What follows is a clear, practical beginner‑level guide to the valve train on a Nissan Skyline R32 (RB family engines: RB20/25/26), how it works, why valve maintenance (valve clearance/shim service) is needed, every component you’ll encounter, what can go wrong, and a step‑by‑step procedure to measure and correct valve clearances (shim‑under‑bucket type). I assume the R32 you mean uses the common RB “shim under bucket” design (this is true for RB20/25/26 commonly found in R32s). Always cross‑check exact specs and torque values with the factory service manual for your specific engine variant.

Quick summary (if you only want the big picture)
- The valves open/close to let air/fuel in and exhaust out. The cam lobe pushes on a bucket over a shim to open a valve. A small clearance (valve lash) is required so valves fully seat when hot. On RB engines this clearance is set by selecting the correct shim thickness under each bucket. You measure lash with feeler gauges and change shims where the measured clearance differs from spec.

Why this repair is needed (theory)
- Thermal expansion: parts grow when hot. Valve clearance compensates for that so valves still close when the engine reaches operating temperature. If clearance is too tight the valve won’t fully seat when hot — that causes compression loss and burnt valve faces. If clearance is too loose the valve train will be noisy, cam lobes/buckets will wear faster, and valve timing is effectively degraded.
- Wear and settling: cam lobes, tappets/buckets, valve stems, and seats wear over time, changing clearances. Periodic checking and shimming restores correct geometry.
- RB design uses solid shims under buckets (shim‑under‑bucket). There’s no automatic hydraulic lifter to self‑adjust, so shims must be changed to set lash.

Detailed descriptions of every valve‑train component you’ll deal with
- Camshaft: the rotating shaft with lobes that push on buckets. It sits in journals and is driven by timing belt/chain and sprockets.
- Cam lobes: eccentric lobes on the cam that press on the top of the bucket to open valves. The lobe base circle is the smallest diameter portion (used when measuring clearance).
- Cam caps / journals: caps bolt over the cams to retain them in the head. They must be marked and reinstalled in their original orientation and torque sequence.
- Bucket (cam follower / valve bucket): a hardened cup that sits over a shim and transmits cam lobe lift to the valve stem. On RB engines the top of the valve stem engages a valve spring retainer/keeper; bucket sits on the shim between cam and valve.
- Shim (solid shim): a thin steel disc that sets the gap (lash). On RB engines the shim sits under the bucket (between bucket and valve stem top). Shims come in discrete thicknesses.
- Valve stem / valve: the valve itself (head, face, stem). The top of the stem interacts with the shim/bucket; the valve seats on the valve seat to seal combustion.
- Valve spring: returns the valve to closed position and keeps it seated. Springs must retain tension and be inspected for sag/cracks.
- Retainer and keepers (collets): hold the valve spring on the stem (retainer) with split keepers that lock in a groove on the stem.
- Cylinder head: houses all these parts, provides channels for oil, cooling, and mounting points.
- Timing components (belt/chain/sprockets, tensioner): keep cam aligned with crank. You must preserve timing when removing cams.
- Cam seals, seals & gaskets: prevent oil leaks when you remove covers/cams.

How the system works (simple analogy)
- Think of the valve train like a row of doors (valves) that must be opened briefly in a timed sequence. The cam lobes are like cam‑activated “door openers.” The shim gap is like a small rubber bumper that ensures the door fully closes even if the frame expands when it gets hot. If the bumper is too thin (too tight) the door won’t fully latch when hot; if it’s too thick (too loose) the door bangs when it closes.

Common failure modes / what can go wrong
- Lash too tight → valves run hot, burn, or fail to seat fully → compression loss and poor running.
- Lash too loose → noisy tappets, accelerated wear on cam lobes and buckets, possible broken shims if they slip.
- Worn cam lobes or buckets → uneven lift and inconsistent valve timing/clearance.
- Broken or weak valve springs → valve float at high RPM, misfires, or dropped valves.
- Improper reassembly (wrong cam timing, wrong torques, swapped cam cap orientation) → catastrophic engine damage (valves hitting pistons on interference engines), cam bearing damage.
- Dropped or lost shims → engine damage or valve clearance completely incorrect.
- Dirt/contamination when reassembling → accelerated wear, sticking valves.
- Using the wrong shim thickness or miscalculating replacement thickness.

