Rocker cover oilways

[mdt-son]

Johnny B
The changeover information was mentioned by John Truscott in the cited thread - it may not be accurate, and going by the number of occurences, the changeover may very well have happened in the late 50's. However, it appears certain the oil jets were introduced 1949. First year of a reference to "metering plugs" was 1956, so perhaps the changeover occured that year? I am still puzzled by specificaton of a third metering jet.

Respectfully I have to disagree with your claim there is no pressure feed. Top end lubrication was taken off the end of the plunger pump (up to 1963), which acts as a piston pump. By closing the inlet and moving the piston, enforced of the guide pin, the pump builds up pressure to overcome geostatic height, wall and fluid friction, turbulence and tube narrowing by curves, and the restinance built into the rocker box by way of nozzles (jets and small orifices). Resistance is highly dependable of viscosity. Pressure is maximum at the pump outlet and gradually diminishes as pressure is "consumed" by the resistances causing pressure drops. The remaining overpressure at the exhaust pool feed is converted into kinetic energy, i.e., oil splashes out. Why is there overpressure at this point? The orifice is kept small enough so that resistance at this nozzle is equal or slightly lower than sum of flow resistance at the rocker shafts.

The value of total head losses is determined by adding together the serial continuous and localized head losses. In the case of the rocker box, where there are 5 feeds working in paralell, we use the electric cirquit analogon to find the flow rate through each nozzle. Pressure drop at each nozzle is determined by the one out of 5 nozzles offering the highest pressure drop, which may be the orifices at the rocker arms. Actual flow at each orifice (nozzle), which is what interests us the most, is easily calculated by: dQ/dt = dp_maxoffeeds * (2/R1 + 2/R2 + 1/R3),
where R1 is flow resistance at the rocker shafts, R2 is flow resistance at rocker arms, and R3 is flow resistance at the exhaust pool feed. dp is the observed pressure drop of all rocker box nozzles combined, and dQ/dt is the given total volume flow. Flow through each nozzle is then determined by (dQ/dt)1 = dp_maxoffeeds/R1, etc. , and the important sizing of nozzles is determined by the desired flow branching to each nozzle, e.g., (dQ/dt)1 : (dQ/dt)2 = 1/R1 : 1/R2 = R2/R1.

- Knut

[Joker_Bones]

This rocker box has a 3/32" hole at this location.

IMG_20230731_130242.jpg

This rocker box appears to have a plug/jet in it at the same location with a smaller hole in it.

IMG_20230731_130318.jpg

The third jet?

[mdt-son]

Thank you J_B, yes, so it seems. I have tried to capture the changes to the rocker box oiling system. According to the Fred Neill's book on AJS Motorcycles, "A practical guide covering models from 1945" (Pearson, 1948 , 5th ed. 1958), the 2S oil pump was introduced for 1947. The unrestricted large oil bore at the exhaust valve oil pool (3/32" presumably) was discontinued for 1954, at which time nozzle 018890 was fitted. Earlier rocker boxes may be converted and the method is described (p.15-16). So, we have established that the exhaust valve pool oil feed should have a nozzle 018890 to prevent over-oiling, and that this was standard fitment for 1954 models onwards. My rocker box, which could be similar to J_B's first sample depicted above, undoubtly was made prior to 1954.

So, what about the 2 nozzles of type 018890 cited for the first time in the 1956 spares list and fitted inside the rocker arms? Their fitment was the the last evolutionary step of rocker box oiling mods. The rocker box depicted by Les Howard appears to be of the final type.
How did these oil feeds look like prior to 1956? Were they 3/32" drillings which the factory or privateers blocked by use of brass plugs, with or without flats? We will never know for sure when plugs were inserted. It may have been a field service modification instructed by the company. Iron head models made 1946-50 had no plugs in these two feeds, and it appears likely the first alloy head introduced for 1951 didn't have rocker boxes with plugs either. Apparently plugs were inserted and possibly specified by the works (is there a Service Release Note?) some time between 1951 and 1956. According to the year-by-year list of changes to the singles, cylinder head oiling was increased for 1954 models, albeit without increasing the total flow, and it seems likely AMC achieved this by restricting oil feed to the rockers by partially blocking them. My guess is that rocker boxes with blanking plugs were made in the 1954-55 period.

- Knut