1991 RS 305

Hi, I was looking for some expert advise as I've never measured for new pushrods before. Backstory, engine is a 305 with Vortec heads milled to 56CC. I replaced the factory pushrods about 15 years back with hardened pushrods as I went to guide plates for the roller rockers. I believe I went with a stock length at that time which is 7.200", I'm going through the engine now and believe they were too long. I've measured and believe I need 7.050 length pushrods. I've attached two pictures one with the 7.200" and one with 7.050" using the comp measuring pushrod deal. Just looking for someone to look at the pics and agree/disagree that the 7.050" look like a good fit.

Appreciate any input, Matt.



Stock Length

Proposed 7.050" length. Kinda hard to see, but overall I feel the wear in pretty centered.

skinny z

The idea is to get as narrow a sweep across the valve tip as is possible. .030" is achievable but the narrower the better. This is at the expense of centering the pattern. Narrow and close to, but obviously not off, the edge is better than fat and in the middle.
There are other more precise methods of achieving this instead of marking and running through a couple of cycles, but in a nutshell, this is what you're going for.
What do you have for a valvetrain? Hydraulic lifters? Are you using a checking spring or the running spring?

1991 RS 305

Thanks for the reply. I'm running the stock hydraulic lifters, Not exactly sure what you mean by running spring but engine is on a stand and I'm rotating it by hand... also priming the engine with an oil pump primer.

skinny z

By running spring I mean the actual valve spring. The problem with hydraulic lifters that you're probably aware of seeing as your trying to keep the engine full of oil pressure, is that they tend to collapse under the load of a full spring. This will in turn, change the length of the adjustable pushrod you're using. It's certainly enough to skew the measurement so it's something to consider.
I've used what's called a checking spring, which is really nothing more than a lightweight coil spring you'd get from the hardware store. Enough pressure to keep the components together but not so much so as to push the lifter cup down.
If you're confident that you're not collapsing the lifter with the priming, then go for that narrow sweep. It's hard to resist the notion that centred is better, but for the intents and purposes of correct valvetrain geometry, it isn't.


Checking Spring

skinny z

Examples of narrow vs centred.



7.3" pushrod. Wide pattern and toward the intake side.



7.4" pushrod. Narrow and centred.



7.5" pushrod. Getting wider and moving towards the exhaust side.

Keep in mind that mine isn't stock and I was able to achieve a narrow and centred pattern. But that took some very expensive rocker arms and a lot of trial and error using the Mid Lift Method for geometry. I used the sweep pattern to confirm my other measurements.

Hope this helps.

1991 RS 305

skinny z, thanks for all the help. I can’t be certain I’m not collapsing the lifters so I’m going to take your advise, get the checking spring and go for the narrow sweep. Thanks again for all the info and I’ll let you know what I come up with.

much appreciated, Matt.

skinny z

Keep in mind that the checking springs can induce their own margin for error in that they don't necessarily keep all of the components tightly packed. But that's more when making a comparison with an engine that has bigger spring pressures that you and I are running. So I just run with it when double checking.

AlkyIROC

Watch this video. I'm assuming you already have an adjustable pushrod to measure the length. Buy the height check tool and measure the length required. Using this tool will give you the smallest sweep possible across the tip of the valve. No check springs required. No rotating the engine over to get a sweep mark across the valve tip.

skinny z

I do that with similar tooling. As you know, this would be the Mid Lift Method.

F-BIRD'88

Just curious how much did you mill your cylinder heads down to get a 56cc chamber volume?

F-BIRD'88

Check the valve tip rocker contact pattern using a stock GM SBC rocker arm for comparo. (Just for giggles.

I'd make up and use a pair of pinned hyd lifters . Drill and roll pin the internal plundger so its solid at the correct desired hyd preload plunger height.

I've seen others having a hard time with rocker geometry push rod length with rockers like that.

Longer than OEM valve stem length (eg: +.100" valves) moves the tip of the valve toward the rocker stud. As the rocker stud and the valve are at different angles.

