best grade of aluminum for rocker arm 1

[Rob_G]

Hey guys,
I am new to the board and I need to manufacture a set of rocker arms for my race car and am looking for the correct material for this application.
2024 seems to show up in searches but my main concern is what temper is best for the fatigue that this part will see.

Thanks
Rob

 

[SwinnyGG]

2024 is a good choice if you know your part is going to be subject to a high fatigue load, and any of the artificially aged tempers (T3, T4, T6, T81, etc) are going to provide you with the highest fatigue limit available from this material. It really depends on what is available from your supplier and what processes you plan to use to form your parts.

Also, note that the corrosion resistance of 2024 is poor, so anything 2024 that is inside an engine needs to be anodized.

The real question I have is why are you manufacturing your own rocker arms? I don't know what specific engine you are building, but I can pretty much guarantee that just the aluminum bar stock you're going to need to buy to machine these from is going to cost you more than a proven aftermarket rocker arm setup. Once you include shafts, bearings, rollers, etc you're going to be way way over on cost.

 

[Rob_G]

should the material be extruded rectangle bar?

As far as why, 1st my engine is a 540 C.I. Buick with an aftermarket cylinder head, and anything for Buick is hard to get and 2nd it is very expensive.
I own my own machine shop and have made connecting rods, billet intake manifolds and other various parts for myself since i can.

I have all the parts for the rockers i.e. bearings, shafts and other parts I just need to make the intake rockers because the intake ports are shifted .180

 

[SwinnyGG]

2024 is a wrought alloy, so any form you buy it in, sheet plate bar round whatever, will be cold worked or rolled.

 

[Tmoose]

bad geometry in high stress areas can overcome gorgeous material's high fatigue strength.
Mil spec shotpeening can help fatigue strength performance a lot, too.

Something like these Harland Sharps ?

https://static.summitracing.com/global/images/prod/xlarge/csp-s60565k_xl.jpg?rep=False

If I had rocker arms, I'd be considering Milling 0.188" off one side, and adding a spacer to the other.

 

[WKTaylor]

Why an aluminum alloy in this high temperature high stress/wear application? Due to poor fatigue durability, I tend to shy away from aluminum for this/similar applications.

Off the top-of-my head, I would lean towards 2219-T6xx or -T8xx tempers for a host of reasons...

However, strongly suggest going to SAE website

http://www.sae.org/

for automotive and racing engine design info [texts, article, papers, material specs, etc].



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[SwinnyGG]

A roller rocker arm is not a high wear application. Rockers are replaced because a geometry change or component weight reduction is required, not because the rocker body is worn out. The load on a (properly set up..) rocker arm is also never reversed. There are probably 100,000 performance engines running around right now with aluminum roller rockers, without issue.

I would not recommend 2219. The only real benefit that 2219 has over 2024 is weldability, which isn't required in this application. A rocker arm made from 2024 and one made from 2219 will perform exactly the same, but the 2219 version will be more expensive.

 

[WKTaylor]

jgKRI... OK... but not necessarily agree.

On-line article from enginebuildermag.com.... "Selecting The Right High Performance Rocker Arms"

... ...
As for rocker materials, Elliott offers the following observations:

Stamped steel rockers should only be used on engines with less than .600? of valve lift and at engine speeds below 6,500 rpm. Higher lifts or engine speeds require upgrading to some type of performance rocker (aluminum or steel).

Aluminum rockers are lightweight, easy to manufacture, and have a dampening effect on the valvetrain. The downside is their mortality rate, since all aluminum rockers have a cycle life.

The pros of using steel rockers are that they are more durable (extended cycle life), and stiffer with a slight deflection improvement over aluminum. The downside with steel rockers is that they are not as easy to manufacture, they can be heavier and harder on other valvetrain parts.

The lighter the rocker, the less mass the valve spring and pushrod have to accelerate, stop and accelerate in the opposite direction. But each application dictates how light of a rocker can be used. Removing weight from the roller tip always pays the biggest dividends, as long as the rocker is capable of handling the loads for that application.

