push rod length

Here is my question to Scott Foxwell at StruabTechnologies followed by his response:

My question (long winded - sorry):
Hi Scott,
I saw your video on line (https://youtu.be/o5is9BsH5OU).

First off I really thought you did a great job explaining rocker arm geometry in laymen’s terms. It was well thought out, explained and made into a nice example that can be easily understood. Thank you for your efforts in making this information available.


Second, I have two questions:

Near the end of your video after you have established proper geometry, you then determine the proper pushrod length by putting an adjustable pushrod into the engine and adjust it for the rocker/valve at half lift by adjusting the pushrod against the lifter and rocker arm. I think this works great in a solid lifter that does not deflect when any pressure (even minute) is exerted against it.

In my case, my engine is basically a stocker (1969 AMC 390 with a Comp Cams Hydraulic flat tappet cam – called the Magnum 280). I am running the Comp Cams Pro-Magnum anti pump up hydraulic lifters that are to have .002-.004 preload on them (not .030-.040 like a typical hydraulic lifter).

When I set lifter preload, what happens (per what I have read on how to set preload) is that you need to first set zero lash by spinning the pushrod with one hand and slowly tighten down the poly lock adjuster nut with the other hand until you feel a slight drag or even feel the pushrod to just about stop spinning. Next, tighten down the poly lock adjuster nut another 1/16 of a turn (based on 7/16-20 studs) to get the proper preload of .002-.004.

When setting the lifter up for zero lash (where I just about make the pushrod stop spinning) and then wait a couple of seconds, (as though I am going to now turn the adjuster nut 1/16 of a turn to get the preload established)……but instead now try to spin the pushrod again, It will spin freely. In other words, I think the lifter had bled down, effectively eliminating my zero lash setting that I just established moments before. I believe all I have to do is ignore the bleed down and just turn the adjuster nut to 1/16 of turn to get the proper preload. Does this sound correct to you?

My second question (which goes back to my first question): When you establish the proper pushrod length, wouldn’t the same issue (with bleed down) become a factor in the pushrod length? I would assume that when you adjust the adjustable pushrod that you would want to be careful about not putting any pressure against the lifter because it will bleed down, messing up the measurement. I do realize (as you said in the video) that the pushrods come in .050” so there is some lee-way.

I guess at the end of the day I have found that I do not like hydraulic lifters because doing certain measurements like what you illustrate in your video become not as accurate as if using solid lifters. I assume your motor in the video has solid lifters?

Thanks again for any insight on setting hydraulic lifter preload and for establishing proper rocker arm geometry on a hydraulic lifter engine (allowing for one to then establish proper pushrod length for hydraulic lifters).


Scott Foxwell's response:

Hi Mark,

Thanks for the kind words regarding the video. It’s always good to get positive feedback form enthusiasts like yourself.

Everything you said is correct. I will say this, however; I don’t like using the “spin till you feel drag” method of finding zero lash with hyd. lifters. I agree, a hyd lifter does leave something to be desired as far as “perfection” but it’s the hand we’re dealt, so we deal with it. What I like to do with hyd. lifters is just let the pushrod sit in the lifter and as I tighten the adjuster (or in this case extend the adjustable push rod), I will move the rocker up and down feeling the lash, and as it becomes less and less as I tighten the adjuster, it’s pretty easy to feel where the rocker stops moving and the lash is zero. With the adjustable pushrod, you can just gently extend it till the lash is taken up and use the same method…just wiggle the rocker arm up and down feeling the lash until it goes away. You do this with a very light touch because as you pointed out, you don’t want to collapse the plunger in the lifter. I feel the “spin till you feel drag” method is way too vague. Some lifters may have oil in them, be “pumped up” and exhibit more drag, sooner, while some lifters may not have much oil in them at all and you’re only working against the pressure of the small spring inside the lifter. In that case, you can collapse that little spring and the plunger without feeling much drag on the pushrod at all. I wish I had spent a little more time addressing the specifics of hydraulic valve train but it was our first video and there were a few oversights. You are also correct in that, since the push rods come in .05 increments, this isn’t 100% exact but it will get you a lot closer and show much better results than any other method. For typical hyd lifters where there is a significant amount of pre-load, we just add that amount of pre-load to the final measured length of the pushrod.
Hope that helps some.

Thanks again,

Scott

Scott Foxwell

Visit Us at:

Booth# 109
(423) 391-7774
(805) 350-1159 cell
sfoxwell@straubtechnologies.com

Thanks again tsanchez for the link to Straub Technologies video.  It is without a doubt

The harland sharp rockers pushrod seat is higher than most so it gets a longer pushrod