The timing chains: the Stag V8's most important service item

Why the chains matter so much

The Stag's 3.0-litre V8 has a single overhead camshaft in each cylinder head, and those camshafts are driven by chains. Over time the chains stretch and their tensioners and guides wear. A stretched chain throws the valve timing out, so the engine runs poorly, down on power and rough, and, at worst, a badly worn chain can jump a tooth or let go, with serious consequences inside the engine. This is the failure that gave the Stag much of its unreliable reputation, and it is almost entirely preventable.

A long chain that stretches

Rather than one camshaft belt, the Stag uses a jackshaft sitting in the vee of the engine, driven from the crankshaft; the two camshafts are then driven by single-row (single-roller) chains that are unusually long, and long single-row chains stretch. As the chain wears, the valve timing drifts and the chain runs slack. Left too long it can jump a tooth or break outright, and because the Stag V8 is an interference engine that means valves meeting pistons: bent valves, wrecked pistons, and a routine service item turned into a major rebuild. The jackshaft also drives the distributor and oil pump, so the whole timing assembly is central to the engine, and getting at it means removing the front of the engine (radiator out, timing cover off).

The oil-fed tensioner

The chain is kept in tension by a hydraulic tensioner running on engine oil pressure, working against a nylon-faced guide. That has an important consequence: healthy oil pressure, and a sensible idle speed, are part of what keeps the chain under control. An engine with tired oil pressure or a very low idle is not giving the tensioner what it needs, another reason to keep the lubrication system in good order and the oil and filter fresh.

Listen for the warning

The Stag usually warns you before it fails, and the sign is a rattle from the front of the engine. What matters is when it happens:

• A rattle on cold start-up that disappears after a few seconds, once oil pressure comes up, means the chains are getting tired, plan to replace them within the next few thousand miles.
• A rattle that does not go away once the engine is warm and idling is the serious one: stop driving the car, because a chain about to jump is about to cost you an engine.

The 25,000-mile reputation

The Stag earned a reputation for needing its timing chains replaced at around 25,000 miles, a short interval by any standard, and a big reason the car was seen as troublesome when new. Two things are worth knowing today. First, that interval reflects the original design and tensioners; a car maintained with good-quality modern chains, tensioners and guides, kept properly cooled and with fresh oil, will generally go longer. Second, because the job is involved, most owners treat it as planned preventive maintenance, renewing the chains, tensioners, guides and sprockets as a set on a sensible schedule, rather than waiting for the rattle.

Cooling and oil: the chains' allies

The chains don't wear in isolation. Overheating, the Stag's other great weakness, cooks everything at the front of the engine and does the chains no favours, and tired or dirty oil starves the tensioners. Keeping the cooling system in good order and the oil fresh is part of looking after the timing gear, not a separate concern.

Replace on schedule, not on failure

Renewing the chains is a significant job, not a roadside fix: the radiator comes out, the timing cover comes off, and the chains, tensioner, guides and usually the sprockets are replaced as a set. The final chain tension can only be set correctly with the heads in place, and the jackshaft and cam timing must be aligned precisely, get it wrong and the engine won't run right. Over-tensioning a new chain causes its own damage, so this is work for someone who knows the engine. It is often done alongside other front-of-engine work (water pump, seals, a cooling overhaul) to save doing the job twice. Given what failure costs on an interference engine, a fresh chain is cheap insurance, and keep the cooling system and the cylinder heads and head gaskets healthy too: the Stag's reputation was built on the two of these together.

The bottom line

If you own a Stag, know the history of its timing chains, when they were last done, and with what parts. If that history is unknown and the front of the engine rattles, budget for the job. Done properly and kept cool and well-oiled, the Stag V8's timing gear is reliable; it is neglect, not the design alone, that does the damage.