Setting the Valve Clearances on the Bristol Cross-Pushrod Head

The Bristol six's valve gear is one of the most distinctive things about the engine under an Aceca's bonnet — and one of the least understood. Setting the valve clearances properly means understanding why the mechanism is more involved than a normal pushrod engine, not just knowing where to put a feeler gauge.

Specs — confirm against your car

Clearance figures: not given here. Get the exact inlet and exhaust clearances from your Bristol/AC Workshop Manual, or from the AC Owners Club or Bristol Owners Club.
Setting order: this multi-link system has its own specified sequence — confirm it, and the firing order, against your handbook before you start.
Condition: clearances are set cold.

Why the valve gear is unusual

The Bristol engine's cylinder head uses a hemispherical combustion chamber — inherited, via the BMW 328, from a design that puts the inlet and exhaust valves at an angle to each other rather than in a single row. A hemi head gives good breathing, but it creates a problem: a single, low-mounted camshaft cannot drive angled valves on both sides of the chamber directly with simple pushrods, the way a conventional OHV engine can.

Bristol's solution was a system of rods, followers and bell-cranks that carries drive from the camshaft, across the engine, to operate the valves on the far side of the head — most notably an arrangement of two separate pushrods in series to work each exhaust valve. It's an elegant answer to a real engineering problem, and it is a documented part of why these engines have such a good reputation for breathing and for revving cleanly.

Why that matters for you with a feeler gauge in hand

A conventional OHV engine has one clearance to set per valve: cam follower to valve stem, adjusted at one rocker. The Bristol's cross-drive arrangement has more than one linkage in series for the valves it operates indirectly — which means more than one place for clearance to exist, and more than one adjustment to get right, before that valve is correctly set. This isn't a design flaw; it's the direct, unavoidable consequence of driving angled valves from a single low camshaft. But it is the reason Bristol owners have always found this job more involved than "the average pushrod engine," and why it rewards care and a methodical approach rather than being rushed.

No clearance figure is given here

This guide explains the mechanism and the general method. It does NOT give a specific clearance figure, in thousandths of an inch or any other unit, for inlet or exhaust. I was not able to verify a reliable clearance specification for this engine, and rather than guess, I am telling you plainly: get the exact figures — inlet and exhaust, and the correct setting order for this multi-link system — from your Bristol/AC Workshop Manual, or from the AC Owners Club or Bristol Owners Club, before you touch a feeler gauge.

General method

The overall approach is the same discipline as any pushrod engine, adapted for the extra linkage:

Work on a cold engine — clearances are specified cold, and setting them warm will give the wrong number.
Rotate the engine by hand (never the starter) to bring each cylinder to the correct point in its cycle before checking that cylinder's valves — normally done in firing-order sequence so you're not chasing the engine around twice. Confirm the firing order and rotation method for this engine in your manual before starting.
At each rocker or bell-crank adjustment point, slide the correct feeler gauge between the parts you've confirmed clearance is measured across, and adjust the screw and locknut until the gauge is a light sliding fit — neither slack nor pinched.
Because there is more than one linkage point for the valves driven indirectly, work through the manual's specified sequence for this engine rather than assuming every valve is adjusted at a single point the way it would be on a simpler engine.
Recheck each clearance after tightening the locknut — the act of tightening can shift the setting slightly, and it's a cheap check against a wasted trip round the engine.

Owner's note. Many Aceca owners, including experienced home mechanics, choose to have this specific job done by — or at least first shown by — a Bristol specialist, precisely because the extra linkage gives more ways to get it subtly wrong than a conventional engine. There's no shame in that choice on this engine; it's a reasonable response to a genuinely unusual mechanism, not an admission of defeat.

About the author

An independent classic-car enthusiast in North Yorkshire with decades of hands-on experience on British classics — including the AC Aceca — writing the single-model reference he always wished existed. Read the full bio.

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