This listing is the Oldham Coupling that is fitted to all SOHC engines in the vertical shaft assembly. Two of these are required per engine, and we sell them in various thicknesses. This listing is for the Std 3/!6" thickness version which was always fitted on the bottom bevel and often fitted at the top. See also item number 0684 for various other thicknesses.
These couplings are CNC manufactured by ourselves from very hard EN24T to a high tolerance. The length of the tangs are the standard Norton SOHC size having being copied from good condition (un-molested) original items. Two of these are required per SOHC engine, one is fitted underneath the vertical shaft and one fitted above.
These couplings were originally available in different thicknesses, it being the width of the centre section that can vary - this was to allow for different compression ratio's, where the barrel height may have been amended to allow for a higher or lower compression ratio, in which case it is the top Oldham Coupling that could be changed.
It should be noted that the bottom Oldham Coupling should always be the same width - 0.187" (3/16") - this is important as the vertical shaft has a lip on it at its base, that means it can rest on the inner ring of the lower bevel bearing. Therefore if a thicker coupling was used it would mean the vertical shaft lip would sit proud of the bearing (and I assume this could place extra strain on the lower bevel gears - although how much risk there is of this I am not sure of. . . but it is the way Norton designed it, so must have been for a reason).
However, we offer a selection of different width couplings (both slimmer and thicker than the 'standard' size 3/16" type) to allow for if you are building an engine and need to adjust the coupling to allow for higher/lower compression ratio. This is also very useful if assembling an engine from parts of different years, as there seemed to be many minor permutations of barrel height/crankcase height/conrod length/head to barrel spigot etc etc.
This listing (0683) is for the standard 3/16" type coupling, go to Item 0684 if you require a top coupling of a different width.
Measuring The Required Width For the Top Oldham Coupling
So when measuring your engine on assembly - you should loosely assemble the engine with head/barrel/cambox/vertical bevel assemblies all fitted - but no couplings or vertical shaft fitted. Once everything is tightened down, you should then measure the distance between the top and bottom vertical bevel gears. You then subtract the length of the vertical shaft, the width of the bottom Oldham Coupling (3/16" as explained above), then the remainder of the measurement should be the width of the top coupling you require.
The final important thing to remember when taking this measurement is that it is essential to allow a 'clearance' gap on each Oldham Coupling to allow for expansion - I normally ensure a minimum of 0.015" (i.e. 15 thou) on each coupling - remember to account for this clearance in your calculations (i.e. a total of at least 30 thou gap for both couplings).
To accurately measure the distance you might find it is easier to make a telescopic adjuster - similar to the one shown in one of the photos in this listing. I made this many years ago by boring a steel cylinder approx 0.5" in outer diameter, to allow an aluminium 0.250" shaft to slide up and down inside it, and a knurled bolt screwed in from the side, so the shaft can be tightened in position when fitted, then with a scriber you can mark the position, as you will need to loosen it and allow it to be removed for accurate measuring. A tool like this can be invaluable for accurately measuring.
Email us on firstname.lastname@example.org if this is not clear or you require more inforation.
Last two points when ordering these couplings:
1. You should ensure the coupling slides into the 'female' tang of the vertical shaft completely, so that the top face of the shaft/vertical gear is fully touching the middle section of the Oldham Coupling. If you find the tang of the Oldham coupling touches the bottom of the slot of the vertical shaft/bevel gear - then it is very probable that your original vertical shaft/bevel gear has been modified!! - this was not unusual and I have seen it done on many occasions, I would expect that this means at some point in the past a previous owner has had to change parts or compression ratio and found it caused the vertical assembly to be too long for the cambox - therefore has shortened the end of the vertical shaft or gear to account for this. If this is the case with your bike, it is possible to carefully grind the length of the Oldham Coupling, to shorten it slightly so the base is again in contact - however I must point out - if you do this it is at your own risk! (although it was not an uncommon thing for old owners to carry out, I cannot recommend it). We will shortly be selling our own manufactured vertical shafts to original length's and these will be being made to fit the original tang length - as per these couplings. And for further information - later DOHC engines were normally fitted with couplings with shallower tangs - approx 0.300" in length seems common. If your shaft ends have been shortened to allow Oldham Coupling tangs of this length, it may be because at some point it was mixed with DOHC parts.
2. Wear of your vertical shaft or bevel gears and the fit of the Oldham Coupling?: It is very difficult to be hardfast about what tolerance of fit is acceptable when fitting Oldham Couplings into the verical shaft/bevel gears. Over the years I have measured very many original Norton Oldham Couplings and vertical shafts - and even when they have clearly been unused, there tolerances have varied by up to 2-3 thousandth of an inch??!! However, as a yardstick - when assembling an engine, I normally like it when an oldham coupling tang will push into the vertical shaft/bevel gear slot with a firm finger pressure. It should not lock up, but neither should there be any perceptable play once in place at all. If there is perceptable 'slop' between the tangs then I would suggest your shaft has excessive wear. We have manufactured the tangs on our Oldham Couplings to be in tolerance of the most common standard fit - and we then fitted to a variety of original shafts, and know they will either slide in or just require a light tap. Final point to note - it is not unusual when fitting these into original shafts/gears to find that one end of the slot on the shaft/gear is a 'looser' fit than the other. This seems normal - I think it was a consequence of this engine design that one side of the tang slot wears slightly more than the other - just make sure there is no perceptable 'slop' or rattle, indicating there is too much wear.
Not a cheap item, but reflects the difficulty to manufacture