88305tpiT/A

iTrader: (2)

I am building a 383 and have a new roller cam and lifters that are in the block now. I also have a set of world 76cc chamber S/R torquer heads.

So I got one of the Comp Cams adjustable length pushrods but of course -- I cant seem to get the right length.

When I start at the stock 7.2" GM roller cam pushrod length, the rocker tip starts on the intake side of the valve and just keeps moving toward the exhaust side until max lift.

I think this means that the pushrod is too short because the rocker contact point should go back and forth across the valve tip from zero to max lift. thereby going through a minimum of valve side loading from the rocker (rocker moment arm is always around 90 degrees to the valve).

HOWEVER -- when I actually get the pushrod long enough to get the rocker arm tip to sweep back AND forth across the valve tip I barely have 3 threads engaged on the rocker stud, and the rocker tip is way out on the exhaust side of the valve tip.

I dont think I can get shorter valves nor do I want to since I dont want to machine the heads.

But I was just wondering if I could get away with centering the rocker contact patch over the middle of the valve tip, Even if the rocker angle never gets to 90 degrees WRT the valve tip. This would put side loading on the valve but since I have full roller rockers I thought I might be able to get away with it.

Should I be ok or do I have to pull the heads and get new valves?

Thanks,
James.

88305tpiT/A

iTrader: (2)

Yeah, 7.350 pretty much centers the contact patch but I have to go well over 7.500 to get the rocker to swing through perpendicular to the valve. And at that point I run out of threads on the stud and the contact patch is way on the exhaust side of the valve.

Help?

88305tpiT/A

iTrader: (2)

HMM.. Well I was just looking at comp's website and turned this up:
http://www.compcams.com/technical/Ca...106-07/264.pdf

Here it seems to say to just center the contact patch and makes no mention of rocker moment arm to valve angle.

I guess if this is ok then I would just order a set of 7.350's and be done.

But then I read this:
https://www.thirdgen.org/forums/tech...pushrod+length

So I am still unclear as to which is more important, Centering contact patch or rocker moment arm angle to the valve.

Help!

88305tpiT/A

iTrader: (2)

TTT

88305tpiT/A

iTrader: (2)

Well, I bounced this problem off people at work and no one really had a definitive answer there either.

I just cant figure out which would be worse, side load from a roller rocker across the top of the valve or having the contact patch be very biased to one side of the valve.

Help...

88305tpiT/A

iTrader: (2)

Here is some good stuff I found on this website:
http://www.thedirtforum.com/pushrodlengths.htm

Imagine a line from the center of the pivot point to the valve tip contact point. As that line passes 90° of the valve stem, the wiping action across the valve tip is minimal. So it makes sense to put that point where the spring forces are highest. For best valve guide life that point should be at about 2/3 of peak lift. If the rocker pivot point is in the correct place, the tip should be in the center of that valve at that point also.

Different site says this:
http://www.wighat.com/fcr3/confusion.htm

Pushrod Length- Incorrect pushrod length can be detrimental to valve guide wear. Most sources say that centering the rocker contact patch on the valve stem centerline at mid valve lift is the correct method for determining the optimum pushrod length. This method is wrong and can actually cause more harm than good. The method only applies when the valvetrain geometry is correct. This means that the rocker arm lengths and stud placement and valve tip heights are all perfect. This is rarely the case. To illustrate this, think of the valve angle and the rocker stud angle. They are usually not the same. If a longer or shorter valve is installed, then the relationship of the valve tip to the rocker stud centerline has changed. Heads that have had multiple valve jobs can also see this relationship change. Note, the rocker length (pivot to tip) remains unchanged, so the rocker contact patch will have to move off the valve centerline some particular distance for optimum geometry to be maintained.

The optimum length, for component longevity, is the length that will give the least rocker arm contact area on the valve stem. In other words the narrowest wear pattern. This assures that the relationship is optimized and the rocker is positioned at the correct angle. This means that the optimum rocker tip contact point does not necessarily coincide with the valve stem centerline, and probably will not. What is the acceptable limit for being offset from the valve stem centerline? That will depend on the set-up. A safe margin to strive for is about +/-.080" of the centerline of an 11/32 diameter valve stem. This means that no part of the wear pattern should be outside of this .160" wide envelope. As the pushrod length is changed, the pattern will change noticeably. As the geometry becomes closer to optimum, the pattern will get narrowest. If the narrowest pattern is too far from the valvestem centerline, then the valve to rocker relationship has to be changed. In this case, valve stem length will need to change.

1989GTATransAm

iTrader: (3)

The first one mentioned did not work for me. I bought the Manley style pushrod checker but it was way off for my application. I would have wound up with a shorter pushrod than stock.

The second method is the one I used and what I tried to explain in my other post. I ajusted the pushrod checker until I got the least amount of travel that was within .080" of the center line of the valve. It is a variation of the Miller Mid Lift Principle.

88305tpiT/A

iTrader: (2)

Cool -- Ill do that then.

Thanks.

1989GTATransAm

iTrader: (3)

Forgot to say that the centerline favored the exhaust side of the valve in my situation. If you want to do this correctly get a roller rocker such as the Comp Cams Pro Magnum.

Also and maybe more important get the stiffest pushrod you can afford. Any flex in the valvetrain will cause loss of valve opening and duration. You don't want the pushrod to be poll vaulting. 7/16" studs are better than 3/8" studs for stiffness.

88305tpiT/A

iTrader: (2)

Already have the pro mags and I think 5/16 pushrods and 3/8 studs will be fine for me.