Hey folks, I usually hang out in the C3 forum, but I got some aluminum L98 heads off of an 89 Vette for my 81, and I was curious as to which pushrods I should use? Should I use the stock length pushrods for my 81, or should I use the stock length pushrods for an 89? I'm not sure if the L98's have a different lifter setup, which is why I'm asking. Thanks in advance folks!!

 

If it were me, I'd get a pushrod length checking tool and use it to determine if my current pushrods are right or not.

The 89 came with a roller cam and has, what some believe are, guide plates factory installed. All that I've read says they are NOT "guide plates" and NOT to use them, but to install REAL guide plates on the heads.

I believe those heads are set up for self-aligning rockers arms too, making actual guide plates all the more important.

What the heads have from the factory (the "guide plates") were only used at the factory during assembly of the engine.

You'll need to be sure the springs on those heads are compatible with the cam you're running. Make sure they can handle the lift and have the correct seat and open pressure.

Not knowing how many miles since the last valve job, condition of the valve guides and whether the heads could have experienced over-heating (perhaps warping the deck) are other concerns.

Just my thoughts.

Jake

 

Use the ones for the 81. L98s take short pushrods due to the roller lifters.

 

the roller lifter pushrods are way shorter than the flat tappet pushrods.
try your stock length ones first and check how the rocker arm makes contact with the top of the valve. you may have to adjust this length +/- .100 to get the correct geometry

 

Sounds like a plan. Thanks folks. Some info on my heads; They're completely rebuilt, and have Edelbrock guideplates installed. I'll be using stamped steel 1.5 ratio roller rockers, (They were inexpensive, and they'll do the job) But I'll try an 81 pushrod to see if it works. I don't know if the heads were ever overheated, as I'm not too sure about their history. Cross my fingers and hope for the best. Thanks for the help folks.

 

Sounds good.

The 81 pushrods may very well work, but if they aren't the correct length, you'll get accelerated valve guide wear.

The engine will begin burning oil and at some point you'll be pulling the heads again to have new guides installed.

A pushrod length checker is really inexpensive; Moroso, and others, makes one.

Or you can do it the manual way by scribing a black mark on the tip of one of the valves, installing the pushrod, rocker arm and setting the lifter preload (assuming hydraulic lifters) and cycle the engine through one revolution.

You can then see the travel of the rocker arm tip across the valve by examining where the mark wore off.

Closed valve: the roller should sit slightly inboard on the valve, then at mid lift be centered over the valve, at full lift slightly outboard; half closed centered and fully closed back inboard again.

Hope this helps.

Jake

 

Wow, awesome info. I'm starting to like the C4 forum more and more... I do have a couple pushrod length checkers that I bought with my rocker arms, but they're kind've a pain to use, probably since I've never used them before this. Basically, they're pushrods that you can lengthen or shorten. But do I just start with a basic guess (compare it to my 81's pushrods) then lengthen or shorten from there? And am I correct in assuming that I just tighten the rocker arm until there's a very slight drag on the pushrod as I spin it and adjust the pushrod length to have a slight drag when the rocker arm hits the valve tip in the middle? (Closed valves, TDC) Sorry, kinda new to this stuff, I've never done a valve lash adjustment before, but I think I'm more then capable/willing to give it a shot.

 

Jake, I just started reading your article, and it's clearing up a lot of my questions. Excellent stuff. Thanks!

 

check them for flatness, pits, etc.. runout should be less than .003" MAX in 6 inches..if more ,then there will be problems sealling.. also do a visual around each valve and every passage.. or better yet do a dye penetrate test( non destructive test) on them to determine if any crack prevail. If stocks springs, installed hieght should be 1.700 +/- .015" max. approx diameter 1.234" good for about .480 lift.. and about 85 - 90 lbs seat pressure..(good with the rocker 1.5:1) if needed, change to 981 comp springs. no machining involved..
as far as push rods, the 89 are short and will not work.. use pre 84 version (longer)

 

Were to begin.

There are a lot of different types of pushrod length checking tools on the market. Some are really inexpensive, like the one from Manley for about $15.00, to ones with dial indicators that are a lot more $.

For a street type engine, I wouldn't go with the high-tech variety, the Manley checker will do just fine.

First, you have to understand that readily available (off the shelf) pushrods come in .050 length increments. So unless you need something custom made (are we being **** here?), best you're going to be able to do is choose the set that comes closest to the exact length you need.

Always round UP when selecting a length. A pushrod that is a little too long is much better than one that is a little too short.

I haven't used the Manley pushrod checker in quite a while, but using it is really simple. It's a "pretty" blue color, plastic and resembles a rocker arm. It slides down on the stud, one end is suppose to touch the valve stem tip and the other end the tip of the pushrod.

First step is to make sure that the lifter you're using to check the pushrod on is on the base circle of the cam lobe. This is simple to do:

Say we start with the #1: Use a pushrod that you think is close to what you need. Install it on the #1 EXHAUST and seat it in the lifter socket.

Now turn the engine over while watching that pushrod. As soon as it beings to rise, the INTAKE LIFTER will be on the base circle.

Take out that pushrod and install it on the intake lifter.

If you don't have a pushrod that you think is close, then you'll need one of the adjustable pushrods for that step.

With the pushrod in place, slide the Manley checking tool down on the rocker stud. It self stops. One model is for 3/8th studs and the other for 7/16ths.

You'll be able to tell which way to turn it because one end is longer than the other and, when on correctly, one end will be close to
the pushrod and the other end to the valve stem.

Now, there are two schools of thought on this next step - so just to be on the safe side, I'd check with Manley on this.

One school of thought is that if the pushrod you're using is the correct length, the tool will be flush with BOTH the pushrod end AND sitting flush on the valve stem. There won't be any gap on either end of the tool. If that's that case, you're good to go with that length pushrod.

The other school of thought is if both ends are flush, you add .030" to the length of the pushrod you're using to account for the lifter preload (which should be .030").

If I remember correctly, the first one is the right one. You don't add anything to the length.

So suppose both ends of the tool are not flush?"

If there is a gap on the valve stem end of the tool, use your feeler gauges and measure the clearance between the bottom of the tool and the valve stem. That's how much SHORTER you pushrod needs to be than the one you're using.

If there is a gap on the pushrod end of the tool, measure, and that's how much longer your pushrod needs to be.

Then you'll need to measure the pushrod that you're using and add or subtract what you measured with the feeler gauges to the length of that pushrod.

Get out your parts catalog and look up a set that is as close as possible to the length you measured. Remember, round UP.

Using the adjustable pushrod is much the same. Install it and just screw the end with the adjustabilty until it comes in contact with the Manley tool and both ends are flush. Lock the setting in place and measure the adjustable pushrod length.

The reason some say you add .030 when both ends are flush is because they claim the tool is designed for a .600 lift cam. They claim the difference between your cam's actual lift and .600 has to be determined (subtract your cam's lift from .600 OR if you're running more than .600 add the amount it exceeds .600) THEN you throw in the .030 to get to the final length you need.

I don't buy it, otherwise Manley would have included all that math business in the instructions that come with their tool. I don't recall ever seeing any such thing.

Some of the auto parts stores that also carry performance parts may have one hanging on the rack. Pep Boys is a good bet.

When you asked about rotating the pushrod and feeling for drag, etc., well, that's for setting lifter preload. I've got a doc file on how to do that. When you're ready for it let me know and I'll send it

Hope this helps and hope my memory is still working.

Jake