Tools & supplies you will need
- Factory service manual for model/year (for specs, torque values, firing order, and timing marks).
- Metric socket set, breaker bar, torque wrench.
- Feeler gauge set (metric).
- Metric micrometer (0.01 mm) or digital caliper capable of thin measurements to measure shim thickness (or shim gauge).
- Selection of shims (assortment kit) or ability to order specific thicknesses.
- Camshaft/valve cover gasket(s) and any seals you remove.
- Clean rags, engine assembly lube, magnet or small thin pry tool to remove buckets (if accessing with cams out).
- Labeling materials (marker, numbered bags) to keep parts in order.
- Engine support for camshafts (wood block, rag) when removing cam caps.
- Torque wrench and appropriate extensions.
- Optional: dial gauge for precise lash measurement, feeler gauge holder.

Important safety & correctness notes (read before starting)
- Get the exact valve clearance specs and cam cap torque sequence/numbers from the factory manual for your specific R32 engine. DO NOT GUESS torque values.
- RB engines are interference engines (valves can hit pistons if cam timing is wrong). Mark timing precisely before removing cams and do not rotate the crank with cams removed unless you secure the valves.
- Keep everything immaculately clean. A small bit of debris under a shim or bucket can cause damage.
- If you are not completely confident, consider replacing shims only if you can remove and reinstall the camshafts correctly — or have a professional do it.

Valve clearance measurement and shim replacement — step‑by‑step (typical shim‑under‑bucket procedure)
This is a high‑level but detailed workflow. Follow factory manual for exact torque specs, clearance specs and cam timing marks.

1) Preparation
- Warm the engine slightly then shut off and let it cool to “cold” (many specs are cold measurement). Confirm whether spec is cold or hot in the manual.
- Disconnect battery (safety), remove airbox/intake parts to access valve cover.
- Remove ignition components and anything obstructing cam cover removal.
- Remove camshaft valve cover(s) and clean around openings to prevent debris falling into the head.
- Mark cam timing: align crank to TDC on cylinder 1 compression stroke and note cam timing marks on cam sprockets. Take photos and mark cam caps & camshafts so you reinstall in exact orientation.

2) Rotate engine and measure lash
- With engine at TDC (or as manual directs), rotate the crank so each cam lobe’s base circle is under the bucket for the valve you want to measure. For RB engines measure when the cam lobe for a particular valve is on its base circle — the gap is at maximum and can be measured with feeler gauge.
- Insert appropriate feeler gauge between the top of the bucket and the cam lobe. Note: you measure the gap between bucket top and cam lobe. Record measured clearance for every valve (intake & exhaust) in order.
- Repeat for all valves following firing order/sequence so you get every valve measured with its lobe on the base circle.

3) Decide which shims to change
- Compare each measured clearance to factory spec (e.g., intake spec = X mm, exhaust spec = Y mm — get exact numbers from manual).
- If a valve is out of spec, you will change its shim. Keep track of current shim thickness (you must measure it) so you can calculate the replacement thickness.

4) Remove camshafts (required to change shims)
- Before removing cams, make sure you fully document cam timing alignment (marks, photos). Remove any timing components according to manual or support cam timing as instructed.
- Unbolt cam caps in reverse of tightening sequence (usually a specific order from outside inward) gradually to avoid cam journal stress. Keep cam caps labeled in their original positions and orientation.
- Carefully lift camshaft(s) out and place on clean padding. Don’t let the cam lobes or journals contact dirty surfaces.

5) Remove buckets and shims
- With cams removed you can remove buckets. Keep buckets and shims in order — each bucket has a matching shim that often fits tightly. Use a soft magnet or thin screwdriver to lift the bucket straight up. Don’t pry at an angle.
- Measure the thickness of each shim with a micrometer to record the original shim thickness for your calculation.

6) Calculate replacement shim thickness
- Use the calculation:
new_shim_thickness = original_shim_thickness + (measured_clearance - desired_clearance)
(all values in mm; measured_clearance is the lash you recorded earlier when cam lobe was on base circle; desired_clearance is factory spec)
- Example: original shim = 2.20 mm, measured clearance = 0.35 mm, desired clearance = 0.20 mm
new_thickness = 2.20 + (0.35 - 0.20) = 2.35 mm
- Round to available shim sizes (shims come in discrete steps, e.g., 0.05 mm). Choose the closest size that gets you within spec. If you can’t hit exact, choose the nearest one that keeps you within tolerance.

7) Reinstall shims & buckets
- Fit the new shim into the bucket seat position. Ensure it sits flat and clean. Reinstall the bucket over the shim.
- Keep each bucket/shim for each valve in its original location or install new shim in correct location as decided.

8) Reinstall camshafts
- Carefully lower camshaft back into the head ensuring journals and lobes sit correctly. Reinstall cam caps in their original orientation and positions. Tighten cam cap bolts in the specified sequence gradually to the torque specified in the manual.
- Double‑check timing marks and alignment before moving on. If you disturbed timing components, refer to manual for correct re‑timing procedure.

9) Re‑measure lash
- With cams torqued and timing set, rotate the engine to the positions where each valve’s cam lobe is on the base circle and re‑measure lash with the feeler gauge to confirm you are within spec.
- If any valve is still out of spec, repeat shim thickness selection for that valve.

10) Final assembly
- Replace cam seals or cover gaskets if required. Reinstall valve cover(s) with new gasket, torque bolts per manual.
- Reinstall intake, ignition components, reconnect battery.
- Start engine and listen — a quiet, steady idle with no excessive tappet noise is expected. Check for oil leaks and re‑verify timing if suspicious noises occur.
- Recheck clearances after short run and cool down if manual recommends.

Troubleshooting tips and inspection checklist
- If cam lobes or buckets are visibly scored/pitted: consider replacement of affected parts and check oiling system.
- If you cannot achieve spec with available shim sizes: remeasure and recheck math; you may need to replace with a custom shim or swap to an adjacent shim and accept a small offset within tolerance. If out of tolerance, get correct shim thickness from supplier.
- If a shim is stuck in the bucket, remove carefully with a magnet and pick—do not scratch bucket seating area.
- Always inspect valve springs for cracks or weak coils; replace in matched sets if needed.
- If, after reassembly, engine runs rough or misfires, verify cam timing marks and that nothing was mixed up (cams and caps must go back to original orientation unless marked).
- Keep detailed notes and label everything — cam caps, camshafts, buckets — they’re machined as matched sets.

Common rookie mistakes to avoid
- Not marking cam caps/cams before removal (leads to misassembly and bearing failure).
- Using the wrong clearance specs or measuring at the wrong cam position.
- Rotating the crank with cams removed without following manual procedures (can drop valves into pistons).
- Reusing damaged gaskets and seals — oil leaks cause messy, dirty engines and can contaminate valve train.
- Not using a torque wrench or using incorrect torque sequence.

When to replace more than shims
- If cam lobes show significant wear (flat spots), buckets are grooved, or valve springs are weak/broken, replace those components. Damaged cam or bucket will shorten engine life and can ruin new shims quickly.
- If valves are burnt or seats are pitted, consider valve work (seat/valve replacement) or head rebuild.

Final safety reminders
- Use the factory manual. Exact clearance values, torque specs, and timing procedures vary by exact engine model and year.
- Keep everything clean. Small metal filings or dirt can ruin valve seats and cam surfaces.
- If you’re not 100% comfortable with removing and reinstalling camshafts and setting timing, get a more experienced mechanic to help. Valve timing errors on RB engines can cause valve‑to‑piston collisions.

Useful formulas and quick reference
- new_shim = old_shim + (measured_clearance - desired_clearance)
- Measure clearance with cam lobe on base circle.
- Record every measurement and label parts.

That’s a complete, practical overview for a beginner mechanic: how the RB valve train works, the parts involved, why valve lash needs attention, what can go wrong, and a safe, repeatable procedure to measure and change shims. Follow the factory manual for your exact R32 engine for clearance numbers and torque specs and work methodically.
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1) Theory — what the input-shaft bearing does and how it fails
- Function: the input-shaft bearing supports the transmission input shaft radially (and partially axially depending on design) so the shaft spins concentric to the gearset and clutch. It keeps proper clearances/preload so gears and synchros run true, transfers loads into the case or countershaft, and isolates shaft motion from the case.
- Failure modes: roller/ball surface spalling, brinelling, cage wear, corrosion, or race damage from contamination, lack of lube, overheating, or shaft misalignment. Wear increases radial clearance, causes shaft runout, vibration, whining, gear mis-mesh, accelerated synchro/gear wear and eventual seizure.
- Symptoms: growling/whining that changes with engine rpm (not wheel speed), play at the input shaft, hard or crunchy shifting, metal in gearbox oil, overheating/heat discoloration of bearing, or clunking under load.
- Repair objective: restore correct bearing geometry and preload/clearance so the input shaft runs concentric at specified tolerances, eliminating unwanted motion/noise and preventing gear/synchro damage.

2) Preparation (why each item matters)
- Parts: correct new input-shaft bearing(s), new oil seals, snap rings/circlips, shims (if used), gearbox oil. Bearings must match OEM spec so races and fit tolerances are correct.
- Tools: gearbox jack, press or hydraulic bearing puller/driver, bearing separators, snap-ring pliers, torque wrench, dial indicator (endplay/runout), micrometer (shaft journal), clean solvent, lint-free rags. Dial indicator and micrometer are critical to verify tolerances and preload — guessing causes re-failure.
- Environment: very clean bench; contamination during press-fit or assembly ruins the new bearing.
- Documentation: factory service manual for R32 transmission — for removal sequence, torques, shim/preload specs, part numbers.

