Rocker arm comparison
- Thread starter 63409impala
- Start date
If your heads are 690's you are hitting the same walls that prompted Chevy to go to the 583's in the first place,bigger cam requiring taller springs,and your work arounds are conspiring to mess with your geometry.If someone were to take this into account before they converted the head to screw in studs,they could move the center of the bore of the rocker stud hole away from the valve stem centerline to account for longer valves.This would require a lot of measuring and geometric calculations to determine the amount or would need a junk stock head to perform experiments on for trial and error.Another method could be to rebore the the stud hole so that it is parallel to the valve stem axis so that the rocker pivot centerline distance to the valve stem centerline remains constant from top to bottom,but then you have to worry about side load on the stud.
I did some looking, the first that I have found was Crower when I googled offset rocker arm trunnion. They have .050 or .090 offsets to move the rockers with longer valves.
63409impala
Well Known Member
Found them for big block but only .090 backseat.
Big blocks work don't they.
63409impala
Well Known Member
I'd like to try a .050 and a .090
73684-8 1.75 .050
73688-8 1.75 .090
72897-9 1.7 .050 aluminum
73688-8 1.75 .090
72897-9 1.7 .050 aluminum
So I have read this thread through. What was the conclusion on the rocker studs? Rocker arms have 3 points of contact. Top of spring, Rocker stud and push rod. If any of these change, that changes where the rocker arm touches the top of the spring. So how long are your rocker arms? What is their brand/part number specifically?
63409impala
Well Known Member
They are a harland sharp 1.7 ratio roller rocker for a big block chevy with 7/16 studs
63409impala
Well Known Member
S2001np I believe
I'm sorry, the studs not the arms. Just realized my mistake.
I know this because I just dealt with this issue myself awhile back but I knew my issue was not my rocker arms but the studs I had weren't the studs I needed based on measurements taken. What changes if any have you made to your rocker arm STUDS length and would that make any difference in your case is my question. Every machine shop taps rocker arm studs differently. Did they go to low in your case? I'm pulling apart a set of 690's for a buddy of mine and his aren't done the same as mine were. His seem to work fine it seems but all things to consider where the rocker arm may not be the actual problem. I don't care if you need a 2" valve cover spacer to make taller rocker arm/studs fit, there really is no one size fits all with these girls.
63409impala
Well Known Member
I am not sure as the heads were machined for screw in studs over 20 years ago when the car was built.
63409impala
Well Known Member
Again, I cannot say this enough, thank you all that is along for the ride to try and help me figure this out. I just want to drive this thing. It's not a racecar, just a cruiser that will occasionally get a blast through the gears on a backroad.
Couple of things I've found out through the years:
-Don't get all hung up on having the rocker arm exactly in center of the valve stem. While this seems like the perfect plan, the reality is that as you shorten the push rod to do this, you lose lift at the valve. As long as the contact is from the center to the outer 3rd (w/o running off the valve tip, obviously), you'll be fine.
-Pushrod length, in and of itself, is immaterial as long as the lift at the valve is close to what it should be and the pattern on the valve tip is within reason (see above).
-Bigger/stronger pushrods cure a lot of valve train ills. I won't bore you with the math, but as an example...a 3/8" .080 wall pushrod is nearly 50% stronger ('stronger' as resistance to deflection) than a 5/16" .105 wall pushrod of the same material.
Based on your excellent pics, you have quite a few mock ups that I'd run with no second thoughts....provided the pushrods were stout. Remember that if you're using an adjustable pushrod for checking lengths, these things are about as rigid as wet noodle when it comes to deflection unless you use a light checking spring.
Hope this helps.
-Don't get all hung up on having the rocker arm exactly in center of the valve stem. While this seems like the perfect plan, the reality is that as you shorten the push rod to do this, you lose lift at the valve. As long as the contact is from the center to the outer 3rd (w/o running off the valve tip, obviously), you'll be fine.
-Pushrod length, in and of itself, is immaterial as long as the lift at the valve is close to what it should be and the pattern on the valve tip is within reason (see above).
-Bigger/stronger pushrods cure a lot of valve train ills. I won't bore you with the math, but as an example...a 3/8" .080 wall pushrod is nearly 50% stronger ('stronger' as resistance to deflection) than a 5/16" .105 wall pushrod of the same material.
Based on your excellent pics, you have quite a few mock ups that I'd run with no second thoughts....provided the pushrods were stout. Remember that if you're using an adjustable pushrod for checking lengths, these things are about as rigid as wet noodle when it comes to deflection unless you use a light checking spring.
Hope this helps.
Try to figure out which studs you have and find ones that are longer and test fit one. A few of my friends let me barrow some singles until we found the right length. I had a set of screw in studs I thought I was going to use but found they were 1/4 to short. Ordered the taller rocker stud and problem fixed.
63409impala
Well Known Member
With the adjustable pushrod, I'm using lightweight checker springs. Pushrod wise I'll probably eng up using trend or Manton .130 wall thickness
63409impala
Well Known Member
I plan on trying a couple different rockers including a .050 backseat crower rocker before I make a final decision. Like I said, car is mostly a cruiser but if I decide to play a little I want to be ok.
63409impala
Well Known Member
I need to replace all studs I believe as there is evidence of the old rockers hitting the studs due to too short of a pushrod. Nothing appears to be bent or broken, and compression check as well as leak down was beautiful.