Upgrading a 750cc Commando engine to an 850cc

maylar said:

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.

Click to expand...

Well, yeah, but why?

Ain’t that what gearboxes are for ??

Staytite said:

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" .

Click to expand...

850 Combat is a sensible suggestion.

920 Combat even sensibler …

When I were a lad, many, many years ago a mate of mine had a 750 Commando
with a Dunstall 810cc conversion. I drank too much fine ale at that time to recollect
any comparison with my 850 of that time.

As the 850 is actually only 828cc

and with the diverging opinions as to which is better; 750 for zip and 850 for torque.

Is 810cc the Goldilocks capacity ?

Anybody have any experience ?

Getting back to the original question, I've done the swap. The easy way is to fit an 850 crankshaft, pistons, cylinders and head. The hard way is to re-balance the 750 crank for the 850 piston weight, use an 850 cylinder, and convert the 750 head to 850 bolt pattern. I'd agree with the above posters that the easiest way would be to swap in an 850 engine. I've had the 810 Dunstall kit on a 750, and I don't recommend it. The alloy he used for the cylinder is really soft, so you need to keep re-torquing the fasteners forever. It also has a really thin sleeve that has been known to break. As to overboring the stock 750 cylinder, it's risky to go to far, because if you do, the cylinder is likely to break at the outside front base fasteners, particularly with the early 750s. I never went over .020" over on my 750 race bike for that reason. Having said that, there are cases of owners boring them larger and never having a problem, but it is a known risk to do so. If you go to an 850, the easiest thing you can do to make it more like a 750 in terms of zippiness is to raise the compression ratio. A common way back in the day was to just mill the head. The factory said that removing .075" would raise the CR to 10:1. I'd recommend a bit less for a street bike unless you know you will always have suitable octane gas available. With any serious head milling, you will also have to check valve-to-piston clearance, and probably have to cut valve reliefs in the piston. As an alternative, you can just buy aftermarket high compression pistons that already have valve reliefs. If I were doing it on my bike, I'd just go with the pistons (probably 78 mm bore), and juggle head and base gasket thickness to get the CR right, probably around 9.5:1. I might go to 10:1 if I was also adding a longer duration cam.

Just my humble opinions, based on my own experience. Your choices have to depend on your particular situation.

Ken

Staytite said:

When I were a lad, many, many years ago a mate of mine had a 750 Commando
with a Dunstall 810cc conversion. I drank too much fine ale at that time to recollect
any comparison with my 850 of that time.

As the 850 is actually only 828cc

and with the diverging opinions as to which is better; 750 for zip and 850 for torque.

Is 810cc the Goldilocks capacity ?

Anybody have any experience ?

Click to expand...

Nah. The iron sleeves in the alloy Dunstall barrels came apart.
I believe an 850 can be lightened and tuned as a combat or better. A long time along the owner of a class B altered drag car told me there is no substitute for displacement.

Get some 9.5:1 pistons (milling the head means you can never go back)

Balance the crank

Get a hotter cam

Flow port your head

Get a Maney style exhaust or make sure yours flows freely

Your 750 motor will spin right up and put a grin on your face.

Just make sure your suspension can keep up.

gortnipper said:

Just make sure your suspension can keep up.

Click to expand...

And the brakes…

gortnipper said:

Get some 9.5:1 pistons (milling the head means you can never go back)

Balance the crank

Get a hotter cam

Flow port your head

Get a Maney style exhaust or make sure yours flows freely

Your 750 motor will spin right up and put a grin on your face.

Just make sure your suspension can keep up.

Click to expand...

You sure you should be hooliganing around like that at your age…?

Fast Eddie said:

You sure you should be hooliganing around like that at your age…?

Click to expand...

Listen to you...

Tornado said:

Have heard the 750's are a bit peppier than their bigger, heavier brothers. Yes more torque on the 8, but that comes at a cost of slower spool up.

Click to expand...

Arent the 850s lighter? Same bike, bigger holes

Fast Eddie said:

Well, yeah, but why?

Ain’t that what gearboxes are for ??

Click to expand...

Gas mileage?

I only need one gear from 40-100 mph.

worntorn said:

Arent the 850s lighter? Same bike, bigger holes

Click to expand...

Heavier crankshaft.

maylar said:

Gas mileage?

I only need one gear from 40-100 mph.

Click to expand...

I know, and you’re right.

But it does remind me of the old bike magazine tests that used to make a big deal out of “being able to do 15mph none snatch speed in top gear”...

It’s impressive, I just can’t see why it’s an advantage when you got a gearbox full of gears to play with !

Fast Eddie said:

I just can’t see why it’s an advantage when you got a gearbox full of gears to play with !

Click to expand...

Not needing to change gears may make our dinosaurs more attractive to the current generation. Oh - but you still need a clutch for that one gear ...

Fast Eddie said:

I know, and you’re right.

But it does remind me of the old bike magazine tests that used to make a big deal out of “being able to do 15mph none snatch speed in top gear”...

It’s impressive, I just can’t see why it’s an advantage when you got a gearbox full of gears to play with !

Click to expand...

I'm going to admit to it being a grin factor. Instant throttle response from 2500 rpm where the MKII makes 50 ft lbs gives me a "Yup, it's a Commando" smile every time I twist the grip.

maylar said:

I'm going to admit to it being a grin factor. Instant throttle response from 2500 rpm where the MKII makes 50 ft lbs gives me a "Yup, it's a Commando" smile every time I twist the grip.

Click to expand...

Now that’s a more convincing argument !

My 810 Dunstall Sleeper Mk4 cam 10:1 32mm carbs full dunstall exhaust 5 speed gearbox, is WAY too quick for its gearing...it seems to hit the red line in 5th at 107mph, it is very quick, how they got 130mph out of them is a mystery to me without vigorous regearing...Now im more used to Meriden machines it is very much faster than anything from my past including a blueprinted TSS...Starts easy too....

Fast Eddie said:

I know, and you’re right.

But it does remind me of the old bike magazine tests that used to make a big deal out of “being able to do 15mph none snatch speed in top gear”...

It’s impressive, I just can’t see why it’s an advantage when you got a gearbox full of gears to play with !

Click to expand...

Its a huge advantage to have ample torque for top gear when you are on a climb like this. You really don't want to flog the thing uphill for 7 miles revving in third, unless you enjoy early rebuilds.
We have a lot of grades like this in Western North America.
I did not encounter any in the UK.
So it's a different type of riding altogether.

I'm all for hooligan bikes too, my Herb Becker flowed 650SS is a lot of fun and probably revs quicker than the 750s.
It doesn't do so well on these grades. I wouldn't even consider trying to use it 2 up for this.

I’d just like to point out, all I said was I don’t see any real need to be able to accelerate from 15mph in top. I’m more than happy to use a lower gear for that. Torque in the mountains and for touring two up etc is all a different kettle of fish and a very understandable nicety.

The snatch speed thing always bewildered me too.
It's like winning the slow bicycle race, I would much rather win the fast bicycle race.

Glen