Valve clearances on the AC Light Six
The clearance figure for this engine is unusually well supported, which is a relief on a car this scarce. What matters almost as much is what the job tells you while you are doing it, because on a worn engine an accurate setting is not achievable at all.
Which engine this covers
This page covers AC's own Light Six, chassis prefix AE. If your car has the Bristol 100D2, prefix BE, see the Bristol valve clearances page instead. The chassis guide explains the prefixes in full.
How the valve gear actually works
A single overhead camshaft driven by a long chain, operating inclined valves. The camshaft does not act directly on buckets and shims as a modern overhead cam engine does. It acts on rockers, and the face of each rocker that touches the cam is a roller running on a pin. Everything is carried in the head: the valves, the rockers and the driven timing sprocket.
That construction decides the adjustment method, which is a screw and locknut on the rocker rather than a shim change. It also decides something more useful, covered below.
The camshaft runs in bearings in the head, and a camshaft location bearing holds the shaft in place axially while also providing the oil passage that feeds the camshaft and the rocker gear. Where those bearings are worn, the specialist machines both the head and the shaft and fits shell bearings.
The clearance
Twenty thousandths of an inch, hot, on both inlet and exhaust.
This is the best supported figure in everything the archive has gathered on this engine. It appears in the AC two litre saloon material as a tappet clearance of 0.020 inches when hot, with no distinction drawn between inlet and exhaust. It appears independently in period service data for the 1954 to 1955 Ace and Aceca, which lists inlet and exhaust separately and gives both as 0.020 inches hot. Two sources, two model families, the same number, and the sports source explicitly confirms that the two are equal.
Note the word hot, and treat it as load bearing. Twenty thousandths is a large clearance by modern standards, and set cold as though it were a cold figure it would be substantially wrong. No published cold equivalent was found, and the archive is not going to calculate one, because a conversion invented here would be exactly the kind of plausible number this site exists to avoid.
What the job tells you about the engine
This is the part worth knowing before you start. The specialist who rebuilds these engines reports rocker rollers and pins becoming very worn, which makes accurate valve adjustment difficult, and rocker shafts wearing their bores oval, which makes correct adjustment impossible.
So the adjustment is a diagnostic as much as a maintenance task. If the clearances come up cleanly and hold, the rollers, pins and shaft are sound. If you cannot get a consistent reading, or the setting will not stay put, the problem is not the adjustment and no amount of patience with a feeler gauge will fix it. That is a head off job and it is worth knowing early rather than late.
The camshaft drive
A long chain. Originally an inverted tooth chain, of the type usually called silent, later superseded by a Duplex chain. There is a chain tensioner with a pin, and an adjustable timing sprocket, the sprocket being the means of making fine adjustments to the timing.
On vintage engines the camshaft also drove the water pump. On post war engines the pump moved to a belt from the crankshaft pulley, which is the arrangement in an Ace or Aceca.
There is an oil jet feeding the timing chain, which is worth confirming is clear while the cam cover is off.
Timing marks are on the flywheel, and period data records the inlet valve opening at 12 and a half degrees before top dead centre. No other valve timing events were found.
Other things the head is known for
Cracked and corroded heads are common enough that the specialist pressure tests every one. Valve guides are supplied precision machined in cast iron, and hardened seat inserts are recommended at overhaul along with new valves, which is the relevant point for anyone running unleaded fuel.
Further down the engine, and worth knowing because it presents as a valve gear noise to the unwary, the crankshaft damper is known to disintegrate. The engine had a tendency to vibrate at certain speeds, which AC addressed with a split damper flywheel using rubber rings.
What the archive could not establish
Whether 0.020 inches hot is the factory figure or an accumulated owner convention. Both sources are secondary, and the authoritative answer is in the AC two litre instruction book or with the club. No cold setting equivalent, as above. No published step by step adjustment procedure: the archive can tell you the type of adjustment but has not seen a procedure specifying engine position for each cylinder, or a torque for the locknut, and will not invent one. The chain tensioner type and its adjustment procedure, how to check the chain for stretch, and any replacement interval. Valve timing events beyond the inlet opening figure. And camshaft bearing clearances and endfloat.
Related reading
- AC Light Six plugs & ignition14mm plugs, contact breaker gaps and a timing figure in dispute.
- AC Light Six oil & filterGrade, oil pressure, the bypass filter and the zinc question.
- Cooling the AC Light SixCoolant choice, the water pump fault and reading the gauge.
- Servicing hubWhere each task fits in a full service.
- The AC Light SixBackground to AC's own two litre six.
- Parts and specialistsWho supplies parts for this engine, AC6 among them.
This guide is not a substitute for the official AC Workshop Manual. If anything here is wrong or can be improved, please get in touch.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.