I replaced the rockers on my motor this weekend. It is an 1986 300 I6 and has been freshly rebuilt and I've made some performance upgrades. Upgrades that are relevant to my question here include:
Comp 268 cam
Comp valve springs (part #903)
Edelbrock retainers (Edelbrock part #9724) with new 7* locks (Comp part #601)
Comp hydraulic lifters (part #832)
Comp stock-length pushrods (part #7866)
stock replacement rockers (Sealed Power units from Advanced)
Head shaved very minimally (0.01")
Valves "tipped" by machinist
When I installed the rocker arms, I started at cylinder #1. I put it at TDC so both valves were closed. I installed the rockers one at a time, meaning I left one existing rocker torqued down to hold the fulcrum guide to the pedestals while I installed the other rocker.
I noticed when I was tightening down the rockers that they seemed to compress the valve springs BEFORE the fulcrum was fully seated to the pedestal, and BEFORE I torqued the fulcrums down to 20 ft-lbs. At this point, the pushrods had zero lash (I couldn't spin them) and had preloaded the lifters. After a few minutes, the lifters would bleed down and the pushrods were slightly "turnable", but the valve springs still looked compressed.
I compared the height of the valve springs after installing the new rocker to the height under the old rocker and they were the same. In other words, the original rockers also had compressed the valve springs slightly when the cylinder was at TDC.
My question is in regards to the valve spring compressing. In my mind, it does not make sense for the valve spring to be compressed with the motor at TDC because that would imply that the valves are slightly open. However, when I noticed my original rockers did the same thing, I thought that must be how it is. But I've been thinking about it over the past couple days, and I just can't wrap my head around how that would work.
Even after the lifters bled down following the installation of the new rockers, the valve spring still looked slightly compressed compared to the "fully closed" position (i.e., without a rocker on it).
I wasn't really aware that you could adjust the rockers using shims when I installed the new ones, but it sounds to me like I need to shim the rocker up to prevent it from pushing the valve open when the motor is at TDC.
My thought is that if the base circle on the Comp 268H is bigger than the stock cam, that will result in the valves being pushed open slightly unless the rockers are shimmed up.
I have recently swapped heads on my 86 300, and am in the process of rebuilding a new short block. Adjusting the pedestal style rocker for pre-load was an interesting experience. Evidently many Fords of the late 80's 90's etc use the pedestal system so any YouTube video about setting them on a 302, 351 etc would be a good watch.
The torque value is only noticed because it is what keeps the bolt TIGHT.
It's about finding TDC and ZERO lash. It would probably help if the lifter is pre-oiled and pumped up actually..but no matter.
Once you determine where "zero lash is" note how much you have to tighten the bolt to find torque setting. (1/4 turn 3/4 turn etc)...I found mine to be set at approx 3/4 turn for zero lash to torque value (with the stock Ford Lifters). Of course you need to have the proper length push-rod for your install to get you close..or as you stated..be prepared for the shim process..or even custom grinding a pedestal as I have done on occasion....
What is ok will vary with lifter mfg I understand. I quess what is desired is to be somewhere in the mid-range of lifter total travel..remember when your cam breaks in you will need to check this again.
I'm sure as others chime in you'll develop a better picture...
One thing you can, and should, do is drain the lifters (or at least two of them that you can move from cylinder to cylinder). In the Haynes and/or Chiltons manual, there's a procedure for checking push rod length.
You'll need a feeler gauge.
If I remember right (been a year or so since I did it), you put each cylinder at TDC on the compression stroke (the manual should say, but I do remember doing it wrong and the measurements were different on the exhaust stroke).
Then, with the feeler gauge, you measure the gap between the rocker arm and the valve. If the measurement is within a certain spec, your push rods are fine. If they're above or below, you need longer or shorter pushrods, depending.
As far as shimming/shaving the pedestals, I think this is done (as opposed to different length push rods) if the rocker arm is pressing too far on one side of the valve instead of on the center.
With new lifters you should be able to compress the piston in the lifter a wee bit even after the pedestal bolt is tightened to spec. Check Comp's specs for how much the lifter should be preloaded - usually ~.02" or so. If you can't turn the pushrod when the rocker is tightened you do indeed need to shim the pedestals or get shorter pushrods.
I think I've come up with a plan. The first thing I am going to do (tonight) is run a compression check across all the cylinders. If compression checks out, then I will know, for sure, that the valves are not hanging open. Assuming that this goes well, then this weekend, I will pop the valve cover off again and check the lifter preload. The general methods given by Crane and Comp are similar: mark a reference measurement on a pushrod with the rocker torqued down and the cylinder at TDC-compression. Then loosen the rocker enough that the preload is taken out, give the lifter a few minutes to return to the "neutral" setting (bleed up, I guess?) and make a similar reference mark on the pushrod. The difference between the first and second reference marks is the preload, which in the case of the Comp lifters I am using, should be approximately 0.045".
If the valves are hanging open, then I will (obviously) have bigger problems. At that point, I will take off the valve cover and measure the height of the valve spring as-is. Then I will remove the pushrod, reinstall the rocker, and remeasure the valve spring. If the valve spring is still compressed without the pushrod in, then that means (I think) that there must be interference between the rocker arm and the fulcrum, which I will have to correct using shims. If the valve spring returns to normal with the pushrod out, then that means I need shorter pushrods.
I think I am getting a handle on all this stuff... I guess we'll know more tonight after I run the compression test.