Pushrod length question - how exact must they be?

How much room for error is there with pushrod selection?

I'm going to be swapping my heads, cam, intake, and carb on this weekend. Compcams does recommend a certain length pushrod, though the conventional wisdom on here has always been to check to make sure. I'm planning on doing that, of course, but was wondering how tight the threshold is?

I belive my valvetrain (crane full roller rockers, TW head with stud mounts, etc) is considered 'adjustable'. Should I be able to compensate for a tenth of an inch error?

I'm thinking about simply ordering the ones that compcams recommends, and only shipping them back if necessary. Of course, if the pushrod has to be exact to a really tight tolerance, then I might as well wait and use the right one.

Another question... if I don't use a solid lifter to check (and just use one of my hydraulic lifters), I have to add the preload to whatever length I get right (i keep seeing .030-.060", I believe)?

Also, can you use a hydraulic lifter to check geometry or do you need a solid lifter? That ******* page had directions on how to convert a hydraulic roller lifter into a solid for measuring purposes... any way to do that with a hydraulic flat tappet lifter?

Okay, well I just re-checked the pushrod length and arrived at an 8.75" figure (Trick Flow said the pushrod should be between 8.45" and 8.75", so its long but within reason).

Anyway, my process went as follows:

The instructions Trick Flow included with the head gave a perscribed height at which the center of the roller bearing (the one on the stud), which was 1.15" +/- .05". I installed the adjustable pushrod into the intake side of the #1 cylinder.

My adjustment process basically went as follows... adjust the pushrod to a length which maintained that 1.15" roller bearing height at zero lash and while keeping a uniform and centered contact patch between the top of the valve stem and the roller tip (i used a sharpie marker to 'track' the imprint of the tip on the stem).

This is the 'actual length', and doesn't yet include any adjustment for gauge diameter, lifter preload, etc... not sure how to account for those. My technique to get to zero lash was one I saw on *******.com (in the link posted earlier)... basically tighten the adjuster nut while you wiggle the pushrod... once it becomes noticably harder to rotate the pushrod, you're at zero lash, at that point make another half turn and your good.

Question time... Basically, what did I do wrong? How do I account for lifter preload? How does this 'actual' length translate into what companies like Compcams and Trick Flow sell? Also (probably a dumb question), but the Trick Flow & Crane instruction manuals don't say whether I should use a 5/16", 3/8" or what thickness pushrod... I'm assuming 5/16" but figured I'd ask.

Thanks a ton!

(note, the 'actual length' and 'gauge length' terms were stolen from http://www.compcams.com/information/Products/Pushrods/)

woodsnake said:

OK, why do you think something is wrong?

What springs are you using? What specs on the cam? Did you use regular springs, or light weight springs while checking your PR length?

Are you installing a roller cam and lifters?

Click to expand...

Well, I don't think I did it drastically wrong, just figure there are some details I might not have gotten right... which is why I wrote up (in as much painful detail as I could) my procedure so far.

I do have light checking springs but have used the normal valve springs. I've read conflicting opinions on which is better for adjusting the valvetrain and figuring pushrod length, and since the regular valve springs were on, I used those. Was this a bad idea? lol

(cam is compcams XE274H, .512/.523 max lift, 274/280 duration, 230/236 (i think) adv duration, rpm range 1800-6000.. springs are the upgraded stock Trick Flow springs, lift up to .600, i had the seating pressures in another thread, but summit and compcams said they'd work fine with the cam). Yes, installing new cam and lifters (and heads/rockers).

I'm just mainly worried about little details like how to take into account the lifter preload (how do I do that?) etc.

woodsnake said:

Is it this cam?

http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=879&sb=2

The thing about the light weight springs for measuring your PR length is that a lot of 'adjustable' PR's will bend under normal spring pressure.

Do you have, or have access to a 12 inch digital micrometer? When you order your PR's, you will need to be able to say whether it is the over all length, or the use able length.

I know that when I ordered my trick flow PR's at 6.750, they were actually longer, even though they were laser etched with the right length.

New cams are a world of fun the first time!!

Click to expand...

Yeah thats the one.

I was curious about that, i was actually surprised the adjusting pushrod stayed in place while rotating the engine. Well, I guess I'll see how consistent my results are with a few trials with the normal springs, if theres much variance then it probably means its flexing and I need to use the checking spring.

Any tricks to pushrod length? It seems like they could vary quite a bit from manufacturer to manufacturer...

Well, a guy over at fordmuscle.com and a compcams rep both basically said the best/easiest technique is pretty much the one i described above but you convert a hydraulic lifter to a solid one. Once you do that, you don't need to take into account preload or any of that crap.

So basically, I'd:
1) setup the adjustable pushrod in the lifter, and the rocker on top of it (also I use a sharpie here to color to the top of the valve stem)
2) tighten the rocker's adjustment nut while wiggling the pushrod
3) once the pushrod becomes noticeably harder to rotate, its at zero lash
4) I'd verify the center of the roller rocker bearing was at the right height
5) turn the engine over manually 3-4 times
6) dissasemble the rocker and see what the wear pattern was like. ideally a tight band centered on the stem, if anything slightly pushed toward the intake side (i actually never saw a wear patten that looked completely unnacceptable)
7) use some calipers to measure the extra length in the pushrod

Doing this, on both 1 intake and 1 exhaust, I found I need about an ~8.7" pushrod.

Final questions: Am I missing on any math? And the pushrod should be a 5/16" diameter one, correct?

Thanks! Hope my summary above helps, its actually very easy and makes plenty of sense once you do it a few times.