How to Set Preload on LS Rockers (Trunnion Kit Install)

TL;DR: An LS rocker arm has no adjuster nut, so you cannot "set" preload the way you would on an older Small Block Chevy. Preload is a fixed consequence of pushrod length, and the only way to change it is to change pushrods. A trunnion upgrade kit replaces the factory caged needle bearings inside the rocker with full-complement bearings so the rocker survives real spring pressure — but the kit does not change preload by itself. The correct order of operations is: press in the new trunnions, bolt the rockers down, then measure with a pushrod length checker and buy the length that lands preload in the middle of the lifter's travel.

The thing that trips everyone up: LS rockers are non-adjustable

On an older adjustable valvetrain, preload is a procedure. You back the adjuster off, bring the lifter to zero lash, then turn the nut down a specified amount and lock it. The adjuster absorbs every stack-up in the engine — deck height, head thickness, gasket thickness, cam base circle, valve length.

The LS deleted that adjuster. The rocker bolts to a pedestal, the bolt torques to a fixed value, and the rocker ends up exactly where the casting puts it. That is what "net lash" means: the geometry is set at the factory by making every part in the stack the right size, and the assembler just torques bolts.

One consequence follows. Lifter preload on an LS is determined entirely by the length of the pushrod. The hydraulic lifter has a plunger with a limited travel range — the exact usable stroke depends on the lifter brand and part number — and preload is simply how far into that travel the pushrod pushes the plunger when the valve is closed and the lifter is sitting on the cam's base circle.

Too little, and the plunger sits at the top of its bore with slack in the system: you get a tick, and at high rpm you get intermittent loss of control over the valve. Too much, and the plunger bottoms out in the lifter body — at which point the lifter is a solid steel link, the valve is held off its seat, and you burn a valve or lose compression on that cylinder once the engine reaches operating temperature.

So this is two jobs in one session: rebuilding the rocker so it can live behind a stiffer spring, then measuring what pushrod the engine now wants.

Why the factory trunnion fails, and what the kit actually changes

Each LS rocker pivots on a trunnion — a hardened steel cross-shaft pressed into the aluminum rocker body, riding on needle bearings, retained by clips or snap rings.

The factory trunnion uses caged needle bearings. A cage is a thin retainer that holds the needles evenly spaced. It works fine at stock spring pressures and stock rpm. The failure mode is the cage itself: it is the thinnest part in a load path that gets much harsher the moment you install a bigger cam and stiffer springs. The cage cracks, needles escape, and the debris goes exactly where you would least like it — down the pushrod, past the lifter, into the oil. A trunnion failure is not contained to one rocker; hardened fragments circulate.

A trunnion upgrade kit presses out the factory bearings and presses in a new trunnion running full-complement needle bearings — no cage, more needles, more load-carrying surface, and nothing thin left to crack. Most kits include the trunnion, the bearings, retaining clips, and often a press tool or spacer set.

What the kit does not do is change your valvetrain geometry. A quality trunnion kit is dimensionally the same as what it replaces, so the rocker sits where it always sat. Installing trunnions does not change preload. If your preload was wrong before, it is wrong afterward.

Do this in the same session as any cam swap, which is why it appears in the middle of the in-vehicle LS cam install guide — the rockers are already off, and the press work is essentially free labor.

Three ways to handle LS rockers, compared

For most street and truck builds the middle column is the answer. The factory rocker body is a good aluminum casting; it is the bearing inside it that is the compromise. Move to the right column when spring pressures and rpm are past what any needle-bearing trunnion is happy with, when you want a different rocker ratio, or when you have already eaten a trunnion once — the considerations behind the high-rpm valvetrain guide. Browse rocker arms and complete valvetrain for both paths.

Part one: pressing in the trunnions

This is press work, not hammer work. The rocker body is cast aluminum and it will crack or distort if you drive anything through it with impact.

1. Remove the rockers and keep them in order. Pull the valve covers and back out the rocker bolts. Keep rockers, bolts, and pushrods organized by cylinder and by intake/exhaust — anything reused goes back exactly where it came from.

2. Inspect before you disassemble. Spin each rocker on its trunnion. Roughness, notchiness, or side play means that bearing was already going. A cracked cage or missing needles means the engine has been eating debris — change oil and filter immediately after start-up, and read LS lifter failure before assuming the rockers were the only casualty.

3. Remove the retaining clips. Snap rings or C-clips at the ends of the trunnion. Small, spring steel, and they fly. Eye protection.

4. Press the old trunnion out. Support the rocker body — not the rocker tip, not the pushrod cup — using the kit's fixture or correctly sized sockets. Press straight out. It should move steadily; if it fights you, your support is wrong or the assembly is cocked.

5. Clean the bore and inspect it. Wipe the bore and look for scoring or an out-of-round condition. A rocker body damaged in the trunnion bore is scrap — replace it rather than pressing a good bearing into a bad hole.

6. Press the new trunnion in, squarely. Follow the kit's instructions on which side to press from and which spacer to use, slowly enough that you can feel it going in straight. Stop at the specified depth — the trunnion must be centered so the retaining clips can seat in their grooves.

7. Install the new clips and verify seating. Each clip must be fully in its groove all the way around. A clip that sits proud at one point will walk out later.

8. Check for free movement and no bind. The rebuilt rocker should rotate smoothly, with no notchiness and no measurable side play. Any bind means the press depth is off. Fix it now — a bound rocker overheats its own bearing.

9. Repeat for all sixteen, then lubricate. Oil the bearings so the first start is not also the first lubrication.

Part two: setting preload (which means measuring pushrod length)

You now bolt the rockers down and find out what pushrod the engine wants. This is measurement, not adjustment.

