Check and set the valve clearances (Bristol engine)
The Bristol six sets its valve clearances through an unusual cross-pushrod head: one camshaft in the block works the inlet valves directly and the exhaust valves through transverse pushrods that cross the top of the head. There is more linkage to understand and to wear than on a normal engine, so work methodically to the manual's figures — and if in any doubt, use a Bristol specialist.
Tools
- Feeler gauges
- Ring and open-ended spanners
- Screwdriver set
- A means of turning the engine over by hand
- Torch and inspection mirror
Parts / supplies
- New rocker-cover gasket(s)
- Correct-grade oil to top up any loss
Procedure
Understand the arrangement before you start. The single camshaft in the block operates the inlet valves through pushrods and rockers in the usual way, but each exhaust valve is worked through a chain of rocker, a transverse pushrod that crosses the top of the head in its cast tube, and a second rocker. That extra linkage is why the clearances must be checked carefully and set exactly to the manual.
Work at the temperature the workshop manual specifies — some figures are set cold, some hot. Do not mix the two.
Remove the rocker cover(s) to expose the rockers, keeping everything scrupulously clean; the cast-in oilways and tracts do not want debris.
Adjust each valve only when its cam follower is on the base circle — that valve fully closed and its rocker slack. Turn the engine over by hand and follow the sequence in the workshop manual, bringing each valve to the fully-closed position in turn; never set a valve that is partway open.
Set the inlet clearance at the rocker-to-valve gap to the manual's inlet figure — the feeler blade should pass with a light drag.
Set the exhaust clearance at the point the manual specifies for the cross-pushrod exhaust rocker. Because the exhaust valve sits at the far end of the rocker / transverse-pushrod / rocker chain, take extra care that you are reading the true gap and that nothing in that linkage is worn or loose.
Recheck every clearance after tightening the locknut — the gap often changes as the nut is nipped up.
Refit the rocker cover(s) with fresh gaskets, run the engine and listen. Correctly set, the valve gear gives an even light tapping — not a heavy clatter, and not dead silence.
Warnings
- Do not guess the clearance figures. Inlet and exhaust gaps, and whether they are set hot or cold, vary by engine series — take them only from the correct Bristol workshop manual for your engine.
- The cross-pushrod exhaust arrangement has more wear points than a conventional head. If you find worn rockers, slack transverse pushrods, or clearances that will not hold, stop and consult a Bristol specialist rather than winding on more adjustment.
- This is a valuable and uncommon engine. If you are not confident identifying the base circle for each valve, have the job done by a specialist — a mis-set clearance can burn a valve or lose performance.
Related faults
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.