Camshafts have a reputation for being mysterious. They aren't — especially on a Jaguar straight-six or SOHC V12, where a little knowledge goes a very long way. Our thanks to fellow Jaguar hand Dick Maury, whose notes on cam identification and valve clearances are too good to keep to ourselves. Here's the short, friendly version: which cam you have, what gap to set — and the one sprocket that should go straight in the bin.
Know your cam (the six)
Series 1 E-Types — 3.8 or 4.2 — all wore the same camshafts: two bolts fixing the front of each cam to its chain drive, and oil holes drilled in the underside of the lobes. Clearances: .004 in inlet, .006 in exhaust. If you can't land exactly on the figure, err on the wide side. Up to .010 in suits serious competition use — on the road it mostly buys you noise.
In 1968 the lobe pattern changed — not for emissions, but to hush the tappets: the lobe now meets its follower gently instead of slapping it. You can spot these cams by the groove cut round the front flange. Gap them .012–.014 in, inlet and exhaust alike; the top of that range gives the smoothest idle without noticeable noise.
Then came the four-bolt design — groove gone, oil holes gone too (the tappets are splash-fed, so there's oil enough sitting in the top of the head) — and that layout ran to the last Series 3 XJ6 in 1987. The happy footnote: a later cam drops into an earlier engine for a quieter motor, with no noticeable loss of power or smoothness. Just set the clearance for the cam you actually have, not the year of the car.
Three Jaguar straight-six exhaust camshafts side by side — Series 1 two-bolt, 1968 intermediate with a groove round the front flange, and the later four-bolt Series 2 cam
The SOHC V12: much simpler
SOHC V12 cams are nearly all the same. The first 5,000 engines carry a different part number; after that, one cam rules. Clearances moved with the cars — .014–.016 in the E-Type, .012–.014 in the XJ12 and XJS, .010–.012 in the HE engines — and as with the sixes, the wide side of the range smooths the idle. You'd struggle to mix the cams up (they're different lengths), and here's the instant check: timing mark upright, front two lobes of the right-hand cam pointing skywards — that's your right-side cam. Dick's sweet spot across the SOHC V12 range: .013–.014 in. Quiet, and happy.
A pair of V12 camshafts — right and left as installed. Note the different lengths. Photo: Dick Maury
The sprocket to bin
Sprockets vary more than cams, and one of them is a proper trap. The early two-bolt sprockets — from XK120 days through Series 1 — are fine, their cross-drilled bolts secured with lockwire. But around the 1969 model year Jaguar briefly fitted a scalloped two-bolt sprocket with fewer teeth, spaced wider — less tooth-to-tooth contact just where you want most, and no number for it in the parts books. If you find one, throw it away and fit a round one. After that, life is easy: four-bolt sprockets run happily on two-bolt cams and vice versa — same size and thread, the four-bolt type locking with bend-up plates instead of lockwire. The SOHC V12 sprocket is the same part with a shorter stud, and it carried on into the XJ40 and X300.
Five sprockets: early two-bolt types at left, the scalloped two-bolt sprocket to avoid in the middle, four-bolt locking-plate types at right — the far-right one is the V12 sprocket, same part with a shorter stud. Photo: Dick Maury
The problem, up close: fewer teeth, spaced wider — less tooth-to-tooth contact on the scalloped two-bolt sprocket. Photo: Dick Maury
Hot cams — the honest truth
“Will hot cams wake my engine up?” Only if the engine can breathe. Cams earn their keep by holding the valves open longer or lifting them further — and that only pays if the ports and manifolds can feed them. The V12 is the classic case: cams, valves and manifolds are well matched as it leaves the factory, so it's hard to beat the stock cams on a V12 until a lot of other work has been done. Open the ports and manifolds first; then talk cams.
The six-cylinder is more forgiving: hotter cams can pay for little more than new carburettor needles — UO instead of the stock UM on the triple-carb cars — and those needles are often too rich at full load anyway. A portable mixture meter costs little now: measure before and after, and be prepared to re-profile the needles to suit your own rev and load ranges.
There are knock-on effects, too. Extra lift is the easy win; extra duration brings overlap, an unstable idle and a power band that climbs the rev range — costing off-the-line urge unless you add compression (and larger valves and ports). Higher lift means checking spring clearance — coil bind means machining the spring base, if the head allows it — and bigger lobes can mean bigger tappets, guide machining and oversize shims. There's no free lunch: match the changes to the goal.
The short version
Identify the cam. Gap it for the cam. Bin the scalloped sprocket. Check your mixture before you spend money.
Our thanks again to Dick Maury. If you'd rather a specialist set of hands did all of this, our Classic Workshop at Royal Leamington Spa, close to Coventry, looks after engines like these every week — and if you're building something new round a quad-cam V12, you'll find the tera V12 waiting