PoppyMod,
Please realize that the following is not an attempt to refute and/or take exception with your comments/advice, just an attempt to gain a more clear understanding of the subject.
I am in total agreement with the use of a solid lifter and checking spring for checking certain valve train issues, such as piston to valve clearance, etc. What I'm having a hard time wrapping my brain around is why to use them for checking pushrod length? It would seem that if your cam/hydraulic lifter setup specifies, for example, a .060" pre-load/compression of the lifter, you would be eliminating that preload by using a light, checking spring or solid lifter, or both, thereby giving a false reading of the required pushrod length by .060"-ish? It seems that would show the need for a pushrod that was .060" too short. Am I missing something?
Thanks,
Gene
Hi Gene,
None taken. Some of this may be redundant, but, I want to paint a full picture.
I'll start with, in building/assembling my own engines, I have followed the rule for adjusting the pre-load of hydraulic lifters to be within a range of .020 to .060". There may be exceptions, but, for spirited street engines, this has worked well and have never lost one.
OK. The idea of proper P/R geo (geometry), when set correctly, is to insure as near a 90 deg. relationship between the valve tip and it's rocker point of contact, as possible, when cycled. A correct length is described as the distance between the rocker point of contact with the P/R and it's point of contact with the lifter (no depression of the plunger) in the case of hydraulic. The use of checking springs substituted for the valve springs, are just heavy enough to keep the valves closed, yet, when cycled are light enough as to not depress the lifter plunger, thus distorting the length.
So, let's say we have the checking springs in place, we've installed an adjustable P/R and it's set 6.50" and we have marked the tip of the valve in some way. We cycle the valves a few times. We note the wear pattern is short, that is, it's wear pattern is toward the intake. we remove the P/R and unscrew the tip (making it longer). After several iterations we have a perfect center mark and have determined the P/R should be 6.75". At this point after testing and retesting we're satisfied the wear mark is dead center and about .070, or so wide. This is good.
Now getting back to my suggestive pre-load of .020-.060. How do we get there? I'll use this as an example as these are the rocker studs I run. Having 7/16 X 20 rocker studs, one can determine that one complete rotation (360 degs.) of the nut will travel downward .050". this, based on the fact, you have 20 threads/inch. Dividing 1/20=.050". Now, to add to the formula, you have your rocker ratio as the rocker is not 100% linear at it's axis, is it? It's curved. So, If I were to screw down my adjustment nut 1 full turn thinking my pre-load is good at .050. After all it is between .020-.060....I'd be wrong! Why? because, I'm running a 1.6 ratio rocker, the ".6" has to be considered. Therefore, my ooops! of .050, in reality becomes ".080", by virtue of .050 X 1.6 = .080. now too much pre-load. Too much pre-load can cause reduced engine vacuum.
Simply stated, you use the springs to get an accurate P/R length with which to begin the check. The check being, a centered wear mark with a narrow track and an "ideal" (90 deg) relationship. Thus, within reason, you are getting 100% of the cam lobe's ability to transfer it's total "lift" characteristics. If the push rod is too long coil bind, valves touching can make for a bad day.
I hope this is not too much stuff to take absorb. I hope I answered why it is important to start with a correct base P/R length.
Happy Motoring!
,
