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Reply to "Yella Terra YT6015"

The factory rocker arms have a max lift specification of 0.615 inches; BUT they have a reputation for causing accelerated valve guide wear when used in conjunction with high lift cams. I consider the factory rocker arms good to about 0.530 inch lift.

The other limitations are the 5/16 cap screws that secure the rocker arms to the cylinder head pedestals. If a cap screw stretches, the rocker arm fulcrum will begin to "rock" in the slotted pedestal; the rocking motion will eventually snap the cap screw. So for applications using the factory rocker arms in conjunction with aftermarket (stiffer) springs I recommend replacing the factory 5/16 cap screws with ARP 5/16 inch chromoly bolts p.n. 641-1500 and 1/8 inch thick washers p.n. 200-8587. This is good up to about 400 pounds “over the nose” and a 6600 rpm rev limit.

If valve lift exceeds 0.530 inch, or if a solid tappet cam is installed (requiring lash adjustment) the factory rocker arms need to be replaced.

 There are a few choices in aftermarket rocker arms which mount to unmodified factory rocker arm pedestals. Aftermarket rocker arms fall into two categories; (1) those that mount on studs and are laterally positioned by their push rods using guide plates, (2) those that are mounted rigidly to the cylinder head and are laterally positioned by the orientation of their mounting system. The factory rocker arms are rigidly mounted rocker arms. Rigidly mounted aftermarket rocker arms are described by several names: individual shaft mount, pedestal mount, saddle mount, or bolt down. Whatever they may be called they have two things in common: (1) they mount rigidly to the cylinder head and (2) they are equipped with threaded push-rod cup style adjuster screws for solid tappet lash adjustment or hydraulic tappet compression (pre-load) adjustment.

Rigidly mounted rocker arms have several advantages in comparison to stud mounted rocker arms: (1) rocker arm geometry is not dependent upon the length of the push rod, (2) rocker arm geometry is not altered during lash adjustment, (3) the mounting system is more stable thus eliminating any rocker arm induced valve train instability problems (stud girdles aren’t needed), and (4) rocker arm geometry is easier to set because it can be set while the cylinder head is sitting on a work bench.

I've never been able to find a retail business selling the Yella Terra YT6015 rocker arms, so I suggest substituting them with the Scorpion p.n. 3224 rocker arms, which are basically identical. They have a strong billet aluminum rocker arm body, they have a roller tip and are thus good for higher valve lift (up to 0.650 inch); they provide for lash adjustment via the push rod cup, but they are only specified for 420 pounds maximum spring force due to the 5/16 fastener that secures them to the factory pedestal. That should be all the rocker arm you'll need for any hydraulic flat tappet or solid flat tappet street application, or hydraulic roller tappet application up to about 0.580 or perhaps 0.600 lift.

The YT6321 rocker arm is secured to the cylinder head via a 7/16 cap screw. It would be a possible choice for a rigidly mounted rocker arm if the cylinder heads have been drilled and tapped for studs and guide plates. Since it is secured via a 7/16 cap screw it is not limited to 420 pounds spring force.  

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