Determining Pushrod length???

[68converted]

I have AFR 165 heads with 1.6 pedestal rockers, and when I torque down the pedestal bolts all the way, I can still turn the pushrod. The pushrods I was trying to use are stock from my 91 GT. How do I determine how long the pushrod should be? Or better yet, what length do I need?

 

[N8Miller]

What cam are you running? alot of companies (like my comps) make the pushrods for specific cams. now, if you have stock ones, i would think that any set of FMS hardened ones would help. BUt, since you have an RPM2 intake and AFR heads, my guess is that you have SOME sort of aftermarket cam.

 

[68converted]

What cam are you running? alot of companies (like my comps) make the pushrods for specific cams. now, if you have stock ones, i would think that any set of FMS hardened ones would help. BUt, since you have an RPM2 intake and AFR heads, my guess is that you have SOME sort of aftermarket cam.

Right you are... I put in an XE270HR-14. The stock rods are exactly 6.25"s long. Would comp cams be able to tell me which rods to use? Or do you think AFR would be the source? I assume I would want to go with hardened, as the spring rates are almost double that of stock. And... if I have replaced everything else how important will it be to switch out the lifters? Any benefit to the switch? or disadvantge to not switching? Thanks

 

[Michael Yount]

There are so many variables potentially at work within a particular engine, I wouldn't rely on someone else - even the cam company - to determine proper pushrod length. I'd measure. They make adjustable pushrods that can be used to determine the length that gives you proper geometry. Alternatively, you can tighten down your rocker arm, hold the rocker in contact with the pushrod, and use a feeler gauge to measure the gap between the rocker tip/valve stem. Add that gap, + .03" to your stock pushrod length, and you should come up with a length that will work. Be sure the lifter's on the cam base circle, and that you're not depressing the plunger in the lifter during the measurements. Measure all 16, and pick your length based on the longest. That way you can always use a .030", or .060" shim to adjust. If you end up with them too short - well, that's the problem you're trying to solve. Also, I wouldn't assume a length on the stock pushrod - I'd measure them and use the longest as your guide.

 

[68converted]

There are so many variables potentially at work within a particular engine, I wouldn't rely on someone else - even the cam company - to determine proper pushrod length. I'd measure. They make adjustable pushrods that can be used to determine the length that gives you proper geometry. Alternatively, you can tighten down your rocker arm, hold the rocker in contact with the pushrod, and use a feeler gauge to measure the gap between the rocker tip/valve stem. Add that gap, + .03" to your stock pushrod length, and you should come up with a length that will work. Be sure the lifter's on the cam base circle, and that you're not depressing the plunger in the lifter during the measurements. Measure all 16, and pick your length based on the longest. That way you can always use a .030", or .060" shim to adjust. If you end up with them too short - well, that's the problem you're trying to solve. Also, I wouldn't assume a length on the stock pushrod - I'd measure them and use the longest as your guide.

Thanks Michael,
Unfortunately, I went this route and it is pretty much the same story across all rockers. I can just barely turn the pushrod when at the base circle. The only issue that may throw a wrench into my test is that all the lifters have been depleted of oil, as I just put the motor together. So I would think that using a set of pushrods that is .03" longer than what I have now would be the answer.... or am I getting confused.

 

[Michael Yount]

The lifter being full of oil won't change anything - the plunger spring keeps the lifter in the same position it would be if it were "pumped up". If what you're saying is that properly torqued, the pushrod is just barely 'loose' (there's no measurable gap, but you can spin the pushrod) then they do need to be slightly longer. Sounds like either the cam manufacturer got the base circle just a bit too small, or the head manufacturer left the pedestals a bit high - it happens sometimes. If it were me, based on what you describe, I'd go a bit bigger than .03" longer. Remember, you can shim the rocker as much as .090" higher if the pushrods are a bit too long. It's a bigger problem if they're too short, as you're discovering. I'd go .050" longer.

Here's another thought - I've heard of folks removing material from the bottom of the rocker fulcrum successfully. You'd have 16 to do, but if you remove .03-.05" from the bottom of each fulcrum, you'll essentially 'lower' the rocker onto the pushrod. You just need to be very careful to keep the bottom completely flat. I'd also probably want to mark the valve stem tip, and see what the geometry is doing - is the rocker tip staying in the middle of the stem through it's travel. Unless you look at that, there's no way of knowing which approach (longer pushrods or shorter fulcrums) is going to give you the best valve train geometry to minimize side loading of the valves.

 

[68converted]

....is the rocker tip staying in the middle of the stem through it's travel. Unless you look at that, there's no way of knowing which approach (longer pushrods or shorter fulcrums) is going to give you the best valve train geometry to minimize side loading of the valves.[/QUOTE]


Thanks Michael! I just confirmed what you wrote. I pulled the locator/channel shim out and it was a perfect fit. That shim is .05" Thanks for the help! Very cool to see someone helping on the Tech forum rather than busting on everyone's setup's/car's. Much appreciated.

 

[Michael Yount]

Glad to try and help. Not clear for me though - did you remove a shim from between the channel and the rocker fulcrum, or did you remove the entire channel?