My machine shop has run into a dilemma with the ‘60 Pontiac 389 I’ve got in my old coupe. It’s got an early Isky roller cam, but it’s pretty mild. We were able to reuse the old school valvetrain but I had issues with rockers wearing prematurely, so we want to go to roller rockers. He’s having trouble with the geometry when switching to rollers, and has tried a few different options. Talking to guys on the Pontiac PY forum, I had a suggestion to machine the rocker studs in at an angle to help with this. Does anyone on here have experience with roller rockers in these early Pontiacs? From what I’ve read, ‘59-‘60 motors have unique valve train.
I’m not real familiar with the early 60’s engines, but I think to obtain proper geometry you’ll have to machine the stud boss down after removing the press in studs, thread the stud boss for 7/16” screw in studs, then you can run the roller rockers with some adjustable lock nuts. Definitely have to measure for proper pushrod length to get the geometry just right. There are several knowledgeable Pontiac guru’s on this site, The Rabid Whippet and Rocky come to mid right away, It would be good to get their input, I’m sure they would know right away what to do.
One other thing, I believe this engine had through the stud oiling and not through the pushrod. If you convert to threaded studs you would need to deal with that. My limited understanding is that to do it properly you need to tap the oil holes in the block and install threaded plugs to block the “through the stud” oil passages. Then run the proper length hollow pushrods. I just read a thread that dealt with this issue on another forum but can’t seem locate it. Again, I’m sure there are several other members who know exactly what needs to be done.
Yep, if it has thru stud oiling you'll need to modify the oiling. Butler racing offers the threaded studs and the link to them says what you'll have to do for oiling. Here is their link for the studs: https://butlerperformance.com/i-316...e-conversion-kit-1961-1966-389-421-heads.html
The hollow studs have been dealt with already. The cam and install kit from Isky oils through the lifters and hollow push rods. The stock steel rockers were drilled at the end where the push rod meets. The press in rocker studs are long gone, they drilled and tapped for threaded studs back in the day. But in order to run better ARP studs, the heads will have to be machined down where they screw in. Because mine have been drilled and tapped already, I don’t think it’s be an option to drill and tap again at an angle like suggested on the Pontiac PY forum.
I'm interested too. On my 58 motor I had the heads drilled so I could put screw in ARP big block Chevy studs. I ended up using the stock rocker arms with polylocks.The machine shop drilled the studs, so the oiling would still work properly, Roller rockers would be cool though.
The rocker studs can be re-drilled and be offset. It is done fairly regularly on IMCA cars to keep the valve geometry on small blocks. You should know about how much you need to offset them before you have it done as it is not a trial and error operation. I would think a good shop that specializes in heads could get you fixed up.
Shaun, can you post the link to the PY forum that suggests milling the bosses at an angle? I don’t think that would work even if your heads hadn’t been previously tapped. The reason I say that is because the hole for the press in stud is already in place, all that needs to be done is to upsize it with the appropriate size drill to prepare it for tapping. If you were to change the angle of the boss by milling it at an angle, some of the press in stud hole would still exist. And, with the press in stud hole still existing, you can’t change the axis, or angle of the hole, without making a portion of your new hole too large. Reading back through your original post I’m wondering if the pushrod geometry was off before and that was what damaged the rocker arms? Did they install guideplates previously? You will want to run those if you don’t have them. A couple other thing that could have caused your problem, I think the pushrod hole in the cylinder head might need to be relieved at the back edge and at the top of the pushrod hole. You would angle grind the hole back slightly with a die grinder and a carbide burr. But I wouldn’t do that until you assemble everything and check for proper pushrod geometry and make sure everything has proper clearance.
What is wrong with the geometry? Is the roller too far out on the valve stem or too short? Maybe a different rocker or a different ratio will put you where you need to be. If the rocker dimensions are right, you should be able to get the roller in the right position by adjusting the pushrod length. Whose roller rockers are you using?
