Norton Desaxe cylinder offset

Ariel square 4s used both an off centre cylinder and an offset wrist pin. The same basic design went for over 20 years and while there was 3 changes to the 1000cc models, 4G, mk1 and mk2, there was never a need to revise the offset wrist pin. When building the engine the offset is determined by rolling both sides of the piston on a piece of paper and marking the wrist pin location on the paper. The shorter mark being the side the offset is toward. Front and rear pistons are then fitted facing in the opposite direction. The pressure in the cylinder is even so the piston does not tip and there is no rattle. It is like the storey that BMWs wear out the piston on the underside, does not happen.

David
 
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A lot of modern engines have off set piston pins for less friction and less rattle. They are off set towards the major thrust side of the bore which is to the rear on the commando. Probably the easiest mod is Jim's longer rods and lighter pistons for more efficiency. Pistons with off set pins would be nice for less rattle. But who would make them? Aluminium Nikasil barrels would be quieter with less piston clearance. Finally the angled base gasket changing the geometry, would work well for efficiency, but everything attached to the head would end up in a different place! Personally i would like the longer rods and lighter pistons and the nikasil barrels. Graham
 
With a Commando motor, we are already working with massive torque. The problems lie in finding better ways of using it. The throttle response problem disappears with close ratio gears and lean jetting. But with the low crankshaft balance factor, it cannot be used with high revs. Overall gearing must be high. To my mind, in standard form the motor is faster than any early Triumph 650 or 750. Lighter pistons would help, but longer rods deliver less torque and higher revs.
 
acotrel said:
With a Commando motor, we are already working with massive torque. The problems lie in finding better ways of using it. The throttle response problem disappears with close ratio gears and lean jetting. But with the low crankshaft balance factor, it cannot be used with high revs. Overall gearing must be high. To my mind, in standard form the motor is faster than any early Triumph 650 or 750. Lighter pistons would help, but longer rods deliver less torque and higher revs.
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What drugs you on Willis? 🤣
 
velo54 said:
A lot of modern engines have off set piston pins for less friction and less rattle. They are off set towards the major thrust side of the bore which is to the rear on the commando. Probably the easiest mod is Jim's longer rods and lighter pistons for more efficiency. Pistons with off set pins would be nice for less rattle. But who would make them? Aluminium Nikasil barrels would be quieter with less piston clearance. Finally the angled base gasket changing the geometry, would work well for efficiency, but everything attached to the head would end up in a different place! Personally i would like the longer rods and lighter pistons and the nikasil barrels. Graham
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From memory Ariel 200 twins have off set pistons. Speedway GM and other motors have off set cranks, Toyota cars etc, the list is enormous. I would have Maico 490 bores rebored off set to help reduce wear on the exhaust bridge and alter the port timing a tad. Don't fix it if it ain't broke.
 
Seems to me the people with answers to the question either have Norton original drawings, and/or have made cases. This means, to my knowledge the following, there may be others:

AN
Steve Maney
Andy Molnar
Dick Hunt (cast by Dick and machined by Bruno Perlinski)
 
Many modern motorcycles have fuel injection and engine management systems. I suggest that might be the best way of improving performance. From many years of altering motorcycles to improve performance, I have learned the changing many things makes no difference. In the 1970s, every model of the same motorcycle from certain makers, had different needles and needle jets - standardisation would have been cheaper. The retro Royal Enfield 750cc twins have fuel injection - might not cost much more than carburetors ? Fuel injection is probably not adjusted step-wise, and neither is ignition timing. Four things have improved my Seeley 850 - very careful jetting, steering geometry and gearing - and the 2 into 1 exhaust - we need smooth power delivery - it is more important than more power. More speed in corners makes speeds down the straights higher.
 
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jseng1 said:
No you don't have to change the timing gear or cam - nothing changes there - they stay exactly the same. All you have to do is install a tapered base gasket that's very thin in the back and about 1/16" or thicker in the front. This has the effect of offseting the cylinder centerline in front of the crank centeline about 1mm or so and that's where you want it. The lifter to cam contact patch only moves forward about .015" and that doesn't effect anything. In return you get a motor that runs quieter and puts less wear on the piston skirt as well as being more efficient and making more power.

I checked into offsetting the pin and its a bad idea. Moving the pin forward reduces the piston friction but creates more noise - no one wants more piston rattle.

I am checking into having someone machine a base gasket as described. Then all you have to do is install a thinner .020" thick head gasket to restore to original compression.

So far so good. If I can get the special base gasket we'll see if this all works out or not.
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A tapered base gasket.. wouldn’t that cause a problem for the long barrel bolts to engage ? (850 engine)
 
I don’t want to flog a dead horse but I don’t understand how the force on the piston can be equal if the wrist pin /gudgeon is off center. It may be an insignificant amount but the area of the piston crown will be greater on one side of the wrist pin centerline than the other . Please pardon my ignorance if I am missing something…
 
I’d guess that offsetting the gudgeon pin increases some rocking forces acting on the piston.

That doesn’t mean it matters, within limits that have been found by testing.
 
ludwig said:
A tapered base gasket.. wouldn’t that cause a problem for the long barrel bolts to engage ? (850 engine)
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I've checked and the top of the 850 through bolts can flex one or two mm with just hand pressure. The bottom of the bolts where they thread in is in the same location. I'll have to check to make sure the bolts screw in without binding. There should also be plenty of slop in the engine mounting plates and isos. All of this will have to be tested.

I don't have stock 850 cylinders here to test. If someone would do a quick mock up of empty cases and 850 cylinders - please PM me.
 
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Here's a drawing with the Desaxe layout I'm wanting to test.

Norton Desaxe cylinder offset
 
I suggest some things might be wishful thinking - a fuel injected Commando might be able to get the most out of petrol. What I have been suggesting is - it might take a lot of skill to get the most out of petrol using normal carburetors. Many years ago, some of the guys who raced two-strokes got them going a bit faster by using Lectron carburetors. From memory, they used to get finer adjustment by turning the needles which had flats on them. If you lean-off progressively with a normal carburetor, a stage is reached where lowering the needle one notch causes a miss, usual practice is to then raise the needle one notch. Getting the mixture really right might be more good luck than good management.
I might be wrong about this, but I suggest the slightest bit too rich might have a very big detrimental effect on combustion. An engine management system rationalises all the variables.
All of this stuff about 4 valves per cylinder might not be the reason that modern bikes of similar capacity are so much faster than Commandos.
 
With the wedge shaped base gasket the amount of deflection at the top of the 850 through bolt is .048". The bolt hole for the through bolt is .034" oversize so that gives .017" clearance on each side - reducing the bolt deflection at the top to only .031" I tested it and the through bolt seems to thread in no problem.

And no it doesn't change anything with the timing gears or timing cover.
 
Is the wedge shaped gasket intended to slope the cylinders a bit backwards to gain more leverage ? Atlas barrels are vertical. The offset might be different.
 

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