750 commando piston position

I just want to verify the new pistons I have for my 750 commando are marked left and right but not exhaust I just want to double check on the position of the indents The manual says the indent furthest from the crown goes to the intake side. I am assuming that the larger indents are the intake side and the smaller the exhaust ? Just wondered why the manual didn't say that and if that is correct. Thanks jack

Look at the combustion chambers, you will see that the intake valves are much closer together than the exhausts, also the intakes are rear most. So, the reliefs cut into the pistons should have a corresponding pattern, intake cuts to the rear and closer together, exhaust cuts front most and further apart. Don't make this too complicated allow your natural common sense some freedom and you will do just fine. The workshop manual has some good pictures.

Thanks there are no pictures in any of my 3 manuals all they say is the intake is the furthermost from the piston crown One indent is wider and the other deeper I am assuming that I have it right and the wider indent furthest from the edge of the crown is the intake
Thx
Jack

You are correct. The larger valve cutouts are the intakes. As RoadScholar said, they should be at the rear of the cylinder, and close together.

There are pictures in the factory service manuals. In most of the later manuals they are figures C10 and C11, and they don't really show much detail. In some of the earlier manuals, there is a very good illustration of the valve cutout locations in the piston installation sections.

Ken

Thanks guys glad i had in right it seems intuitive but just wanted to check Thx Jack

750 and 850 engines are a desaxe type engine. In this case the repositioned piston center was grown only to the rear toward the intake side of the engine. The original position of the valves was NOT moved. The final result is the INTAKE cutout to PISTON edge (crown?????????????) is always larger.= REAR
IE the exhaust cutout is almost at the edge of the piston.= front
See example in lcrken pix above
Since the original (STOCK) 850 pistons were much shorter than 750, they did not require valve relief cutouts.

Ok thx for the drawing very helpful I did an 850 and there were no cutouts for the valves these pistons are also flat across the crown what is the difference to the 750 valves open further?
Jack

dynodave said:

750 and 850 engines are a desaxe type engine.

Click to expand...

Dave
Apologies for the off-thread question - though it is related...
I have noticed the crank is offset in the fore/aft direction previously - never heard the term "desaxe" before, but looked it up. Interesting!
Have all Norton heavyweight twins (500 to 850) had this feature?
Was it a specific design intent, i.e. to lengthen the induction and power strokes?
...or was in a convenient solution to some geometrical problem Norton was facing?
Cheers
Rob

robs ss said:

dynodave said:

750 and 850 engines are a desaxe type engine.

Click to expand...

Dave
Apologies for the off-thread question - though it is related...
I have noticed the crank is offset in the fore/aft direction previously - never heard the term "desaxe" before, but looked it up. Interesting!
Have all Norton heavyweight twins (500 to 850) had this feature?
Was it a specific design intent, i.e. to lengthen the induction and power strokes?
...or was in a convenient solution to some geometrical problem Norton was facing?
Cheers
Rob

Click to expand...

An understandable question. 500-600-650 all grew to the practical limit using a symetrical bore increase, considering the limited clearance between the valve/push rod tunnel positioning. These engines (sorry for NON commando content) are all small bolt pattern barrels.
Therefore the growing process to 750-850 made the bore...grow only to the rear, hence the BIG bolt pattern barrels which in many places I have mentioned that the "growing" was only by moving the rear set of barrel bolts.
This general principle carried over in two stages for the 750 (medium) and 828 (large) bolt pattern heads.
Finally, since the crankshaft/camshaft/pushrod/rocker gear were not moved it turned the engine into a Desaxe designation...... (if I am correct) for the last two sizes only.

Thanks very much for enlightening me Dave - I have learnt at least one thing today
Cheers
Rob

The definition is given here as follows;
http://www.ashonbikes.com/content/d%c3% ... a9-engines
This has two useful effects: one is a reduction in friction, improving the efficiency of the motor. The second is that because the peak forces on the pistons are smaller, they can be lighter, which in turn allows for lighter conrods and smaller bearings, also reducing internal friction.
Désaxé is often written De Saxé or something similar with claims made that the idea was invented by a Frenchman of this name. This is not the case, and it`s unlikely as one meaning of désaxé is a deranged person - roughly translated it means nutter! The other meaning is off-centre, which describes this cylinder arrangement.
There are other consequences. A small increase in the stroke compared with the crank throw has to be accounted for at the design stage. The power stroke takes longer, which is good for a high revving engine as it allows more time for efficient combustion, but it also makes controlling vibration more difficult as the secondary out-of-balance forces become more complex and asymmetric. The 1970s Hesketh V-twin used a désaxé design which was largely responsible for that bike`s excessive vibration, and the new VW VR6-derived Horex engine is also désaxé, although for packaging reasons rather than to deal with piston sidethrust.
The Ariel Leader two-stroke achieved désaxé advantages but by using gudgeon pins offset in the pistons - the cylinders were still centred directly above the crank.
So, you either are a Nutter as described above or have an engine that this out of balance- is this familiar to anyone :?: