Fix those noisy tappets!
Excessive tappet noise is an indication that something needs attending to. Even the B series engine, renowned for tappet noise, is quite quiet in the valve gear with newly reground cam and followers.
Initially you should check the tappet adjustment (you probably already have). If you have a B series engine, see below regarding the clearance to use. See also the page on valve clearances.
If that doesn’t reduce the noise, you need to check for worn valve gear. This will have been caused by previous owners not keeping the tappet adjustment correct – it’s an infrequently needed job, but in addition to the specified intervals it should be attended to immediately if tappets get noisy or the valve gear will hammer away to serious wear.
Places where wear occurs initially are in the rocker shaft and bushes, cam followers. If worn & pitted followers are not replaced soon enough, they grind away at the cam lobe. This can happen to such an extent that the valve effectively stops opening!
If you have access to a dial gauge, the simplest way to check for cam lobe wear is to measure the lift at each pushrod (having temporarily removed tappet clearance) and compare it with the specified lift for the cam profile. For the standard 1275 exhaust lobes and the standard MGA & B camshaft this should be 0.268″ (being the listed cam lift plus the checking clearance of .016″)
I have preferred to run the B series tappet clearance at 0.013″ (cold), as the 0.015″ (cold) specified in the manual results in the clearance being taken up right at the end of the cam lobe’s opening ramp. At 15 thou, any wear then results in the clearance being taken up at higher velocity (with attendant higher wear rates) whereas at 13 thou the clearance can increase to 15 thou and still be on the ramp. This is about how much it will wear if the tappet clearance is checked at the intervals specified in the manual. In my experience, the reduced clearance has made negligible difference to the engine’s performance and idle, and even in cars driven hard it has not had any effect on valve life.
Interestingly, further evidence that .013” is a better clearance to use in the B series is provided if we look at the A series. The latter tappet clearance is specified at .012”. The BMC cams all used the same opening ramp detail so a direct comparison with the B series is relevant if the rocker ratio is allowed for. Doing this, we find that 12 thou on the A series rocker ratio would be the same point on the cam as 13.4 thou on the B series rocker ratio.