I had the opportunity yesterday to go to well established frame and motorcycle builder that has the specialty in CB750. Here is his web site: http://www.cycleonemanufacturing.com/ . He had a large pile of CB750 motors in his shop along with a stack of approx. 10 to 12 cylinder heads. We were looking at different years and there different "Oil Control Orifice". We did not positively identify the exact year of each cylinder head but we did identify three different types of "Oil Control".
The first was just a small circular milled out area that had just an angled hole drilled in the direction of the oil galley that the stud goes through. There were two of these, just like current set up and location. These of course were non removable or replaceable. We were guessing these were early year cylinder heads.
You're correct: those were the earliest K0 types. The rocker towers themselves had the metering holes inside them, which were easily plugged with gasket debris, having no screening of any kind. These are mostly gone now, due to poor oiling performance. These rocker towers are also unidirectional, and often do not have an oil "exit" on the far end of the casting, so no O-ring recess is found in the head at their far end(s). There is a "left" and a "right" side rocker tower, and they must not be switched, lest the top end lose all oil flow. If a later, symmetric type cam bearing/rocker tower is used on these heads, a plug must be made and installed into the oil journal hole above each head feed, with a metering hole (like .037") drilled through the plug. Otherwise, so much oil enters the top end unrestricted that the crankshaft bearings starve, quickly. The oil PSI also drops to about 20 PSI in that situation.
The second cylinder head we identified had the same small circular milled out area but had a metal disk seated into it with a very small raised "nipple" in the center. This small disk was not aluminum because it had some rust on them and a magnet stuck to them. (I know, not very scientific). We did not put very much effort into it but was not able remove them. The "nipple" did not protrude enough to get a hold of to remove them.
With some more work, you'll be able to remove the jets. They have a dome on the other side, and the steel orifice plate you see is crimped around a collar and a 13-hole perforated screen on the other side. These are the later K0-K1 jets, often also found in K2 (like mine). These are the most efficient screen types, with metering holes usually of .0365" size. These are used with the symmetrical rocker towers, and there is an O-ring on both sides of the rocker tower where an O-ring seals the unused hole against the head to capture the oil pressure. These towers are interchangeable, left-to-right.
You will not usually see these oil orifices in any current parts fiche from Honda: they disappeared around 2002 from view. To my knowledge, you never could get separate ones from Honda as a spare part, either.
Then the third cylinder head we identified had the current circular milled out hole to except the current aluminum "Oil Control Orifice".
We looked through the rest but that was the only three types that we could find. Again not very scientific but to his more trained eye all the center metering holes in all types of Oil Control Orifice were the same size. All used the same 5.8 x 1.9 o-ring or 5.6 x 1.9 o-ring.
The large oil orifice in the K0-K1 heads can use either the 1.9x5.9, 2.6, or 1.9x3.9, or 2x4mm O rings as their seals, so long as the rocker towers in use are the symmetric type. In many of these heads, the outer O-rings MUST be the smaller size because the recess in the head is fitted to the max 4mm OD.
In all heads after the K2, the 1.9x5.9 or 2x6mm O-rings can be used on both sides of the rocker towers, as the heads were spot-faced to fit the 6mm size.
In my research at looking up the current stock or part number: 12238-300-000 Orifice, Oil Control I found that on most all fish files from places like (Partzilla.com, Hondapartsnation.com, Hondaparthouse.com, and many more) that this part is listed to be used in over 35 models of CB750, from 1969 to 1978 and used in K, F and A models.
I thought this was odd since we visible seen three different types but the fish has the part used in all Honda CB750 models.
This has got me a little confused as to when they actually started using them and in what models. Hope this info is useful to any that might be interested.
Also, like 70CB750 I now offer these for sale in the "Services Offered" section.
