SWITCH AS-MAGNETIC 1752301 - Caterpillar

3500 Family Of EnginesReference: Engine News, February 1986, Page 6, "Stronger Cylinder Heads Used".Two different cylinder heads are used on the above models. The oil drain hole is these heads has a different size depending on where the crankcase breather is located.
Early engines have the breather mounted on the front timing gear cover. The heads on these engines have a drilled drain hole 25.4 mm (1.0") in diameter. These former heads should only be used on engines with the breather on the timing gear cover. These heads have no cast boss on the left side. See illustration
Later engines have the breather (sometimes two breathers) mounted on a valve cover(s), and the head drain hole is a cast hole that is noticeably larger than the early drilled hole. This hole on later engines is larger because all the breather fumes from the entire crankcase must go out these holes while oil must also drain through these holes at the same time. The new heads are easily identified by the presence of the cast boss on the left side. See illustration. These heads can be used with the breather on either a valve cover or on the timing gear cover.
How to identify cylinder heads with the two sizes of oil drain holes.If a former head is used with the breather on the valve cover at this head location, it is possible that the crankcase pressure could be enough to restrict the flow of oil out of this head. Oil could then accumulate around the valve mechanism. Excessive oil consumption and carbon buildup on the valve heads could result.
If you are installing either an 0R2671 or 0R2621 Remanufactured Cylinder Head Group, be SURE to determine the size of the oil drain hole in the head if a valve cover breather is used with this head. Some of the core heads used in these Remanufactured Heads do have the smaller drilled drain hole and therefore should not be used with a breather at that head location. One solution may be to move the breather (or breathers if two are used) to another valve cover on a head with the cored drain hole if other parts on the engine at this cylinder location do not interfere with the disposal tube.
If necessary, the 0R1362 Remanufactured Cylinder Head Group can be used. All of these Remanufactured Cylinder Heads have a core head with the larger cored oil drain passage and will work with the breather (breathers) at any location(s). When this head is used, be sure to use the 2R6806 Connector in the fuel line and that the injectors have fuel screens. See Reference Article.