Today I swapped the camshaft in my brother's van from a mechanical camshaft to a hydraulic camshaft. During the procedure I made some interesting discoveries about the differences between the hydraulic and mechanical camshaft valve trains that I have not seen discussed elsewhere.

First, a picture showing the different lobe shape between a hydraulic lifter profile cam and a mechanical lifter profile cam (hydraulic on the left):



Key differences: hydraulic cam lobes are pointier and the rear bearing surface has the groove all the way around the cam for oil supply. The mechanical lifter cam has softer points and only a hole through the camshaft for oil flow.

The next difference is fairly major and I was surprised to see it. The oiling holes in the rocker arm shaft for the hydraulic valve train are drilled at a different orientation to the rocker arms than are the holes in the mechanical lifter rocker arm shaft (hydraulic on the left, mechanical on the right):



Accordingly, the oil passages in the hydraulic rocker arms are noticeably different than the oil passages in the mechanical lifter arms:

Hydraulic:


Mechanical:


The hydraulic rocker arms have a circle at each end of the oil groove on the inner diameter of the rocker arm shaft surface and the groove itself is longer. The mechanical lifter groove has a circle only on the top and the groove tapers out much sooner than does the hydraulic rocker arm groove.

When installed correctly, the oiling holes in the hydraulic rocker arm shaft point directly at the head, parallel to the rocker arm shaft bolts. The larger groove in the hydraulic rocker arms allows the rocker arm to get oil flow near continuously during operation.

Perhaps it was only me, but I had been under the mistaken belief that one could swap to a hydraulic cam and keep the rocker shaft for the mechanical rocker arms. This would make swapping a hydraulic cam into a drool tube head easier due to the difference in the rear rocker shaft bolt:



I believe that the differences in the rocker arm shaft oil holes means one MUST use the hydraulic rocker arm shaft with the hydraulic rocker arms since the mechanical rocker arm shaft puts the oil in the wrong place. I believe using the mechanical rocker arm shaft will result in insufficient oil reaching the groove in the hydraulic rocker arms.

If one wished to install a hydraulic cam in a drool tube head equipped motor, all one would have to do is drill out the bottom of the hydraulic rocker arm shaft rearmost bolt hole with a 3/8 drill bit. Clean out all the shavings to avoid the shavings working their way into the lifters. The rearmost mechanical rocker arm shaft hold down bolt can then be used with the hydraulic rocker arm shaft.

Another interesting thing I discovered was that at least Autozone gives a different part number for a hydraulic cam fuel pump vs. a mechanical cam fuel pump. The fuel pump on my brother's van happened to be bad, so I bought a new pump but matched it to the hydraulic cam. I compared the two pumps side by side and didn't see ant noticeable difference. Maybe it is nothing, but the van is running now.

Hopefully this information will assist someone in the future who is contemplating installing a hydraulic camshaft in their slant six.

EDIT- sorry about the stupid Photobucket watermarks. They weren't there when this thread was created.

Great pics and writeup! This ought to be a sticky. Difference in fuel pump is inlet/outlet angle, and it's by year/application rather than valvetrain type.

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Too many people who were born on third base actually believe they've hit a triple.

FYI Reed, the "stepped" rear bolt you have from the hydro rocker assembly became a standard feature about 1978-1979 on mech cam
225's. I have a couple of "late" heads and a 1978 "447" head, and a 1979 "447" head I have both came this way. If I get some time I might have to pull these heads and rocker shafts and compare to the hydro motor I have, to see if oiling holes are drilled differently as stated.

The lobes are probably pointy on the hydro cam to allow the lifter to accelerate faster, so it will pump up. Currently there are more hydro cam cores available new than mech cam cores, which makes for a good "cheat" for people getting a new grind who want/need more oiling to the rockers and don't want to install a grooved rear bearing.

Good write up! Thank you!


