
Not that I'd be bothered either way, most of the bikes I own/ have owned have the wrong engine
 
The 20M3S engine cases would give it away at 50 feet. (Having a camshaft timed breather and hose off the case)

It would soon be forgotten by the time you hit fourth gear.
 
Deckard said:
It will bolt in. I don't know how much you've researched, but here's a few tips:
That 750 engine does have 30mm intake ports, but you should try to find '74 850 intake manifolds to mount 32mm carbs. They were made to mount 32mm carbs to a 30mm head. Good flow and more go.
Change the trans output sprocket to 19 tooth. Your 850 trans has a 20 tooth sprocket. That would be fine for highway cruising on a 750 (Interstate), but 19 tooth sprocket was the norm for 750's and better for all around fun. Depends on what you want for this project.
I doubt you'll need to bore the cylinders more than .020 over to clean it up, but that's up to your machinist's advice. The original pistons of the era s/n should be replaced with later design anyway.
The 750 motor has a crankcase breather on the drive side of the crankcase into the oil tank. No need to do anything special about that, just different than 850. Run the hose to the oil tank and all good.
The 750 motor s/n does pique my interest. It's an early '71 s/n and the area where the numbers were stamped on the crankcase moved from the curved area on the front
of the left drive side case to the flat area rearward. That change was somewhere in the 145XXX timeframe. Just wondering where the numbers are on this engine.
Click to expand...
Thanks for the reply. I'm attaching some pictures but to answer your question I believe the VIN is on the curved area.

Considering a 750 20M3S engine in an 850 frame/trans


Considering a 750 20M3S engine in an 850 frame/trans


More pictures:
 
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Would appreciate any input, going to go pickup the engine tomorrow. Apparently the donner bike is fairly complete and while I'm only really after the engine and possibly final drive I'd like to know what else I might need from that donner 750. Thanks!
 
Grab the lot if you can depending on price of course what's not usable will be saleable, primary side including clutch and Alternator will be usable as will the oil tank and electrical equipment as the s/n denote '71 model, this will include the handlebar switch gear etc again as your project will not be a restoration most bits will be usable.
 
Unless the head steady and center stand were upgraded to 850 HW, they're not worth bringing home. Not sure if you have the headsteady for your 850 chassis, but definitely don't put a 750 headsteady on engine.
The '71 side stand won't fit and was the poorest design ever for Commandos. It probably doesn't have one, most fell off when circlip was lost.
I like the '71 rear tail light style much more than 850's, but they didn't have provision to mount US plates unless it's been upgraded for that. Aftermarket plate mounts are available
'71 Tach/ Speedo cups are polished and gauges are Smiths, so prettier than 850.
750 Front fork triple T's should never be used on an 850 frame. Deadly combination or rake angle
'71 750 had drum brake front unless upgraded to disk. Disk 'mo better
All other stuff is pretty much the same as your 850
 
How did it go with picking up the engine? Were you able to get any other parts while there? I've only just seen this thread but look forward to your build.
 
Sirwompus said:
Would appreciate any input, going to go pickup the engine tomorrow. Apparently the donner bike is fairly complete and while I'm only really after the engine and possibly final drive I'd like to know what else I might need from that donner 750. Thanks!
Click to expand...
Isn't a Donner a kebab?
 
If I was near enough and had the cash I would buy a project like this...in fact if you are in France and have one...

Of course like most of us I would then spend too much on it and end up with something completely different from what I had intended!

Have at it!
 
Sirwompus said:
I have a 850 frame/trans (302861) and found a 750 (145074) engine for sale a few hundred miles away that I plan on buying. Besides the obvious negatives of resale value etc I wondered if anyone could comment on compatibility issues. Could I use primary drive components from the 750 to mate the two? The cylinder/barrel looks rusty, would a .060 overbore be unsafe on that engine or are liners the only option? From what I've read it's generally doable but would like any relevant input you might have.

Thanks in advance, I'm new to British motorcycles and wouldn't blame any of you for thinking this is a fool's errand but I like learning.
Click to expand...
As long as you can get it titled in your state there is no actual issue.
 
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