Yella Terra

Roger Vinci at Yella Terra High Performance Engine Parts agrees with the other rocker suppliers that more and more racers are choosing steel rocker arms and shaft systems for high rpm applications. “Keeping the valvetrain as stiff as possible while maintaining the lightest weight over the nose of the rocker is our goal. We have one of the smallest nose wheels in the industry. The lightweight technology in our Ultralite LS rocker designs allows the use of lighter spring weights.”

Vinci says strength is of utmost importance in a rocker arm (far more important than its appearance or packaging!). “We use a 2024 T3500 [2024-T3510 or -T3511?] alloy in all of our higher end rockers. Some manufacturers start with an inferior alloy and anodize it to make it look good. But anodizing an alloy can reduce its strength up to 50%. That’s why we do not anodize our rocker arms.”

Vinci says creating as much lift as possible via the rocker arms is paramount to stabilizing the valvetrain. “We know that utilizing the leverage of the rocker shortens lifter travel, which lowers friction. It also lowers lifter and pushrod inertia and can greatly increase the rate of lift of the valve as comes off the seat to initiate and sustain airflow. This can allow the engine builder to use lighter pressure valve springs, which we believe is always more favorable when possible.”
... ...


NOTE. 2024 cannot/should-not be die-forged, so plate or bar or extruded shapes are necessary raw stock. 2219 is a suitable die forging alloy ... IF You want mass production [precision] die forgings generally produce the optimum grain-flow for durability, strength and relative ease of manufacturing. I agree: try to avoid anodic coatings due to early crack initiation. Post machining/NDI shot peening will usually help fatigue life [retard crack initiation] dramatically.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[SwinnyGG]

This dude is building a one-off set of rockers for his own project. He does not care about forging capability.

Look at the rest of the comment of the guy you're quoting- he's talking about rockers for a NASCAR engine, which are shaved of every possible gram, and are under much more extreme temperature, RPM, and static load conditions than a rocker for just about any other possible engine. OP isn't building that type of engine.

Failure of aluminum roller rockers in engines of all stripes are very very rare- google if you don't believe me. The failures that do occur are usually due to setup mistakes or parts that collide because of bad choices by owners.

Personally, I wouldn't be eager to design a set of rockers myself- I definitely agree that it's a highly stressed part and would be easy (and expensive) to get wrong. But ultimately, neither you nor I can stop him if he's determined to do it himself.

 

[Rob_G]

jgKRI, thanks for the backing. You are right this is a one off project.
I have made my own aluminum connecting rods with no failure on a 700 hp motor, so I am not too worried about doing a rocker arm. As far as the design there are 1000's of aluminum rockers to reference off of. I am not trying to re-invent them.

Rob

 

[WKTaylor]

Rob G...

Ever considered buying aluminum rockers from a reliable commercial source [good reputation]; and then modifying [trimming, thinning, etc] those rockers for lighter weight?

OK, OK, OK... I get-it: You're in it for the personal design experience. For some weird reason I thought this was for a possible commercial application.

I'll butt-out.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[SwinnyGG]

jgKRI, thanks for the backing. You are right this is a one off project.
I have made my own aluminum connecting rods with no failure on a 700 hp motor, so I am not too worried about doing a rocker arm. As far as the design there are 1000's of aluminum rockers to reference off of. I am not trying to re-invent them.

Well it sounds like you have experience with highly stressed parts. Good on you and good luck.

I do have a couple of suggestions... if your rockers are going to operate such that the pushrod and spring forces are not centered on the rocker stud, (I'm assuming these heads you're using are stud mount, but I know little to nothing about big block Buicks) I would seriously consider going to a shaft mount rocker and skewing the axis of the shafts so that there is no torsional load in the rocker- this will make them MUCH stronger.

Also, make sure to consider how some of the parts (valve springs, rocker shafts, etc) will deflect at very high speeds which you can't simulate- valve springs do crazy things at 7,000 RPM, so make sure you provide clearance for that.

Last thing is make sure they are set up correctly- it sounds like you've built some big-boy engines before so I don't mean to patronize- but aluminum rockers are MUCH less tolerant of things like incorrect spring pressures or incorrect pushrod lengths than steel rockers are. That stuff needs to be dialed in.