-D.Idiot

The lobe difference is why you can put hydraulic lifters on a solid cam, but can't put solids on a hydraulic. The ramp is steeper on a hydraulic because the lifters absorb the shock.

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Official Cookie and Mater Tormentor.

Thanks for the compliments. I figured these differences were important enough to be worth documenting. I have found that there isn't really much info about the hydraulic motor and the differences between it and the mechanical cam motor. The limited info I have been able to find indicates that there were some noticeable weight savings implemented in the hydraulic motor as well as the obvious switch to hydraulic lifters that would make it desirable for a daily driver. I plan on building a fuel injected hydraulic motor for my brother's Duster project and I will document more of the differences as I get into that.

I was aware that the stepped bolt came into use around 77 with the switch from the drool tube heads to the peanut plug heads. I figured since most people out there use the drool tube heads (and because that is what is on my brother's van) I would just include the info about adapting the hydraulic rocker arm shaft to the drool tube head. I have successfully used a late 70s peanut head on a hydraulic motor short block and kept the hydraulic valve train and not had to modify the rear rocker shaft bolt hole. The rear rocker shaft bolt is not really a function of mechanical lifter vs. hydraulic lifter, but rather a function of pre-77 vs. post-77 (or whenever the peanut head came into use).

Peanut plug head comes around in the 1975 model year, so castings will start from mid-1974 to prep for new cars being sold about September 1974 for the 1975 model year. So far the peanut plug heads I have from 1976 motors all have the straight bolt like the 1974's, so it's a refinement later to the peanut plug but after cast crank engines were introduced.

-D.Idiot

Ahh, I see. So there was a few years production of peanut plug heads that used the older fatter and longer rear rocker arm shaft bolt, and the stepped bolt was phased in around 77. Interesting. I wonder what the motivation was to change to the stepped bolt?

Sounds like a trivia question for Mr. Weertman....

I'd really be curious what spurred this on...Were the designers starting that early on the hydro motor retrofit/designs?


-D.Idiot

Possibly. Dan can provide more details, but I know that the late 70s was a time of many development projects for the slant six such. However, my 1984 factory service manual says the lifters only need 3-4 PSI to pump up, and that is easily obtained using the older non-stepped bolt design. I can't imagine that the stepped bolt design was necessary simply for greater flow or oil pressure reasons. I am tempted to take a bandsaw to some of the spare heads I have to investigate if there is any difference in the stepped vs. non-stepped bolt oil passages, but I don't have the heart to cut up useable parts.

The "stepped" bolt, with 2 different size holes and using a different size thread, makes it impossible to install the rocker arm shaft incorrectly.
DD

It is nice that the bolt makes rocker arm shaft installation a one-way-only proposition, but was that the only reason the change was made?

The reduced diameter of the "stepped" rear bolt does allow for a little better oil flow, past the bolt and into the shaft.
DD

That's ringin' a bell for me…I thought there was something in the History of Chrysler Corporation's Slant-Six Engine (W.L. Weertman) stating that the change was made so the rocker shaft could only be installed the right way round (rather than relying on looking for an unreliably-present little flat on one edge of one end of the shaft), but I don't find such a statement in that document. I know I read it somewhere! I'll ask WLW and see what he recalls.

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Too many people who were born on third base actually believe they've hit a triple.

As an aside, the mechanical rocker shaft that we removed from my brother's engine did NOT have the flat on the top front of the shaft AND the shaft was installed upside down. Goes to show you that the stepped bolt was a good idea.

As far as oil flow past the rearmost bolt, would it be worth it to grind down the diameter of the bolt on the non-stepped-rear-bolt assemblies?

The amount of opening between the rocker bolt, rocker arm shaft & support pedestal is something to inspect and adjust. (if needed)

It does not take much of an opening to allow oil to pass but things like shaft mis-alignment and the bolt's shank diameter can restrict the flow.
Pull the rear bolt and look into the hole with a bright light... you will see any significant restriction.

More important... did the hydraulic cam have a groove in the rear journal?
DD