Rebuilding, Swapping, Buildup, & Restoration


Valvetrain Geometry

Two things to watch out for on valve train geometry:

1) Any lift over .520" will cause interference between the spring retainer and the top of the valve guide. You can machine the valve guide down to eliminate this interference. Up to .540" is ok as long as you check the valve to piston interference with clay. This is with the stock 10.5:1 pistons. With the 8.5:1 pistons it's more, but that kind of cam won't work with a 8.5:1 compression.

2) You basically want the tip of the valve and the tip of the pushrod to be as close to the factory position as possible, to maintain proper valvetrain geometry; if one is raised and the other is lowered, you may have the proper preload at the lifter, but the rocker arm will be further along in its travel (or further behind) from where it is designed to be, and at the extreme it can cause extreme wear on the rocker tips, or bind on the rocker bridge.

Things that can throw this geometry off are valve jobs (usually raises the tip of the valve), different-length valves (raises or lowers valve tip height), reduced base-circle camshaft (lowers pushrod height), milling the block or the heads (raises pushrod height), different-length pushrods (raises or lowers pushrod height).

Many performance cams use a reduced base circle, and thus will need pushrods that are longer by the same amount (well, actually half that amount, unless you machine the heads, and then it depends what you machine... it can get messy; that's why they have adjustable-length checking pushrods, and shims to make all the valves happy.).

This is an item that Mondello will gladly sell to you. It looks like a U-shaped piece of square steel, and from the middle another short piece of steel descends. With an untouched assembled head, or the correct measurements, you can make your own.

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This is used to check the valvestem height after a valvejob is done. The outer arms of the tool rest on the valve-cover surfaces, and the middle finger of the tool is supposed to just contact the top of the valvestem. If the valvestem is too long, you cut it down to fit; the valvestem would be too long because a valvejob cuts down the valve seat, which puts the valve higher in the head. This is to account for the non-adjustable rocker arms on the Olds; if the valvestem is too high, then the pushrod will be placing too much preload on the lifter.

On second thought, the arms of the tool might rest on the workbench that the head would be resting on. That is where it should rest, anyway, because you want to measure the position of the valvestem in relation to the cam, not the rocker-cover rails. Doing a valvejob raises the valvestem in relation to the entire head, but surfacing the heads will lower the entire head in relation to the camshaft, so these two often even each other out.

You can also use a $10 pack of shims (placed under springs) to get the proper preload for each lifter. Crane makes these shims (along with others), and it won't take more than an hour to set up. They're only $10 a pack. I bought two just in case, and ended up using two from the second pack.

If you use +.050" pushrods with a -.100" base-circle (-.050" radius) cam, the tip of the pushrod is exactly where a stock-length pushrod on a stock cam would be, so valvetrain geometry wouldn't be a problem. You can only go to about .060" with the shims anyway (there are two thicknesses, .020" and .030", if I recall correctly; maybe it was .015" and .020"). The job is a little tedious, but once you know the clearance figure you want for optimal preload, it goes pretty quickly (maybe two hours tops, if you're taking your time).

And no, it doesn't improve hp, but the lifter preload may keep you out of valve float at higher rpms where you otherwise couldn't go, and most of all, it will stop the valves from clattering!

The one who says you don't need it either doesn't know what you're talking about, knows what you're talking about but has some alternative method for establishing proper valvetrain geometry (i.e. shimming the rockers), or knows what you're talking about but doesn't care enough to actually check these measurements. You can always ask him how he establishes the proper valvetrain geometry on an Olds engine; if he says Huh? or Aw, ya don't need to do that, then run, don't walk, from that man. If he has another answer, listen carefully.

[ Thanks to Bob Barry, Jim Chermack for this information ]