Has anyone else thats running chromoly pushrods noticed they don't expand at the same rate as the cylinder(like stock components)?

The effect being that they infact open up the valve clearences as the engine approaches tempereture, instead of closing up like aluminium pushrods do.

I've noticed that if I set my valves at 0 lash, the expand to about .006" at 80 degrees. It's the only way I can have nearly quiet running, and not have the valve clearence gaps look like they've been set with a hacksaw blade.

Cheers,

Being a martensitic steel Chromoly (4140) expands about 20% less than normal (non-martensitic) steel. Steel in general expands about half as much as aluminium and aluminium alloys.

So what you have described is to be expected as the difference in thermal expansion rates between the push rods and case is worse when using Chromoly rather than stock push rods, assuming stock push rods are a low or medium carbon steel.

Don't know if this helps at all, but any chance to use the crap I learnt at uni is a good excuse.

Yes it does help, thanks.

There is a forum listing on the http://www.shoptalkforums.com  forum about setting valve clearences for turbo drag bugs (which mostly use chromoly PR's). But they never mentioned that chromoly as being a reason to set clearences diferently.

Anyone else running chromoly push rods?

Yep i run mine and it is noisy, run 0.006" and it seems to get noisier over time. I was alos told that because they are denser they also rattle more, eg don't absorb the noise as easy. eg try a shaking a rubber bouncy ball in jar complared to a ball bearing


That doesn't amke sense, but hope you know what iam trying to say. (to many :beer at the work lunch today

I run them, valve lash at 0 cold. I set it so I can spin the pushrod, gets loose as it warms up... I reckon the aircooled.net alloy ones would be the go, pricey but no noise, and will work with duals, also light and easy to cut.

I'm actually planning on using aircooled.net's aluminium PR's on my new turbo engine. Light weight is the key, but still strong enough for 6500 RPM. Keeping down the valve train weight is what it's all about.

My current 1641cc turbo uses custom length pr's with the scat 1.25:1 pro comp rocker gear.

Just curious, but is everyone using pre-assembled chromoly standard length pushrods, or custom cut to lengh ones?

Cheers,

Cut to length is really the only way to get the geometry right

stock rods are NOT steel.

cut to length cromoly

What are stock push rods made of?

The stock pushrods are aluminum, and were designed to expand at the same rate as the motor.

I use stock pushrods and rev it well passed 7k every gear every race.

Its all about the geometry and valve spring load, get it right and the stock will last fine.

How do you get the tappets right if your engine is at various temperatures ??

I've set mine at 0 lash, so that I can still turn the rods with my thumb and finger, then I just drive gently till its warm (as we all should). It warms up inside of 5 minutes.

I used to actually start the car and idle it till it was up to tempreture(when I had a company car and my bug was a weekend drive), but thats not always possible now the car is a daily driver.

I'm not too concerned that the clearences are too close when its cold, as i check them regularly (every 2 weeks or so).

Don't you get sick of checking every two weeks ?

Do they change all the time? or do you just check for piece of mind sake.

Na, they don't change at all actually. It's just a piece of mind thing.

I've been doing a lot of work on the car recently, having just converted from multi-point efi to full sequential efi, and having to completely re-tune the efi system.

Normally I don't work on it all the time, and when I said 2 weeks i think i meant 4 weeks (for valve checks that is).

I used to run chrome moly pushrods in my 1915.

yes I checked them hot and cold, yes the clearance is bigger hot. I used to set them to about 1 thou cold. About the smallest I could measure.

Yes aluminium expands nearly twice as much as steel/chrome moly so that is the problem.

Yes my pushrods in my 1915 were noisy, especially in a Baja with an open engine. It got nicknamed the diesel.

I have just put these in a new motor I sold .I will now call the guy to let him know about this ,thanks for the post