Original pushrods used in '71 400 4BBL?
 

Hey all. The original #96 heads on my '71 Firebird Formula 400 have been throwing rockers / bending pushrods and guide plates. They've done this both before and after a long-block rebuild. This rebuilder botched things, a new mechanic I trust is now telling me that these original heads are shot, they tested horrible.

I found a "new" pair of #96's (date code four weeks after the originals) and my current engine builder says they are solid. But he'd like to have the original pushrod specs as a reference, the new valves sit a little higher and he has to make sure there's no clearance issues, etc. The old rebuilder put replacement pushrods in, I don't have any of the originals.

Pontiac, of course, is notorious for having many different pushrod lengths. Does anyone have a shop manual or something that specifies what length of pushrods this motor originally used: a 1971 YS code 400 4-barrel with AT and AC, casting code 481988, block cast 3/31/71...? The Wallace Racing site says all V8's '69-'79 were 9.171875" (9 11/64"), but this site says '55-'79 -- were all 9.146". Is it one of these? Something else? Thanks as always.

Dont know the length but I do know that all 55-79 pushrods were NOT the same length.

Yeah, and I don't know if '69 - '79 V8's were all the same, either. I'm just getting a lot of contradictory info, looking around the net. That's why I thought I'd ask y'all.

Should be 9.145” long. A few thousandths of an inch different either way means nothing and will work fine.

Pretty standard part, just buy a set of new ones for that year engine and you’re good.

The early 326/389/421 engines used 8.672” and RAIV engines used 9.130” so there’s really not a bunch of different lengths.

Did you go with an aftermarket cam with more lift than stock?

Excessive head milling can cause you issues, as can using a cam with non-stock lift. Have you tried switching to poly-locks and setting your rockers to zero lash plus 1/2 turn?

On the bright side, you have a '71 Formula :)

I believe that using a stock pushrod with OE rocker studs and OE rocker nuts could be the problem;
once a head is milled, that brings the rocker studs closer to the crank;
once a block is decked, that brings the rocker studs closer to the crank;
Reduced distance between crank and rocker studs means (at least to me) that OE rocker setup is no good.

In my words, the OE setup essentially bottoms out the rocker nut against he "bottle neck" rocker stud - that bottoming out of the rocker nut requires OE distance for OE pushrods (and rocker nut) to work properly.

I prefer to use Chevy Big Block rocker studs with sinch nuts tightened manually - not to a torque spec;
OR
OE rocker studs with poly locks, again set up properly.

Maybe your mechanic thinks the heads are shot because the geometry has changed sufficiently to require different rocker parts?

Verify valve spring coil-bind clearance, retainer-to-guide (or valve stem seal) clearance. If you have stiffer-than-stock valve springs, maybe the pushrods are just too flimsy. Perhaps you're floating the valves.


WHAT TEST?

Valve sealing? Valve guide clearance? Cooling system pressure-test? Gasket-surface warpage? Something else?

Yup.

As alluded to earlier, it would be nice to know WHY he thinks the heads are "shot".

I should have mentioned that the last time I came running to you guys (a couple months ago), it was to ask for replacement cam recommendations. I went with a safe choice, Summit's 2800. The new builder has the card for that cam and set up the heads accordingly. We are going with poly-locks by ARP. Stock rocker arms, then whatever pushrod size the mech calculates with the new geometry -- he'll put one head on the motor, just a couple bolts to hold it in working position, then hand-crank the motor to (1) make sure there's sufficient clearance with the stock valve covers and (2) figure the optimal pushrod length.

Years ago before I knew very much about Pontiac engines and learned the hard way that they used several different lengths of valves throughout the years, depending on chamber size, I had someone build a set of heads for a 400 for me. We were working on a set of 72CC heads that should have taken the 5.10 length valves. He used a set of valves that had a length of 4.98. It proceeded to bend nearly every pushrod. Pontiac also used a 4.88 length valve in some deep chambered 455 heads. If someone put 4.88 length valves in your 96's, I can see how it would bend pushrods.

As B-man said, Pontiac really only used one length of pushrod on a D port engine from 67-79. I would be looking at your original heads and measuring the valve length. Your 96's would have come factory with a 4.98 length valve. I would also check the springs to make sure that they were not coil binding and that you had plenty of retainer to guide clearance. I would also convert to Big Block Chevy rocker studs and nuts if you haven't already so you can properly adjust your valvetrain.

