Cylinder axis in 750/850 Norton?

Re: Cylinder axis wrong in 750/850 Norton?

This is not a critical post, just the observation of someone who spends a lot of time with Norton twin parts.

If you lay 650 base and head gaskets over a 750 case and head you can see what Norton did. By luck I have the second production Norton 750 engine to look at.

The squish band at the back of the 750 head is as wide as the difference in the cylinder bore of the 650 and 750, 5mm. The base studs of the cylinders are all the same except the rear four, which are moved straight back.

Norton spent as little money as possible. It would have been a lot of engineering and cost to move the valve gear-cam-crank relationship. The cheapest route was to make a new cylinder that only required moving a few holes and adding clearance to the existing crankcase for it to fit. Maybe the smaller diameter Atlas head bolts saved them some money.

Looking at the cylinder head it is easy to see that the 5mm of extra bore was all added to the rear, the cylinder center would only have to move backwards half that amount, 2.5mm.

If you lay a 500-650 head gasket over a 750 head, it is easy to see the combustion chamber is in the same place on both engines and was not moved.

If moving the center of the cylinder made the engine run more quietly that was a free bonus for what was initially sold as a very low-compression, single carb sport-tourer.

Somehow the Japanese and others must have figured something else out, because they are using cylinder axis in front of the crank axis on current consumer goods. Of course they are not selling anyone half-century old air-cooled engines either.

Re: Cylinder axis wrong in 750/850 Norton?

beng said:

If you lay a 500-650 head gasket over a 750 head, it is easy to see the combustion chamber is in the same place on both engines and was not moved.

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Try laying a 650 gasket onto a 500cc iron head, like this ?
It was the bore that was moved back, as already mentioned earlier...

Iron 500cc head, later 750cc gasket.
(All the iron 500cc dommies had spiggotted cylinders.
As did all 600cc dommies, early 650's and early Atlases.)

Re: Cylinder axis wrong in 750/850 Norton?

I always thought the offset in the combustion chamber was to maintain adequate gasket surface between the combustion chamber and pushrod tunnel.

Re: Cylinder axis wrong in 750/850 Norton?

bwolfie said:

What about an oval bore. Worked well for honda.

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Not for long.

Re: Cylinder axis wrong in 750/850 Norton?

Since this is idle musing on Beng's part......

How about solving this 'problem' by running the engine backwards! now the problematic offset is actually a desirable offset!

..........of course you will need a gear in the primary drive to maintain clutch direction, a new cam profile and maybe a few more things......

And it will also have effects on handling and stability I may not have throught through yet ;-)

Re: Cylinder axis wrong in 750/850 Norton?

Some cycle have had the crank rotation opposite the regular direction for help with wheelie tendency. The best I can glean why most moderns have piston pins or bores shifting to forward bias is to help higher rpm above Norton tolerance so may not do a thing in that regard for Nortons as something else would break first any way, like crank or cases or drive train. Norton did the smart thing for mo power even if not the best thing for hi rpm, like over 10grand. Even so the best I can glean is bore shift still only good for a few hundred more rpm so nothing very dramatic gained.

Re: Cylinder axis wrong in 750/850 Norton?

Dances with Shrapnel said:

I always thought the offset in the combustion chamber was to maintain adequate gasket surface between the combustion chamber and pushrod tunnel.

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The combustion chamber originally was never offset, as Rohan's photo shows, the bore was moved back at the same time it was increased in diameter, keeping it's front edge mostly in the same place in relation to the pushrod tunnel.

When the bore center was moved back 2.5mm for the 750, that 2.5mm becomes the hypotenuse of a right triangle with 65/25 angles, the 25 degrees being the amount the valve centerlines are rotated from straight forward.

So when 750 pistons are machined, the valve notches have to be moved off the bore centerline which is now one millimeter away from the plane of the valve stems. The 500-650 engines had the bore axis laying on the valve stem centerline so when pistons are made for them no offset is needed.

This matches drawings used by Heinz Kegler to have Mahle make pistons for Dominators in the 60s, and drawings used by USA manufacturers of Norton pistons for the 750 engines.

Now modern manufacturers seeking gains in power and efficiency are moving the bore center more than 2.5mm, Honda has one engine aimed like a gun to shoot in the direction of crank rotation 14mm. Maximum pressure would not be on the compression stroke, but early in the power stroke when the fuel is all burned and pushing the piston back down the bore, so if the piston and rod are all straight with the crankpin at that moment, then the highest instant of possible friction between the cylinder and piston is minimized.

