Drive Chain

An auxiliary engine chain can be as critical as the cam timing chain

A short chain may drive only the oil pump or balance module, but loss of that drive can remove lubrication or allow internal contact. Its loading follows pump torque, oil viscosity and crank-speed changes.

The service scope depends on how it shares sprockets and covers with the main timing system.

Drive-chain applications

Chain construction and pitch

Bush, roller, silent/inverted-tooth and other chain types use different engagement. Pitch, width, plate profile and link count must match sprockets. Surface hardening controls pin and plate wear.

Do not shorten, join or substitute an internal engine chain. Endless and riveted designs require their specified assembly process.

Kit-content checklist

Chain “stretch” is joint wear

The steel plates do not normally elongate like elastic. Wear at pins and bushes increases pitch across many links, making the chain longer and mismatching sprocket spacing. Tensioners take up some slack but cannot restore pitch.

Measure a specified number of links under the defined tension or compare tensioner extension using engine limits.

Sprockets reveal load history

Hooked, sharp or uneven teeth show chain/sprocket wear. Fretting at a key or shaft, bluing and displaced timing marks indicate further trouble. Inspect crank sprockets that drive multiple chains.

Replacing worn components as a matched set preserves contact and phase.

Chain-line alignment and thrust

Drive and driven sprockets must share the intended plane. Missing thrust washers, reversed sprockets, incorrect crank end float or a distorted pump housing shifts the chain sideways. Polished plate edges and one-sided guide wear are evidence of misalignment.

Measure offsets with the specified straightedge or datum and verify shaft end float. Do not add improvised washers to centre a chain; that can alter oil-pump engagement and fastener clamp.

Backlash and torque pulsation

Oil-pump torque changes with pressure and rotor position, while crankshaft speed fluctuates between firing events. The tensioner and guide layout keep the slack span controlled through those pulses. Excess clearance lets the chain strike the cover; over-tension overloads pump and shaft bearings.

Check guide position and tensioner extension at the stated engine position rather than pulling on the chain by hand and judging it against another engine.

Tensioner types

Fixed guides establish path; spring and hydraulic tensioners control slack on the non-driving span. A ratchet may prevent collapse when oil pressure falls. Check feed holes, check valves, plungers and locking pins.

Do not release a tensioner before the chain and guides are correctly placed. Forcing it back without the reset procedure can damage the ratchet.

Symptoms and diagnosis

Oil pressure is part of chain diagnosis

Use a mechanical pressure gauge at specified temperature and rpm when safe. A warning switch may only identify very low pressure. Slow pressure build can leave hydraulic tensioners collapsed.

Check oil viscosity/approval, filter non-return function, pickup seals, strainer, pump and relief valve. Do not blame a tensioner without finding why its supply is weak.

Oil-pump resistance and cold starts

Cold, excessively viscous oil raises pump torque and chain load. A blocked pickup causes cavitation; a jammed relief valve raises pressure. Debris can lock pump gears abruptly.

Use the specified oil and repair contamination or coolant/fuel dilution. Turning a suspect pump independently can reveal binding but must follow its inspection method.

Timing marks and coloured links

Coloured plates assist initial alignment to marked sprocket teeth. Because ratios and link counts interact, they may not line up again after one or many hand rotations. Recheck crank/cam/module timing at the defined datum, not colour alone.

Use factory marks and locking tools; do not create replacement paint marks on an unknown previous setup.

Removal preparation

Establish engine position

Set the nominated cylinder and crank position before releasing tension. Lock crank, cam or balance shafts as required. Support the engine when mounts or structural covers are removed.

Protect the lubrication system

Clean the exterior, plug galleries and stop gasket debris falling into the sump. Record guide, spacer and oil-nozzle positions. Do not rotate independent shafts if piston/valve or balance timing can be lost.

Pump, pickup and module inspection

Measure oil-pump end/side clearances, cover scoring and relief-valve movement. Inspect pickup tube cracks, O-rings and strainer blockage. Check driven shaft keys/splines and module bearings.

A new drive on a worn pump may fail from the same torque load or continue low pressure.

Assembly and fasteners

Install sprockets fully on clean keys/dowels, route chain across guides and set marks. Prime tensioner/pump as specified, then release the tensioner only when all fasteners are secure.

Use new torque-to-yield, prevailing-torque or staked fasteners where required. Correct thread lubricant and angle are essential for clamp load.

Cover sealant and sump cleanliness

Apply the stated bead width around oil holes and joints. Excess sealant squeezes inward, detaches and can block the pickup. Replace formed gaskets and crank seals using protected lips and correct depth.

Clean the sump of chain/guide debris; inspect the filter and cooler path before closing.

Hand rotation and first start

Turn the crank in its normal direction through the specified cycles. Feel for binding and verify timing at datum. Check guide seating and allowable slack; do not use the starter as a timing test.

Prime lubrication, start without revving and confirm pressure immediately. Stop for rattle, warning lamps, leak or abnormal vibration.

Safety and roadworthiness

A broken oil-pump or timing-related drive can seize the engine and cause sudden loss of power. Recover rather than driving with pressure warnings or severe chain noise.

The chain is internal and not directly inspected at MOT, but emissions, warning lamps, oil leaks and vehicle reliability can be affected.

Practical engine-drive-chain FAQs

Q: Is an oil-pump chain the same as the timing chain?
A: No, though it may share sprockets/covers and some kits combine related parts.

Q: What does chain stretch mean?
A: Wear at pins and bushes increases pitch across the chain; the plates have not simply stretched elastically.

Q: Can only the chain be replaced?
A: Inspect sprockets, guides, tensioner, pump and fasteners; matched worn parts often need renewal.

Q: Why does it rattle only at start?
A: Tensioner leak-down or delayed oil pressure is possible; check oil/filter/pickup as well as chain wear.

Q: Do coloured links realign after one turn?
A: Usually not. Use fixed timing datums after hand rotation.

Q: Can the engine run with an oil-pressure warning?
A: Switch off immediately; loss of oil-pump drive can damage the engine within moments.

Q: Can thicker oil quiet the chain?
A: Use only the specified viscosity/approval; masking noise can increase cold pump and chain load.

Q: Should a hydraulic tensioner be compressed in a vice?
A: Use its controlled reset and locking procedure; forcing can damage check/ratchet parts.

Q: Why inspect the sump?
A: Broken guide material and metal can block the pickup or recirculate into bearings.

Q: Must the oil pump be primed?
A: Follow engine procedure; dry start can delay pressure and damage a new drive.

Q: Are chain fasteners reusable?
A: Many are one-use, staked or torque-angle parts; follow service data.

Q: Can excess sealant cause failure?
A: Yes. Squeezed-out sealant can block oil pickup or galleries.

Q: What proves successful repair?
A: Correct timing, free hand rotation, clean oil system, prompt pressure and quiet leak-free operation.