The '55 - '67 used a 20-degree valve angle. In 1967, the valve angle was reduced from 20 degrees to 14 degrees for better breathing. 1967 was the last year for closed-chambered heads. The "670" head was 1967-only and the only PMD head to have a closed chamber with the new 14 degree valve angle. The 400 2-barrel kept the 20 degree valve angles for '67; starting in '68 all Pontiac V8s went to the 14 degree valve angle. Is it possible the roller rockers you're trying to use on your early 20-degree heads are actually for the later 14-degree valve angles? jack vines
That’s good to know on the BBC rocker studs at least. I think the geometry might be why the stock rockers were failing on me. Hopefully someone has done it without drilling/tapping the studs at an angle! Here is the thread: https://forums.maxperformanceinc.com/forums/showthread.php?p=6490164 It looks like he used 1/2”-13 threads which may be large enough to deal with the existing stud hole. It could be that this has been an underlying issue. I managed to ruin multiple rockers, and eventually broke off a stud because one was worn. This was within about 2000 miles. It does not have guide plates necessarily, but it’s got an Isky rev kit down in the valley that I would say acts as a guide plate for the push rods. I believe the rocker had good contact with the valve tip at first, and as the valve goes up the rocker is barely touching anymore. I believe the machine shop tried a couple different ones for Pontiac as well as SBC and maybe Ford. Nothing was seeming to work.
I read through the thread, thanks for posting that. It sounds like he used SBC rockers for that engine. He didn’t say what angle he milled the bosses at but it did sound like he somehow figured out all the geometry before machining. Hard to diagnose without being there but if it was me I would get a couple pairs of early 60’s and post 67 Pontiac rocker arms and mock everything up to see how it looks. Use light checking springs on one cylinder so you aren’t fighting the spring pressure. Make sure there isn’t any contact on the pushrod anywhere as it cycles through its lift. Watch the rocker arm tip and see where it sits on the valve stem as it cycles through. Answer is in there somewhere. I have a set of adjustable Pontiac pushrods I made for determining the proper pushrod length, not certain if they would work with your roller rockers but your welcome to use them if I can find where they are buried. I believe there are also adjustable pushrods sold for this purpose.
If the roller on the rocker is starting off with good contact (near the center) of the valve and barely touching the valve (meaning moved to the edge of the valve), you need to change your pushrod length. The pattern on the valve should be very small. Pushrod length will help make the pattern smaller. If it is too far out or in, then you need to adjust your geometry. If the pattern is tight and out some but not too much, that usually works. The loads on the valves are consistent. When the pattern is wide the load point shifts all over the place and wears out your parts.
This may not be your problem, but I ran early '65 heads on a 455 once. This was back when I had more time then money. I drilled and tapped the holes for screw in stud with the 1/2-13 threads. Where I failed was using the 455 pushrods, this set the rockers and pivot balls too high on the stud. The studs were 3/8" thread on top with a 7/16" shoulder where the ball pivots set. With the wrong pushrods, the ball pivots were half off the 7/16" shoulder. I made two pass at the drag strip before it started missing. All the intake valve tips were mushroomed out, three or more studs tore up, and rocker arms mangled. Without the proper support of the ball studs, the rockers were all over the place bouncing around and beating the tar out of the valves. The same thing happens without rush rod guide plates. The plates need to be up top as close to the rockers as intended.
I am putting together a 1960 389 engine, have modified heads for screw in studs for adjustable valvetrain with oiling through pushrods. The SBC Howard Cams 90007 (stamped steel) rockers look to be very close to the originals in shape and geometry. I used the original stud locations and angle but milled down bosses about 0.1". ARP 290-7201 studs were used but needed modifications. I have not fully assembled but expect that I will need custom length pushrods (I am also using slightly different length modified BBC SS valves). There also was an ebay seller that had special studs without the hex that could be used, but I think you had to put helicoils in, which did not sit right with me. Using the 1/2" studs provides more bite/support in the oil gallery area just below the bosses. I believe I used 11* wedges on the head gasket surface to do the studs, and 20* wedges to do the valve seal machining. Some slight shimming was required on one head as they were not quite the same.
If you are wearing out the stock ball rockers, it seems that you have a pre-existing geometry problem. Roller rockers won't fix that. It seems that pushrod length may be the main culprit, and also ensure the guide plates are working as intended. Are you running taller than stock springs? Or machined the spring pocket and they now sit too low? Something is not lined up right, and the travel of the rocker over the valve tip seems excessive, which indicates geometry is not as good as it should be.
It sounds to me to be a push rod problem too. Pontiac used different length pushrods and I believe the change over was 67'. I would check online to see what the difference is in length and then measure yours. I've seen guys put the later style into an early motor and have problems, can't interchange the push rods. At one time you couldnt buy replacement early push rods..... but I would bet that might be the problem.
Thanks for the advice everyone! The cam/lifters/push rods all came as a kit back in the day from Isky, and were marked for 55-60 engines I believe. Not to say that the push rods still aren’t wrong! I’ll talk to the machine shop some more and see how he’s testing the geometry of everything. This motor has the Isky RR3 Super Street cam. Not too wild, .440 lift 286 duration.
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