It will move the contact point of a rocker unless the rocker is redesigned.

1991 RS 305

I wish I knew, I had a shop about 15 years back mill em down to 56cc's. I just had a different local shop go through the heads last year and verify they were 56cc's. I wish I had taken a picture as the guy at the machine shop said it was visually very apparent they were shaved, so it must have been a decent amount.

1991 RS 305

Got ahold of checking springs and tried to get a pattern, couldn't seem to get below .040"... the pattern was hard to read. I'm using a sharpie and not getting decent wear marks. I'm going to try a dry erase tomorrow and maybe it'll wear through that a bit better.

The reason I went down this path of measuring pushrod length was way back when, when I originally ran this setup, I had a tick from under the valve cover after running about a month or so. Turns out one of the rocker arms slid off the side of the valve and wore the valve tip and guide plate. I went back to a 1.5 self aligning rocker arm on that cylinder and ran it for the next 15 years with without issue. I'm going to attach some pics of the damage here and am wondering if anyone has an opinion as to what happened? Did I over rev, maybe I didn't adjust the valve lash correctly. Just curious what you all think.

Thanks again, Matt

1991 RS 305

I picked this tool up and am very pleased with how easy it is to use. I'm measuring that most valves measure 7.050" and a few are closer to 7.025. I'd really like to stick with a single length pushrod and nothing custom, please keep in mind this is a stock lower 305 with some slightly hotter heads/cam... nothing crazy high perf. Any thought on sticking with the longer for all valves? Seems Trick Flow has a pushrod that measures 7.050 ( https://www.summitracing.com/parts/tfs-21407050 ). Comp also has one but it's $60 more expensive. I'm assuming Chromoly is a good "hardened" pushrod material to be used with guide plates?

Thanks again for any advise, it's greatly appreciated. Matt.

skinny z

If there's no preload on the lifter, and by using that tool there shouldn't be, to your measurement you can whatever preload you'll use. So a longer pushrod will be needed than the length of the adjustable pushrod.
Although the math is off a little (the true formula is a little more complex) you can calculate the amount the plunger gets pushed into the lifter by using the number of turns applied to the rocker nut by the thread pitch.
A 3/8ths rocker stud has a 24 TPI pitch. So one turn equals ~ .040".
Your adjustable pushrod can have that amount added.

skinny z

As for the damage above, where the pushrod holes in the cylinder enlarged so as to allow the guide plate to properly do it's job? The pushrod with have witness marks if it was binding on the head and the guide plate. It appears to a binding issue.
It almost appears to be some deliberate clearancing with a grinder. Or the rocker stud was loose and allowed to move around.

1991 RS 305

Thanks for the input! The holes in the head seem to be plenty large, no wear marks on the pushrods. The rocker arm had slid off the the center of the valve to the edge and wore down the edge of the valve. When I removed the valve cover, the rocker arm was crooked, riding on the edge of the valve and the damage was done as you see in the pics. I just threw a factory self aligning rocker on that valve and drove it that way for the next ten years or more without issue. I'm guessing I didn't have it adjusted correctly. I'm just going through the engine again and attempting to prevent that from happening when I go back to a 1.6 roller on that valve (now that it's been replaced).

Thanks again for your help skinny, you've been a great help. I'm going to add the preload to the pushrod length and get em ordered.

skinny z

Question for you in regards to the rocker stud and guide plate. Did you get the engine assembled or was the guide plate upgrade your work?
Reason I ask, as I mentioned earlier, is that the groove on the stud, because of it's concave nature, looks to be done with a grinding stone. And the guide plate was clearanced similarly. Factory tolerances being what they are may have been such that those two studs weren't on the proper centre to centre distances. With the guide plate being fixed, something had to be modified to fit.

Not that it matters now and I happy to hear you're getting it sorted out but my curiosity has the better of me.

Glad I could help in whatever limited capacity that was.

