Old Heads vs New Heads - 401

[Scotty54]

77 Cherokee 401. I need to rebuild my heads at 100k - a stuck or burned valve in #2 cylinder. I have been offered a set of early heads for free (pre-1974), already rebuilt. I know these have the adjustable non-bridged rockers. Can these be swapped with the bridged-rocker heads or are there other considerations like pushrod length or alignment dowels to be concerned about? I have not researched the casting numbers yet. Figured someone on the forum has made this swap before. The new heads are from a 401 so valve size is the same.

[Srdayflyer]

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[rocklaurence]

My understanding is the heads are all mostly the same with the exception of the chamber sizes. So, you'd have to make sure that your piston and head combo' doesnt create high or low compression ratio's. I believe that the Later Smog heads are created to produce a 8-8.5 /1 compression.

[tgreese]

Adjustable? Seems unlikely. Looking at the '72 TSM, that style of rocker has a has no bridge, but it still has hydraulic lifters. From what the book shows, the change is a difference between a ball-shaped pivot and the bridged pivot.

Jeep72ValveTrain.png

Whether you can convert from one style to the other, I don't know. I'd guess it depends on the rocker pedestal height. However, I think it would be fine as long as you use the complementary parts (ie pushrods, lifters) that go with that style of rocker. They may be no different - you could look to the online sellers and see if the same parts are used for both style of rockers.

[Srdayflyer]

i woud bet the earlier head has a different squish chamber for higher compression as the EPA got into the pollution control game in 71-72 and beyond, i posted a thread on the 4.2/3.8 vs 4.0 6cyl. heads a lil while ago showing the marked difference between the open chamber 71-72 and later and the closed chamber heads pre 72 with pics, adding higher compression not that 9.0-9.5 makes a whole lot of difference but i noticed the difference in low end torque in my cj. and still run regular, premium comes into play around 10.0 + ratios, call those amc guys or anyone who does amc mods in your area ,IMO

[OldFarmTruck22]

Adjustable? Seems unlikely. Looking at the '72 TSM, that style of rocker has a has no bridge, but it still has hydraulic lifters. From what the book shows, the change is a difference between a ball-shaped pivot and the bridged pivot.

Jeep72ValveTrain.png

Whether you can convert from one style to the other, I don't know. I'd guess it depends on the rocker pedestal height. However, I think it would be fine as long as you use the complementary parts (ie pushrods, lifters) that go with that style of rocker. They may be no different - you could look to the online sellers and see if the same parts are used for both style of rockers.

The later Bridged rockers are designed to give the recommended lash for hydraulic lifters when tightened all the way down, VS. the earlier non-bridged rocker that does require a human know how to set lash for the proper lifter operation.

[tgreese]

Hmm. Unless I missed it, there is nothing in the '72 TSM about adjusting the rocker arm studs or nuts. Both the studs and the nuts are bottomed and torqued to spec. No adjustment. Seems this is no different from any other "zero lash" hydraulic lifter design - you have a fixed length pushrod, and the lifter expands (pumps up) to remove any valve lash. Not an expert on this topic - happy to be corrected, if you can point to a reference that contradicts this.

[Scotty54]

Tim - I rebuilt my first AMC engine (a 304) in 1980 and after a 2500 rpm break-in I adjusted the rockers with the engine running (and oil squirting all over the headers). This was after an initial setting per the TSM. The running procedure was to loosen each rocker nut until they started clacking, then tighten them until they stopped, then tighten another 1/4 turn. This was a common procedure with small block Chevy's also, and maybe I got the idea from one of my Chevy buddies. At that time I had all my machine work done at Jeg's and they may have have provided the info (back then Jeg's was just a small parts store and machine shop in Columbus, Ohio). However, I did the same thing with my 401-powered Spirit drag car. It's what I learned and it's always worked, and has also been done by other AMC racers. I have always had a valve cover with a long hole I cut out in order to reach the rockers but retain as much oil as possible. But you are right - the 72 TSM says nothing about valve adjustments but simply a torque figure for tightening the rocker arm nut. My 77 TSM has even less info about the rockers. Maybe I have been wrong all these years. I have some research to do!