My 390 was on a dyno last week. Guy tells me it will run smoother if I put shorter pushrods in 3 of the cylinders.

Took out the pushrods (shaft rockers, hydraulic lifters) and all were 9.59". 3/8 ball both ends.

I suspect these are wrong. Even though some of the pushrods are too long, I wonder if most are too short.

This engine is likely a 67 Fairlane (or thereabouts) and the pushrods are supposed to be 9.62 stock.

Laying a straightedge along the valve stems, I see that a couple are .100" taller than the shortest ones.

My plan (so far) is this:

1. shorter pushrods for the valves that were a little too tight. (9.56" to replace the 9.59s that were there before)

2. 9.62 pushrods for most of the rest. (Hoping the valves will make use of the extra .030 opening)

3. Deal with the very tallest and very shortest valve stems. I may be wrong about other stuff, but right here

I know I am stumped. Do I go shorter pushrods on the very short ones to get the rockers hitting them right?

Or longer pushrods to get them to open more? And the same idea for the one or two very tall valve stems.

Longer pushrods to center the rocker end on the valve stem? Or keep the 9.59 pushrod because

they did not cause a miss before I took it apart?

This hurts my brain, maybe yours too if you have read this far. Maybe somebody knows about these things and can

set me right. Any comments welcome. TIA

BTW this seems to be a stock cam. Carter AFB. Daily work truck, F250.

 

First, with an engine rebuild, all bets are off on pushrod lengths.

With hydraulic lifters, the valves will not open more than the cam tells them to, unless they are loose or bottoming out.

As for what to do about the rest of the issues, when the heads were redone, the machinist should have matched the stem heights. And gotten them to where the rockers should be at the right angles. Others here can help more about what the stem heights should be.

 

Thx Krewat. Got it back together. I put shorter pushrods in the 3 cylinders that had a miss.

Both intake and exhaust, since I didn't know which one was too tight. For the rest I put in the pushrods that were in there to begin with.

Clicked for a minute after I started it up but then the clicking stopped. It idles noticeably smoother than before.

If it drives as well as it idles then I will consider it done.

 

Not a fan of just swapping out pushrods without first verifying the proper length.
The "hydraulic" part of the lifter is designed to take up a limited (but fair) amount of play between the lifter and rocker arm, (even compensating for stem ht) but as such, will only take up that play as needed and within its limits. A longer pr could in fact give more lift, but that could also mean that the valve stays open when it should be fully closed. Simply put, you can't just install longer pr to increase lift. That's the job and purpose of the cam.
Sounds like what you've done may have improved things, but I strongly recommend measuring and confirming what length pr should be used.

 

For the 390's hydraulic lifters (normally) shorter (.060) and longer (.060) push rods are used (generally) to compensate for well understood wear-related geometry changes in the valve mechanisms.

I like using a lifter bleed down tool on the rocker arm to ensure completely bottomed during adjustment - keep the lifter fully bottomed, then insert gauge to check valve stem to rocker arm clearances -- it is at this point where you can determine if different push rods are needed.

I also like measuring the cam to verify lift (requires removal obviously) --- helps engineer plus it determines whether you need certain upgrades (e.g. springs).

Also ---Mucho important getting preload right on these old gals.

 

Thank you AZ. I only put shorter pushrods in, longer ones seemed like they might do harm. Runs much better but still a regular small miss on idle. I would like to chase that down if possible.

F350, I am beginning to understand more about hydraulic lifters. What you suggest is something I want to try. Where can I get a lifter bleed down tool?

And what rocker-to-valve clearance is correct to give the right preload?

 

When you have preload there is no rocker to valve clearance. If I understand what F350 1990 is saying, he is telling you how to measure the clearance before you bottom out the lifter, not how to measure the preload on the lifter.

Typical preload is between .030 and .040".
If you have a performance cam or lifters you should find out what preload the manufacture recommends.

It will be time consuming but I recommended in another post to buy an adjustable push rod made for determining the correct length push rod you need based upon the preload you want. This is the easiest way I know of to determine proper lifter preload on an engine which has a rocker shaft valve system...

PDF from Crane Cam is below

https://www.google.com/url?sa=t&rct=...96jy2Tc2RcXpUu