1991 RS 305

I took the heads to a local shop and had guide plates and screw in studs installed, then I put the heads on and adjusted the rockers. If I remember correctly, the rocker wasn’t just off to the side of the valve but was riding at an angle, the roller tip wore the edge of the valve down and I believe the rocker wore that groove in the stud. Be It was a long time ago (2007ish). I had a tick coming from under the valve cover a few weeks after I installed the heads and the damage seen was the end result. At that time I was just happy it didn’t end up dropping that valve into the engine. I ran a factory self aligning rocker on that very valve and rocker stud until last summer (maybe that was a bad idea) when I removed the heads again and had a shop go through the heads and they replaced the damaged valve, guideplate and stud. I’ll try to locate the old rocker that was on that valve tonight and see what the damage looked like on it. I’ll post a pic if I find it.

Matt.

1991 RS 305

skinny, looks like I saved the rocker arm from way back when. I’ll attach a couple pics here in case you’re curious. You can see the rounded off roller tip and wear where I believe the rocker was making contact with the stud.

skinny z

Having a quick look at it, I suppose it could be the rocker arm working back and forth that more or less sawed it's way through the stud.
As for the roller tip running off the edge, that's the point of having guide plates in the first place. To keep the rocker square to the valve.
The misalignment should have been caught by the shop doing the work. In their defence, maybe it looked to be a non-issue when assembled.
BUT, I'll say that the cut on the stud still looks deliberate as the wear on the rocker arm doesn't appear to match up in the amount of wear that the stud has.

At any rate, that's a moot point now. You've caught it with minimal damage. The labour involved is part of having cars as a hobby.

I may have mentioned having a set of new Vortec's that, right off the hop, I had machined for guide plates and the requisite screw-in studs. In retrospect, I'd skip that and go with self aligning rockers. At the power levels and RPM I was running back then, it was perfectly fine the way the factory designed it. Besides the expense, there were issues with pushrod binding and alignment using the conventional guide plate approach. I eventually had to make the move to an adjustable guide plate (Isky split versions) and that further complicated the setup.

I'm happy to see you're back in business (aren't you?). Be sure you've dialed in the geometry or more troubles might bite you further down the road. Do a few sweeps to confirm.
I'm interested in what you end up with and how it looks. So will others. You're subject line is a hot topic.

1991 RS 305

I like your thoughts on the Vortec's you cited. As they say, with age comes wisdom and that's certainly true. I'm almost back in business, once the pushrods arrive I'll install them and get the engine back in the car. The engine bay needed a respray and the engine had a few leaks that come with age. Decided to tackle this valve train geometry issue as well while everything was apart and skinny, you and others have been a great help and I appreciate all the input and shared wisdom.

Thanks again, I'll keep you posted. Matt.

1991 RS 305

I missed something from your earlier post, I should have been more thorough when looking through the parts. While was working on the engine tonight, I was struggling with lash on one valve. The pushrod felt like it was binding and sure enough it was what you mentioned above. I grabbed a picture. Kinda hard to see but it’s riding on the bottom of that hole. When I looked at the pushrods closer a couple do have wear further down as you had said to look for. I had inspected the holes last week when you mentioned this and didn’t notice any wear, I thought they looked to be pretty large but clearly I was incorrect there. Now I suppose I need to get the Dremel out and enlarge the holes, or possibly go with self aligning rockers.

Would you take the heads back off to enlarge those holes or do you think you could take great precaution and do it with the heads on?

Thanks again, Matt.

skinny z

I'd be inclined to go with a self aligning rocker.
This would remove the guide plate from the equation. Any contact with the cylinder head is inconsequential after that considering on previous iterations of the SBC, the holes in the head did the alignment.
Put it back the way GM intended. You'll find that the SA rocker should play nice with the holes in the cylinder even if there's contact seeing as GM built in that way.