A fairly common thing to do is install a set of 5.10 length valves in the 96's and that will gain you more retainer to guide clearance. At that point, you would need to measure your pushrod length. It would likely take the 9.20 RA4 length pushrods to correct the geometry but this would need to be checked with a pushrod length checker. One other thing to look out for is the pushrod hole in the head. If you are running a cam with over .500 lift, things get pretty close as far as the pushrod and pushrod hole. If you are running 1.65 ratio rocker arms, you will also want to check for clearance and possibly enlarge the hole as that can cause issues.

You're on to something, I think. The old setup bent pushrods on both heads. The machine shop who redid the replacement #96 heads -- a great old one, Kelly's in Culver City CA -- gave me a box with the old valve train parts. I just put the valves on calipers and they measured 4.95 --

There are really only 2 length pushrods. Various manufacturers vary a few hundredths on the actual lengths. There is the short 8.xxx” pushrods for non-HO/RA Dport heads with the short valves/short installed height springs and 9.xxx” pushrods for the HO/RAIV with 1.77” installed height springs.

The longer pushrods were available in 5/16” (HO) and 11/32” (RAIV. The shorter pushrods were 5/16”. If installing the longer valves in non-HO/RAIV heads (to accomodate higher lift cams/taller springs, you can use the 11/32” dia pushrods on Std Dport heads by opening up the guide plates. You just tap 3/8” dia socket down into the pushrod slot to clearance the guide plate flanges.

This times 1000. Bending pushrods is almost always mechanical interference from incorrect retainers or springs. The retainer hits the valve guide or the spring stacks solid. Once this happens, the valve cannot open further and the pushrod becomes a pretzel. The engine is telling you the previous head guy was an idiot.

There were only 2 common lengths of pontiac pushrods, ignoring any RAIV or special low volume engines. There was one length for '55-66 engines and another for '67-79. The Summit 2800 should use the stock pushrod for your engine unless special rocker arms or other valve height issues come into play.

This is what i´ve found on Pontiac V8 push rods:

1955-60 uses 9.136" push rods #540661. (TRW 48056).
1967 (2:nd design)-79 uses 9.145" push rods #9789677. (TRW 48056).
1961-1967 (1:st design) uses 8.715" push rods #546695. (TRW 48069).
1968 RAII uses 9.137" push rods #9794043. Dia. .340". (TRW N/A)
1969-70 RAIV uses 9.205" push rods #545703. Dia. .340". (TRW 48139).

HTH

The stock installed height on a D port head other then a 670 / 997 casting RA1 or a # 96 casting 1968 1/2 RA2 motor is 1.586”, but note that I have seen as little as 1.560”

Stock springs in all but the two heads I listed are good for .456” lift and will be safe from a coil bind issue with that as a max for lift.

Your 2800 cam should work with stock springs and the correct stock installed height.

If you started off with a 1.586” installed height and a freshen up valve job was done that used the full diameter of the each valve -.005” then you installed height will be about 1.609”.

With stock used springs and a .470” lift Cam your open pressure will be about 258 psi .
This is not enough to bend push rods , and it’s not enough in general to get a motor safely to 6000 rpm no less if your only running .440” lift as with the 2800 cam.

A 1.609” installed height will give you a .594” retainer to guide clearance if the heads where cut for positive type valve seals .

The stock retainer to guide clearance as seen in my photo is about .550” , so even that is safe for your summit 2800” cam and even if you where to run 1.65 rockers, but the stock springs would likely be a issue.
The correct valve length for you 96 castings ( and many others) is 4.980”

The stock installed height on 1968 1/2 RAII # 96 heads valve inner springs 979044 are 1.726" and outer springs 979045 are 1.776".

If your referring to what I posted then please re-read it, as that was one of the two exceptions in terms of heads that I listed for D port heads in my post.

That should be 9794044 and 9794045.

Thanks, i stand corrected.

Since you mention it, the 1968 RAII heads are O-ports.

Yes your darn right, but i guess I could have used a longer definition to differentiate them

Thanks to everyone for your expertise on this.