If the cylinder is aimed into the crank rotation as on the 750 Norton, then at the point of max cylinder pressure during the power stroke the rod is already at a less efficient angle trying to push the piston through the side of the cylinder wall.

So the crank degree at which maximum pressure in the engine takes place is found, and that is the degree at which you want your connecting rod to be parallel with the cylinder wall, so more of that power can go straight into the crankpin and less lost to friction.

Re: Cylinder axis wrong in 750/850 Norton?

beng said:

Dances with Shrapnel said:

I always thought the offset in the combustion chamber was to maintain adequate gasket surface between the combustion chamber and pushrod tunnel.

Click to expand...

The combustion chamber originally was never offset, as Rohan's photo shows, the bore was moved back at the same time it was increased in diameter, keeping it's front edge mostly in the same place in relation to the pushrod tunnel.

Click to expand...

Yes, my bad choice of words, that is exactly what I was trying to convey. Thanks.

For what it is worth, when bumping up to much larger bores and high compression ratios, this gasket area between the cylinder edge and pushrod tunnels becomes problematic.

Re: Cylinder axis wrong in 750/850 Norton?

Great thread, thanks Beng for coaxing out those who know.


When there is a gear driven primary, the engine has to turn the opposite direction of a chain driven primary. As mentioned, that fact may have something to do with the Japanese' decision to offset forward.

Re: Cylinder axis wrong in 750/850 Norton?

I think the cylinder axis in the commando engine is offset in the correct direction to produce more torque, which is sensible in a long stroke engine. I suggest a lot depends on mindset, some of us are fixated on high revs, big ports and short stroke to get performance. I know that I have been, and it might not always be the best answer. I suggest the main weakness in the commando has always been the gearbox.

Re: Cylinder axis wrong in 750/850 Norton?

Why would moving the cylinder back give more torque ?
Something the 850 in particular is not short of..

A common trick that appeared in proddy race bikes, Kawasaki Z1's in particular apparently, was to resleeve the cylinders slightly forward of the usual sleeve position.
This gave the engine in effect slightly advanced cam timing, and more jump off low revs. (only a slight effect, but any advantage can give you an edge)
Once the scrutineers became aware of this, they were vigilant of looking for it....

Re: Cylinder axis wrong in 750/850 Norton?

It was suggested that moving the cylinders forward to advance the cam timing was done on a bike used in the Australian Castrol 6 Hour race in the 70s. I do know that the identifying numbers of the bikes to be used in that race, from a certain shipment were sent to a dealer from Japan. Moving the barrels back improves angularity which suits low revs better.
As far as scrutineers detecting that the barrels have been moved - you must be joking !

Re: Cylinder axis wrong in 750/850 Norton?

acotrel said:

As far as scrutineers detecting that the barrels have been moved - you must be joking !

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I believe they were supplied with a reference set, as a genuine spare part, showing how they should have been manufactured.
Once compared, any cheating was obvious.

Re: Cylinder axis wrong in 750/850 Norton?

For what its worth with pros and cons of being forward or backward of the axis, RGM's 920 pistons have the gudgeon centre line offset slightly more to the rear. So, there may be something beneficial in what Norton did with the 750 onwards to the 850.

Re: Cylinder axis wrong in 750/850 Norton?

Ive an idea they did the offset bore on the kwickersakis to unmask the inlets , so WAs to the rear .

I never used to believe in my 850 engine, however now I think it is pretty good. The way I've tuned it, it delivers massive torque, and the only problem with that is gearing for it. I cannot understand why people fully hemisphere the heads and shorten the stroke. Triumph engines are inferior except for the separate inlet and exhaust cams. I suggest there are two ways to go - top end, lifting the rev range - or build on the strengths of the design and increase the torque output. I consciously chose the latter, it is much cheaper and probably just as effective.

Re: Cylinder axis wrong in 750/850 Norton?

ML said:

For what its worth with pros and cons of being forward or backward of the axis, RGM's 920 pistons have the gudgeon centre line offset slightly more to the rear. So, there may be something beneficial in what Norton did with the 750 onwards to the 850.

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I think that's more due the piston availability, not intentional design on RGM's part. As I recall they use English Ford pistons for their 920 kits, just as Fair Spares used to, and they come with the pins slightly offset. When I used those pistons for a 920 conversion some years back, the original Ford pistons had no valve relief cutouts, so you could have fit them either way. I fit them so the offset cancelled out some of the Norton cylinder offset, and then machined the valve reliefs to match. I think RGM does it the same way. According to Steve Maney, if you fit them the other way, so that the offsets add, they are extremely noisy.

Ken