67 Atlas slightly smoky.

Hi Robert........I found the attachment lurking in the very old Dominator Service Notes. It contains some useful numbers and good advice but does not quite tell you how to test if your spring needs replacing. Except by checking the length. I believe that fitting a pressure gauge to the timing cover is one way of confirming that both the oil pump and pressure release valve are doing their jobs.

Over many years I owned and rebuilt 6 Atlas models. Generally they all smoked a little due to the lack of oil seals on the inlet valve guides. The fix for my later bikes was either changing the head for a Commando version or fitting modified guides to take oil seals.

The physical property of steel is that it has a particular Young's Modulus. That is - the stiffness does not change as it gets older no matter how it has suffered (unless it is physically cracked). However, the shape may change. So if one spring has the same dimensions as another (wire size, coil size, number of coils) they will both have the same stiffness (change in load per unit change in length) (as long as they are not overloaded and permanently squashed when tested). But they might not provide the same force when fixed in length if one is shorter than the other.

So all you need to know is: what is the original length supposed to be? If yours is close to spec, it should be as good as new.

Tricky! All I can suggest is that the stiffness is proportional to diameter raised to power 4

So 5% difference sure side is about 25% stiffer... (1.05 Ã 1.05 x 1.05 x 1.05... whatever that is...). It soon mount up.

E = 13400 tons per square inch in old money.

What thickness of compression plate will you be using, Robert? Obviously there is an optimum relationship between the arc of the rocker arm, and how it contacts the head of the valve stem, but the very fact that there is adjustment means that it is a little variable. A much cheaper way of taking up the slack is to install lash caps on the valve stems. Cheap as chips by comparison, adding 1mm, or 0.040" to the length of the valve stem, they may well restore the geometry without the need to change your pushrods. Before stripping, I would be inclined to check the sweep of your rocker adjuster against the valve head, to see if it is as it should be at the moment. If not, this may be an opportunity to correct it.

I know purists will say lash caps add weight to the valve train, which is a retrograde step from a performance tuning point of view, but I recall from your previous posts that you prefer a softer tune, so this will not be an issue, and will have the benefit of not wearing out the head of your valve stems.

TBH Robert, I think it will be down to trial & error, whilst first of all ascertaining how accurate your existing geometry is. Quote from RGM; I am sure they won't mind, and I am going to have to type it, because this forum will not allow me to copy & paste; how annoying is that?! Re lash-caps 'it can also be useful in certain situations when lengthening the valve improves the rocker geometry' As the earlier geometry was considered to be in need of improvement, the changes you make may well improve it;

Or not......

When the valves were lengthened post '66, and pushrods shortened, that is in effect what you would be doing by 'a' adding lash caps which lengthen the valve; 'b' adding a compression plate which in effect is shortening the pushrods. In theory one would roughly cancel the other out. As yours are the longer pushrods, you may struggle to find any longer ones; but may not need to.

Ah, I thought your bike was an early one! Post 125871 is the later one with longer valves. However, the principal should still work, without changing your pushrods.

Doh.... you put '67 in the header! Blame my foggy old brain...

It applied to the 650s as well. Looking at RGM's parts shop, they list the same inlet valve for all Dominators, 88, 99 & 650 from 1960-'67. It then changed from engine # 125871 for the 650 as it did for the 750; the 500 & 600s being gone by then.

It may be a re-design in 1960 created the issue, or possibly the change of valve was simply to increase the valve head diameter, with the introduction of the 650, and 88SS & 99SS. I think the latter is more likely, because they had introduced a hotter cam in 1959, upped the compression in 1960, twin carbs were available and the 650 was on it's way.

Maybe the geometry was never perfect, but did not manifest itself as an issue on the early twins, in their soft state of tune? I am surmising here of course, hoping that others who actually know will pop up and put the record straight!

Not sure about the downdraught head connection. Bear in mind the 600SS never had it, having twin carbs on the old head with splayed inlet manifold, whilst the 500SS gained the downdraught head in 1961, a year after the 650 was introduced, and a year after them all changing to the same valves.

AMC had no doubt been meddling for some time, given the merger with Norton in 1953. The correction to better geometry came during Norton's time at Plumstead.

Previously ian_cordes wrote:

Not sure about the downdraught head connection. Bear in mind the 600SS never had it, having twin carbs on the old head with splayed inlet manifold, whilst the 500SS gained the downdraught head in 1961, a year after the 650 was introduced, and a year after them all changing to the same valves.

AMC had no doubt been meddling for some time, given the merger with Norton in 1953. The correction to better geometry came during Norton's time at Plumstead.

Hello Ian the 650 was built from november 7th 1960 well before any SS models 500 or 600, and the model 88ss and 99ss are built from april 1961 with 650 crankcases push rods and camshaft and followers , and early model 88ss did not have the downdraft heads until later in the year of 1961 the 650ss was sold from around august 1961 some odd ones even earlier ,and twin carburetor model dominators were sold as far back as 1954 if you had the cash norton would supply itfor you, yours Anna J

Previously robert_tuck wrote:

I am also interested in when the poor geometry occured, Did it start with the Manxman or when AMC took over production around 63 and the 750 only? If then how come it took many years to come to light??.

Hello now tell me how this bad Geometry started with the 650cc manxman when Norton had taken the trouble to make a downdraft head all based on the Manx norton valve Geometry its bad geometry may of started with the more economical plumstead built machines as production was by then going pear shaped, by late 1965 yours anna j

Previously philip_hannam wrote:

Hi Robert........I found the attachment lurking in the very old Dominator Service Notes. It contains some useful numbers and good advice but does not quite tell you how to test if your spring needs replacing. Except by checking the length. I believe that fitting a pressure gauge to the timing cover is one way of confirming that both the oil pump and pressure release valve are doing their jobs.

Over many years I owned and rebuilt 6 Atlas models. Generally they all smoked a little due to the lack of oil seals on the inlet valve guides. The fix for my later bikes was either changing the head for a Commando version or fitting modified guides to take oil seals.

hello early 650 and 750 twins did not need rubber seals as the inlet side as DRAIN HOLES down the sides of both valve springs if these Drain holes get blocked then Oil cannot drain way from the Inlet side, the Exhaust Side oil runs down into the two built in push rod tunnels to lubricate the camshaft followers so there is no build up of oil on the Exhaust side, so try checking the drain holes within the cylinder head on the inlet side, and the one that runs down the back of the cylinder block into the timing side cases , and the only valve guides you need are Bronze ones witch are better then old cast iron ones witch can break with the engine running, yours anna j