How far can you mill a head before pushrods are too long?

[jamyers]

Like the subject line says, how far can you mill a small-block head before the pushrods are too long and the lifters don't have enough room to pump up properly?

I see where folks are milling .075 - where's the point where valvetrain geometry becomes an issue?

 

[CNC-Dude]

Aftermarket pushrods are available in .050 increments for a reason. Many people make the incorrect assumption that they are starting with a perfectly machined factory engine when they start a modification of this type. The factory allows a certain amount of +/- tolerances for all the machined portions of an engine. Its quite possible, that to start with, you could have an engine/head that just "squeaked" by the QC guy, or you could have one that should have been rejected, and he passed it anyway because it was late on a Friday, and he didn't want to fill out all the paperwork associated with it if it failed. You first need to establish a baseline to know where you stand as far as your rocker geometry is concerned before you begin to mill your heads more than about .025-.030. Many people also assume incorrectly, that because they don't see any immediate ill effects from milling their heads excessively, that all is OK, and they have been successful with their mods they have done. Also, all modern hydraulic lifters are the same, there is only about 2-3 companies that make them for the entire OEM and high performance aftermarket. If you look at any of these aftermarket cam companies websites or catalogs, you will see that they only recommend between .025-.045 lifter preload to properly adjust their lifters, and when you preload them to .100 or .125, and wipe a lobe off of the cam....who's fault is it. In a non-adjustable valvetrain, you can see how this is very critical to maintain the correct lifter preload. Also trying to maintain proper preload and correct geometry is even more of a issue when you don't have an adjustable valvetrain. A bad valve job where the valve tip heights are not consistant, can also throw off your lifter preload and geometry as well. So, there is more to this than just resurfacing that can create these issues of rocker geometry and preload. The best thing to do is to only cut the heads a minimal amount, or buy aftermarket heads that have a more desirable chamber size you are seeking to start with. If you are doing a valve job, and putting in larger than stock valves, don't try to sink them in an effort to offset the installed height in some way. Many performance based shops know all this, but also do your homework as well so you don't end up overspending to fix a problem later on that got overlooked....

 

[jamyers]

So the short answer is: "it depends". I can live with that.

OK, I've gone blank - with non-adjustable rocker arms, how do you tell/check lifter preload? Assuming valvestem height and all else is within specs.

 

[CNC-Dude]

You can start off by placing a straight edge across the tops of all the valves on an assembled cylinder head to see how cosistant they all are from end to end. You need to have a dial indicator with a magnetic base, and a lifter that has all the oil bleed out of it. Make a 2" wide by .125 thick steel plate, and drill a small hole into it along the edge where you can bolt it to the valve cover rail. You just have to mock up and check one at a time. Rotate the engine where the heel of the cam is against the lifter, as you normally would to adjust the valve. Slowly tighten the rocker down(either pedestal style or other)until you feel all the slack out of the pushrod, but not so much that it starts to push the plunger down in the lifter. At this point, put the indicator onto the pushrod end of the rocker and zero the dial, then finish tightening the rocker the rest of the way down. The reading on the dial will show you how much the lifter is compressed or preloaded. If its too much, depending on whether its a pedestal mount or shaft style, you can get shims from any of the cam companies to raise the shaft or pedestal back up(much cheaper than pushrods), considering they are all pretty equal on to another. If you have several that compress the lifters more than others, you might have to either tip the valves on the others so they all will match, then you can evenly shim them up, or you could sink the valves on the ones that are too much(within reason). Its sound like its real time consuming and complex, but its very simple, especially if you are pretty mechanical minded.

 

[jamyers]

Makes sense - Thanks!