Now if you would rather not go with an SA rocker, then I'd say pulling heads is the way to go. And rather than a dremel, it's a 1/2" drill bit so as to ensure adequate clearance through the entire pushrod travel path.
Then you can go about using the guide plates. It may be also, that a fixed guide plate won't cut it. In that case, an adjustable version will be needed. You have to remember that there's no guarantee that the studs in that GM head are exactly where they're supposed to be. This could have been the cause of all of your grief in the first place. Guide plates with a spec distance between the on-centre spacing is trying to match up with studs that aren't where they're supposed to be. It only takes a few thou to really mess things up.

https://www.compcams.com/8-adjustabl...hrods-cpg.html

AlkyIROC

I'm not sure how bad the stock alignment is on the SBC but I know on a BBC with canted valves, it's far from perfect. BBC guide plates are a lot different than the SBC ones and I've cut and repositioned a couple of them to reposition the rocker more over the valve. Pushrods have a rough alignment on a SBC going through the holes in the head. If they are small holes with just enough room for the pushrod then that's going to be the alignment. Installing guide plates to relocate the pushrods means they may bind in those smaller guide holes and the holes should be enlarged.

On my BBC aluminum Dart heads, BBC already use guide plates which isn't an issue however I also increased the size of my pushrods to reduce deflection from the high valve spring pressures. Wasn't a big deal until I switched from pedestal rockers to shaft rockers. The shaft rockers position the rocker directly over the valve tip and there is zero side to side movement. That was great until I went to put the pushrods in only to find that a few of the rockers with their perfect alignment now caused the pushrods to rub up against the sides of the holes going through the head. I think I had 2 on each head that needed to be modified. Lots of safety precautions to catch aluminum but a die grinder with an aluminum burr bit and a bunch or trial fitting was all it took to keep the pushrods away from the sides of the holes in the heads.

The only time you really need guide plates on a SBC is when you use heads where the holes through the heads are the size of the pushrod to keep them aligned. Heads that use self aligning rockers are not like that but that also means that guide plates will not perfectly position the rocker over the valve and if they do, they may force the pushrod too close to a hole wall so like on my BBC heads, modification will be required.

1991 RS 305

Skinny, Here's the latest. I put the factory 1.5's on 4 valves today and it moved that pushrod much closer to center. In case you're curious as to what side of the hole the pushrod was riding on, with the comp 1.6's the pushrod was riding super close/ in some cases rubbing on the side of the hole in the head closest to the valve. It wasn't off to either side but consistently closer to the valve side or exhaust port side of the head. I just wanted to make clear what side of the hole it was riding as you seem to make sense of theses situations. For some reason if I had to guess I would think the 1.6 would push it the other way, clearly I'm wrong!

I have the factory 1.5's that came on the engine (that's what I was testing with today). Do you see any value in upgrading to a say a comp 1.5 self aligning? Only reason I ask is I've heard the factory stamped rockers can be inconsistent. Maybe for the type of build I'm working with it's not that beneficial to spend the money. If not I'm ready to throw the factory units back on. Lastly, may seem like a dumb question, but I'm assuming I can just remove the guide plates and use the same screw in studs, correct?

Thanks again for the help, Matt.

skinny z

There's no telling where the rocker ratio is made up between ratios of the same brand or ratios from different brands. Is it on the pushrod side, the valve side, or a combination of the two? Add in different styles of rocker arm and any prediction goes out the window. My Crower 1.6 differ from my COMP 1.6 on where they contact the valve tip. So there's that.
Although contact with the head when using guide plate is definitely wrong, it's where the rocker tip contacts the valve that's the real concern.
That brings us to what is a preferred rocker style. At the very least I'd go for a roller tip. In my view, it makes setting up the geometry (by way of pushrod.length) easier. There's an element of reduced friction as well although some may argue that the LS7 utilizes a shoe type rocker arm. That said, any upgrades I've seen tend to go for a full roller although experience there is limited.
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skinny z

EDIT: I missed your rocker stud sans guide plate question. Yes, the stud can be used without the guide plate but be sure to check if the stud's base threads aren't protruding into the intake port. It may require a little trimming.