I'm just wondering why most people are running the chromolys instead of heavy duty aluminum (see links below) for close to stock applications that are not high revving or using stronger valve springs?

I couldn't find discussions about using HD aluminum (only stocks: http://www.914world.com/bbs2/index.php?sho...p;#entry1422350 )

In my recent 2056 build I used chromoly pushrods but didn't change to 911 swivel feet with 1.7 arms which I realize now was a mistake (still have 2.0 arms and new 2.0 adjusters). I'm getting ready to change over to the 1.7L arms and swivel feet.

I've been unhappy with the valvetrain noise I'm having, but I think most of that has to do with NOT changing to 911 swivel feet adjusters and 1.7 L arms. I can really hear this chatter with the 2.0L adjusters and I'm hoping that switching to 911 swivel feet will remove this because of the larger contact area.

As a result of changing the arms I get to re do valve train geometry and order up some new pushrods. One thing that has bugged me about the chromoly pushrods is they start at zero lash cold and end up at some amount of lash when hot.

After the switch I'm going to install my existing cut to length chromolys (which will be the wrong length) for a test run to see the sound difference just from the arm change and from there (with the help of this discussion) decide which route to go for pushrods. This conversation may be a moot point but I wanted to see what the reasoning is behind the use of the chromolys, the whole increasing valve lash thing has been bugging me.

I would expect that increased lash at operating temperature would reduce valve lift and duration slightly which would reduce performance. Since most people are trying to increase performance I wouldn't expect the chromolys to be advised. With the amount of care and thought that is given to every aspect of these engines by builders like Jake and all the others on this forum that recommend the chromoly pushrods I figure I must be missing something.

For close to stock engines it would seem the strength of the chromoly's is not the advantage but rather being able to set the lash to zero.

The .095 wall 2024 T3 Aluminum HD pushrods (58-62Ksi tensile) will be (theoretically) almost half the weight of the .058 wall chromolys (4130/4135 - 140Ksi tensile). Approx 3.8 grams/inch length for Aluminum vs approx 7.4 grams/inch lenght for Chromoly. Area difference is .08356 in^2 for .095 wall 2024 T3 Al HD vs .05776 in^2 for .058 wall chromoly HDs (assuming 3/8" OD)

I see there is a clear strength difference but again, for a slightly more than stock engine why would I want to use the chromolys when I'm essentially losing some duration and lift by having added lash at temperature in addition to the added noise I'm seeing (which again may go away entirely from the 1.7L arms with swivel feet).


There is also a price difference (aluminum HDs almost 2x as much as chromolys) but an extra $80 is not that much in an engine build.


As always, thanks for your insight and advice,

Andy


references:
HD aluminum: http://vwparts.aircooled.net/Heavy-Duty-Al...cnhdalprcut.htm

HD double taper aluminum: http://vwparts.aircooled.net/ACN-Dual-Tape...thdalprcut.htm)

Manton PDF: http://www.mantonpushrods.com/Images/Manton_Catalog.pdf

Expansion rate!

Chromoly's are safer to run is why I think a lot of professional engine builders use them in their motors. Less chance of burning a valve and the engine coming back because it got hot; like a fail safe to some degree.

You will lose lift with them over aluminum. The perfectionist's way to do it would be to set the chromoly at zero lash with the motor hot.

About noise, steel is far less resonant than aluminum so they seem to be quieter warm than aluminum cold.

That is interesting. I was about to order push rods and lifters. I didn't realize there was this much of a difference with the chromoly push rods. I had not heard that they get that loose when hot. According to one read the gap increases to .012"! this sounds huge to me. I am guessing this is why you are encouraged to use the swivel feet. They must be more tolerant to large valve lash gaps. Is this correct? Do these really get noticeably louder? I would be interested in hearing from others that have used these.

If you set hot lash to zero with chromoly P-rods it's pretty likely you'll never get the engine started once it's cold. Think about it.

Ham whats your thoughts, for us weekend techs ????????

Set 'em to zero cold. Check the lash hot to see how much they're picking up from heat. Only have to do that once to know what the hot lash is.

Jake went through this years ago and settled on zero as being just right for iron jug engines.

The co-efficient of expansion is different for the CM push rods as Slits has said.

When the lash is set to .006 with the Alum. they will expand to zero when hot.

CM does not have the same co-efficient.

I see what you are saying now. Setting it to zero hot would mean they'd be open/tight by like .012" cold.

It' doesn't add up though that steel would be expanding twice as much as aluminum , I would believe the other way around?

Regardless though, it seems aluminum would lead to the most lift at temperature and still be able to start the motor, this being advantageous for exhaust valves especially.

I've thought about what would happen if you ran aluminum pushrods for exhaust valves gapped to .006 cold and chromoly for the intake gapped to 0 cold, since a hot engine doesn't need as much fuel.

The jugs, case and heads get hotter than the PR's and therefor expand more. That's why the steel PR's gain lash.

The aluminum PR's gain some length from the heat they do pick up and it's enough to keep pace with the rest of the engine.

Properly setup and configured chromoly pushrods are just as quiet as aluminum. If you don't use 911 swivel feet and a solid rocker shaft spacer kit, they will be louder.

The solid rocker spacer kit is often overlooked, because people don't realize that as much noise can come from lateral movement as from lash. They also don't realize that a loss of lift and duration can come from lateral movement of the rocker during an opening event.

I run zero lash and solid rockers with aluminum center sections. I set lateral movement for the entire arrangement at .004 cold and its as quiet as you'll ever get a solid lift engine.

Components aren't everything, its the performance mindset that most people just don't have anymore.

So the main reason for choosing chromoly over aluminum is that there is no chance of the pushrod expanding enough to keep a valve open if the engine temperatures get too high? (as nathan said in post 3). Most of the other discussion has been about where to set the lash hot vs cold for chromoly, but little over why the chromolys are preferred.

I'm already running a set of solid spacers but I have to check/redo them when I replace the arms and redo valvetrain geometry. It'll be a good time to drop the engine and take care of some leaks.