3) Ordered procedure with theory for each step (bench and car work combined)
1. Safety and disconnects
- Disconnect battery; support vehicle; remove driveshaft/halfshafts, starter, shifter linkage, and any wiring crossing the gearbox.
- Why: prevents electrical shorts and gives clear access; disconnecting linkages prevents damage when gearbox is moved.

2. Remove transmission from vehicle
- Support gearbox with a trans jack, remove bellhousing bolts, lower gearbox straight back away from clutch/flywheel, then lower to bench.
- Why: you must remove the transmission to access the input shaft and bearings; careful straight pull prevents damaging the input bearing by forcing it axially while still mounted.

3. Drain gearbox oil and remove external covers
- Drain, remove inspection covers and selector shafts as required to access internal assemblies.
- Why: clean work and inspection; removing oil avoids contamination and lets you inspect metal debris that indicates bearing failure extent.

4. Disassemble to access input shaft bearing
- Remove snap rings, retainers, and any gear assemblies (mainshaft, countershaft pieces, synchronizers) that block input shaft removal. Keep parts ordered.
- Why: the input shaft/bearing is usually retained by internal circlips or a pressed race; you must remove blocking components without damaging shafts or gear faces.

5. Remove input shaft (or remove bearing in-place if the design allows)
- Use a press or bearing puller/separator to press the bearing off the input shaft or out of the case. Heat the housing slightly if necessary (carefully) to free a pressed bearing.
- Theory: bearings are interference fit; forced removal avoids shaft/journal damage if done concentrically. Do not pry on races — apply force to the race you are moving.

6. Inspect and measure components
- Inspect input shaft journal(s) and bearing seat for wear, scoring, flat spots, discoloration. Measure journal diameter with micrometer and check housing bore with appropriate gauge. Inspect bearing inner and outer race contact surfaces.
- Why: if the journal or bore is out of tolerance, a new bearing will fail quickly; repair may require shaft replacement, sleeve, or bore rework.

7. Clean components and housing thoroughly
- Solvent clean all parts; remove all particles; dry with lint-free rags.
- Why: any contamination introduced during reassembly will cause immediate bearing damage.

8. Fit new bearing correctly
- Press the bearing on/off the correct race: when pressing onto a shaft, press on the inner race only; when installing into the housing, press on the outer race only. Use correct drivers so force does not transmit through the rolling elements.
- Why: pressing through the rolling elements damages them; seating the race against its shoulder ensures positional accuracy and axial location.

9. Set preload/endplay and reassemble retaining hardware
- If the input bearing uses shims or an adjustable collar, set the shim thickness or preload to factory specification and verify with a dial indicator for axial endplay or torque for rotational preload. Install new snap rings/collars to the specified seating.
- Why: correct preload or endplay keeps the bearing in its designed operating condition. Too much preload causes heat and premature failure; too little causes bearing slop and noise.

10. Reassemble gearbox internals
- Reinstall gears, synchronizers, shafts, and selector mechanisms in original order; replace seals and gaskets. Torque all retaining nuts/bolts to factory spec.
- Why: correct reassembly ensures gear alignment and that no secondary cause (bad seal, worn synchro) is missed.

11. Test-run checks before vehicle installation
- Rotate input shaft and mainshaft by hand, check for binding, free rotation, and absence of play. Use dial indicator to re-check runout if necessary.
- Why: verifies assembly integrity and early detection of misassembly.

12. Refill gearbox with correct oil and reinstall
- Reinstall transmission into vehicle, reconnect linkages, driveshafts, and wiring. Refill with the specified transmission fluid.
- Why: correct lubrication and reconnection prevents future failure and restores normal operation.

13. Road test and final checks
- Listen for noise, verify smooth shifting and absence of play. Re-check fluid level after warm-up.
- Why: confirms repair success under load and normal operating temperature.

4) How the repair fixes the fault (concise)
- Replacing the input-shaft bearing removes the damaged rolling elements and worn races that allowed extra radial/axial play and runout. A new bearing restores the designed clearance/preload and concentricity of the input shaft relative to the gearbox case and countershaft. That eliminates the root causes of whining/vibration and incorrect gear mesh, stops metal-on-metal wear, restores smooth synchro engagement, reduces heat generation, and prevents premature failure of adjacent gears and synchronizers.

5) Prevent recurrence (brief)
- Replace seals and contaminated oil, ensure clutch alignment and pilot bearing condition, verify shaft straightness, and use correct torques and preload specs. Contamination, misalignment, or incorrect preload are the usual repeat causes.

Note: follow the R32 factory service manual for exact torque, shim, and preload specifications and for any model-specific removal order.
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