1. Get the cylinder you are measuring onto the cam's base circle. Both valves fully closed for that cylinder — that is the only position where the lifter plunger is unloaded and the measurement means anything. Rotate the engine by hand at the crank bolt, never on the cam bolt and never on the starter.

2. Set up an adjustable pushrod length checker. A checker is a threaded pushrod that lengthens and shortens. Install it in place of the pushrod for the valve you are measuring, with the rocker fitted and the rocker bolt torqued to its normal specification — 22 lb-ft is the widely used LS rocker bolt figure, but check current service data for your specific engine and the instructions for any aftermarket rocker or stand.

3. Find zero lash. Lengthen the checker until all the slack is out of the system — the point where you can still spin the checker with your fingers but feel it just beginning to load. Then stop. That length is zero lash: pushrod touching, lifter plunger not yet pushed into its bore.

4. Add preload. Continue lengthening the checker by the amount of preload you want. This is the number the whole job turns on:

Check the target preload range against the specification published by your lifter manufacturer — brands differ, and short-travel race lifters have a much narrower window than an OE-style hydraulic roller.

5. Read the checker. Remove it without changing its setting, measure it with a caliper or micrometer, and write the number down.

6. Measure more than one cylinder. Valve lengths vary and a machined head varies more. Check at minimum one intake and one exhaust at each end of each head — four measurements per head is a reasonable floor, and measuring all sixteen is not wasted effort on a fresh build.

7. Buy pushrods to the length you measured, not the length that was in it. Pushrods come in fine increments. If your measurements spread across the bank, choose the length that puts every cylinder inside the window rather than the one that is perfect on a single cylinder and marginal elsewhere.

What changes pushrod length (and why you must remeasure)

The factory pushrod length is only correct for the factory stack. Every one of these moves it:

• A different camshaft. A different base circle diameter changes the lifter-to-rocker distance directly — which is why pushrod length is a line item in the LS cam swap cost guide and a decision point in the LS cam brand comparison.
• Milled or aftermarket cylinder heads. Removing deck material shortens the stack.
• A different head gasket thickness. Same effect, smaller magnitude, still real.
• Different lifters. Short-travel and race lifters do not share the plunger height or preload window of an OE-style lifter.
• Different valves or a valve job. A valve sitting deeper in a freshly cut seat raises the tip and shortens the required pushrod.
• Different valve springs and retainers. Not a length change directly, but stiffer valve springs are exactly when the trunnion upgrade stops being optional.

Pushrod length is an output of your engine's stack-up, not a catalog lookup. Anyone quoting a length off a forum is quoting their engine.

Reassembly, torque, and first start

Torque the rocker bolts to specification, working evenly across the head rather than down one end to the other. Oil the trunnions and pushrod cups. Confirm every pushrod is seated in its lifter and in its rocker cup — one resting on the edge of a lifter instead of inside it will bend the moment the engine turns.

Rotate the engine by hand for two full crankshaft revolutions before going near the starter. Every valve should open and close with no hard stop. A hard stop is a valve contacting a piston or a pushrod out of its cup, and finding it by hand is free.

Reinstall the valve covers with fresh gaskets. Start it, bring it to temperature, and listen. A hydraulic roller valvetrain with correct preload goes quiet almost immediately once the lifters fill.

FAQ

Can I adjust preload on stock LS rockers? No. They are non-adjustable, net-lash rockers with no adjuster nut. The only way to change preload is to change pushrod length — or to fit an adjustable aftermarket valvetrain.

Does a trunnion upgrade change pushrod length? A correctly made kit is dimensionally the same as the factory assembly, so it should not. Measure anyway: trunnions usually go in alongside a cam or a set of heads, and those absolutely change it.

What preload should I run on an LS? Most street builds target near the middle of the plunger's travel. Follow your lifter manufacturer's number over any general figure — it differs by brand.

What happens if preload is too high? The plunger bottoms out and the lifter behaves as a solid link. The valve is held slightly open, cannot shed heat into its seat, and that cylinder loses compression and eventually burns a valve — typically once hot, not on the first start.

What happens if preload is too low? A tick, because there is lash in a system designed to have none. At rpm you also get instability, and the plunger gets hammered rather than loaded smoothly.

Do I need a pushrod length checker, or can I count turns? On an adjustable valvetrain you count turns. On an LS there is nothing to turn, so a checker is the tool — inexpensive, and the only way to actually know.

Can I install a trunnion kit with a hammer instead of a press? No. The rocker body is cast aluminum and impact will distort the bore or crack the casting. Use an arbor press and the kit's fixture.

Do I have to do all sixteen rockers? Yes. The ones you leave alone are the ones that fail, and one cracked cage puts hardened debris through the whole engine.

Ready to order

• Rocker arms — trunnion upgrade kits, rebuilt and complete rockers
• Pushrods — fine length increments, hardened and heavy-wall, plus length checkers
• Valvetrain and valve springs — springs, retainers, seals, hardware
• Lifters — OE-style and short-travel hydraulic rollers
• Camshafts and cam stage kits — the reason most people are in here
• Gaskets and seals — valve cover gaskets and grommets
• LS engine parts — everything else the job touches

Confirm your engine on the fitment lookup before ordering.

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    {"@type": "HowToStep", "name": "Install a pushrod length checker and find zero lash", "text": "Fit an adjustable length checker in place of the pushrod with the rocker torqued to specification, and lengthen it until all slack is removed without loading the lifter plunger."},
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