8/1/2002
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Understanding Cam Science, Wear And Oiling
By Doc Frohmader
Through the ’50s one aspect of American engines went through quite a number of changes – cams. The experimentation and changes were the result of the move from flatheads to OHV engines. While flathead valvetrains got relatively debugged, OHVs were new stuff and presented issues not seen before. Increased engine speeds, performance levels and engine demands created a whole new generation of engineering.
For many engines, the top end of the valvetrain was not lubricated through the push rods, but through internal passages. At the bottom of each passage was the cam and cam bearings which, in combination, metered oil to the top end. In most cases, this worked fine unless the passage got plugged, or until the engine was rebuilt. When rebuilt, several common vintage engines had trouble because cam and bearings had to be a set.
One set was a cam with a grooved journal and a bearing that had a specifically-sized orifice. Oil from the main galley passed into the cam groove, and a constant supply of pressurized oil was available to the bearing. In turn, the bearing orifice would meter the oil allowed into the vertical passage and supply the right amount to the top end.
The other set-up used a cross-drilled cam and slotted cam bearings. Oil flowed from the galley into the cam in pulses – one every time the hole in the cam crossed over the oil hole in the cam bearing from the galley. Then, as the cam rotated, part of the oil in the cam would pass into one of the vertical passages as it passed the slot or hole in the cam bearing on that side. As the cam rotated more, another portion of the oil was delivered to the other vertical passage. The obvious problem was the one side would tend to get more oil than the other – if the slots or holes in the bearings were the same. To compensate for this, bearings were made with a longer slot or larger hole on the second side.
You can see why when ordering parts for a shaft-mounted rocker equipped engine, you want to make sure cam bearings match the cam, and bearings are installed correctly. Ford actually got this wrong in 1963. They installed a cross-drilled cam in some engines with bearings designed for grooved cams. One side was wet and the other dry. They sent out a circular showing how to install a cup plug with a drilled orifice in the head on the wet side (under the rocker assembly) so the other side would get lubrication.
The other major problem you may face with vintage cams is the materials used. For all intents and purposes, there were three types of cam materials: steel, hardenable cast iron or chilled iron. Very few steel cams were found, but both hardenable and chilled iron were regularly found. With few exceptions, modern cams are made from hardenable cast iron, and most you’ll find for vintage engines will also be of the same material.You must, however, know for sure. The reason is the cam and lifter materials have to be compatible.
I don’t recommend using forged lifters on any cam. This was a bad idea, one that at least Ford tried, and which resulted in fast-wearing cams at best. If you run chilled iron lifters on a hardenable cast cam, the harder lifter will eat the cam. If you run hardenable lifters on a chilled iron cam, the harder cam will chew up the lifters.
The solution is to purchase your parts from a reputable and knowledgeable source. You want someone who knows the difference, will state in unequivocal terms exactly what they are selling, and accept the responsibility for mismatched parts. If they don’t know or won’t cover you, go elsewhere.
The Ford/Edsel (FE) line engines, such as the 390 and 428, have an interesting tale around cams and oiling. I’ve seen any number of FEs with low oil pressure and someone scratching their heads and wondering why. Solutions involving enlarging passages help, but the real source is wear and clearances.
When assembling, look at the cam-to-bearing clearances and determine if there is excess clearance (.002˝ to .004˝ is considered acceptable). Solutions include a different cam, sorting through bearings to get smaller bores, or using unfinished or oversized bearings and machining them after installation for the right clearances.
In some cases, an oversized bearing is available from some suppliers which fits tighter in the block and will reduce the internal size as a result of compression. These bearings are typically made to use in a block which has been bored out and requires a larger OD bearing. There have been some engines with loose bearing problems in the past and certainly there have been damaged blocks and these items helped cure them.
Because we see fewer shaft-mounted rockers these days, another problem has developed. Oil is regulated to the top end, but this pre-determined flow assumes the parts are in proper condition. Rockers pass oil over the push rods, valve stems and pivots. They self-regulate oil flow.
When the rockers wear or the shaft wears, the clearances between shaft and rocker increases and more oil flows out of each pivot point. This can cause low oil pressure elsewhere, can cause dry push rods or valve stems, and in at least one engine, can result in so much oil in the valve covers it can’t drain back fast enough and the pan goes dry.
It goes without saying that it’s not worth damage to the rest of the engine to use "ragged out" rocker assemblies. You can get replacement parts new for many. For the rest there are sources like Rocker Arm Specialist that can rebuild any original parts and make them better than new for reasonable money.