While most of us would love to own a twin cam-powered classic, we can’t all afford to buy, run and maintain cars with this type of high-tech engine. Instead, a lot of our readers are probably more than happy with a less highly specified pushrod motor. And there is, of course, an upside — pushrod-equipped engines are simpler and easier to maintain and tune than their twin-cam (or quad-cam) relatives. Indeed, they are perfect for those who love to tinker with their cars.
While pushrods form a very simple part of a car’s valve-train, correct adjustment is critical. The pushrod itself is a tube with a fixed cup at its top end. This fits onto one tip of the rocker arm while an adjustable cup on the bottom end fits onto the tappet. Adjustment, achieved through screwing the lower cup in or out, simply increases or decreases the useable length of the pushrod.
Of course, when we say that we are adjusting the tappets, this is not actually true — we are actually adjusting the pushrod length or, indeed, adjusting valve clearances.
It might sound complicated, but the actual method of adjusting valve clearances is relatively straightforward, and only requires a minimum of tools — a screwdriver, appropriately sized spanners and a feeler gauge.
Making a Start
The first job is to pull out your owner’s handbook (or workshop manual) and discover the correct valve clearances. Most manufacturers will list both hot and cold valve clearances, but in practice, the hot settings are the best ones to use — which means, of course, that the valves will need adjusting on a hot engine. So, be careful if you don’t want to burn your fingers, and make the adjustments quickly before the engine cools down. You can always practice the technique on a cold engine beforehand.
On some makes, inlet and exhaust valves may require different clearances — check with your handbook. And if you can’t work out which valve is which, simply use the inlet and exhaust manifolds as a guide — each port should line up with its valve.
In order to check valve clearance, you’ll need to turn the engine over (if this proves difficult, remove the spark plugs) until the valve you’re checking is fully closed. A useful tip here is to remember the rule of nine, which is applicable to four-cylinder engines. As an example, if you turn the engine until valve two is fully open and as far down as it will go, minus two from nine and valve seven will be fully closed. For a six-cylinder engine, the rule is 13 — in the above example you would then subtract two from 13 and valve 11 would be fully closed. (This, by the way, won’t work for V-engines — check your handbook for the maker’s recommended order.)
Once you have ensured that the valve is fully closed, check the existing gap with a feeler gauge.
To make the adjustment, loosen off the lock-nut — do not unscrew it completely. You can then adjust the screw until the correct gap is obtained. Check again with the feeler gauge and when satisfied that everything is correct, hold the screw with a screwdriver and tighten up the lock-nut with a spanner. Do not over-tighten. Once set, it pays to recheck the clearances one more time just to ensure that nothing moved during the final tightening of the lock-nut.
If the engine has been allowed to cool appreciably during the adjustment procedure, the setting will not be satisfactory — so make sure you’ve practised enough times to handle the adjustments quickly and efficiently.
Once finished, you can check your adjustments by turning the pushrods one full rotation — checking to see if the resistance alters at any place during the rotation. If you do encounter resistance, this could mean incorrect adjustment or either a pushrod or adjuster could be out of true — the latter will, ideally, mean replacement of the affected parts.
Do’s and Don’ts
A few final tips — when adjusting valve clearances it is important to get them correct. If the pushrod is adjusted too tightly it will hold the valve away from the valve-seat. This will be felt as either very low or zero compression in the engine. That will make it very hard to start the engine, and the valves will become vulnerable to the hot gases exiting around them — which, in turn, will lead to burned valves.
Go the opposite direction with an adjustment that is too loose, and valve lift will be reduced. Probably not a real problem — providing you don’t use your classic for racing — but you’ll soon get fed up of listening to the tappet banging the pushrod into the rocker. As well, over a period of time loose adjustment will inevitably lead to distortion and run-out of the pushrod — and that will make future adjustments difficult.
Words: James Black
This article is from Classic Car issue 230. Click here to check it out.