This guide captures ‘bygone’ advice on on how to adjust valve clearances on a overhead valve engine Classic Car. It is part of a series I am building that describes what to do on a Classic Car Basic Service. I will be covering overhead cam engines in a separate post.
- How often should you check Valve Clearances
- How do push rod OHV Engines work
- Why are Valve Clearances important
- Preparing to adjust Valve Clearances
- Remove the Rocker Box cover
- What sequence to adjust Valve Clearances
- Adjusting Valve Clearances ‘on the rock’
- Adjusting Valve Clearances by TDC
- When valves are side by side – Cross Flow Engines
- Valve clearance adjustment on V Engines and Flat Engines
- Adjusting Valve Clearance
- Checking for wear on Valve Rockers
- How to use Valve Adjustment Tools
- Replace the Rocker Box cover
- About this site
Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.
Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.
Adjusting the valve clearances of your car is not as complicated or as difficult as it may seem. The job requires only a few tools, some common sense and maybe up to an hour of your time.
Correct valve clearances are essential to keep your engine in a good state of tune. The only way of telling if the gaps are set correctly is to go through the procedure of checking them.
How often should you check Valve Clearances
The recommended interval for this is usually between 10,000 and 15,000 km (6,000 and 10,000 miles). If not, make it part of your 10,000 km service.
The vast majority of classic cars use push-rod operated ohv (overhead valve) engines.
The procedure for checking the valve clearances on an ohv engine is quite straightforward, and varies only in detail between one car and another.
The procedure for checking valve clearances on an ohc ( overhead camshaft) engine varies widely from car to car, and I will cover this in separate post.
How do push rod OHV Engines work
In all four-stroke engines the camshaft is driven at half the speed of the crankshaft. It has on it a number of pearshaped cams which indirectly open the valves in the cylinder head.
Each cylinder normally has two such valves, one to permit the entry of the fuel/air mixture, the other to permit the escape of exhaust gases.
If combustion occurs too late, the piston has already begun its journey towards the bottom of the cylinder. The energy in the fuel is not properly used, fuel is wasted and overheating occurs. The engine sounds flat and lacks power.
An ohv engine has small cylindrical blocks known as cam followers ( sometimes referred to as Tappets) located on the cams. Bearing on top of the cam followers are the push-rods.
As the cam on the camshaft reaches its peak, it pushes the cam follower upwards and the cam follower moves the push-rod in the same direction.
The top of the push-rod then forces one end of the rocker arm upwards. The beam action of the rocker on its shaft means that the far end of the rocker arm then presses downwards on top of the valve stem, opening the valve inside the combustion chamber in the cylinder head.
The cam is shaped in such a way that it holds the valve open for a pre-determined period. Then as the peak of the cam passes, the spring on the valve forces the valve shut. The rising valve stem then reverses the direction of the rocker, the push-rod and the cam follower.
Because of the extreme temperatures at which a valve operates while the engine is running, a pre-set clearance between the top of the valve stem and the rocker is essential so that the metal can expand.
Although the clearance is measured in only hundredths of a millimetre or thousandths of an inch, without this gap the valve will expand to such an extent that it cannot close properly.
Why are Valve Clearances important
It is important to make sure that the gap is correct. If the clearance is too large, the valve gear will be unduly noisy and excessive wear will occur. The engine will also develop less power because the valve will open later and close earlier than it should.
On the other hand a valve clearance that is too small is even more serious. In this case the valve will not be able to close fully and eventually both the valve and the valve seat will burn out. Soon the perimeter of the valve will become so severely damaged that it will fail to make anything like a reasonable seal in the cylinder head and serious damage to the cylinder head and perhaps the block will result.
Not all rockers are like those in the pictures that follow yet they are the most common. An alternative type is where there is no rocker-shaft as such, but where the rockers take the form of boat-like steel pressings with a central nut holding the assembly in place. The British 3-litre Ford V-six engine for example uses this type of rocker assembly.
Preparing to adjust Valve Clearances
The job of adjusting valve clearances involves making a tiny adjustment to the position of the rocker arm so that the gap between it and the valve stem is correct.
This clearance can be measured only when the valve is fully closed. But because you cannot see when this happens you need some other method (and there are several) of telling when a particular valve is closed.
The commonest method is to use the rule of nine or rule of thirteen (covered in next section below). This is a means of telling which valve is fully closed by seeing which is fully open, for example if No 8 is open then No 1 is closed.
But before you start work you need to know which basic method is recommended by the manufacturer of your car. Looking ion your workshop manual may answer the question however below I have captured ‘OnThe Road’ overview of popular cars up to 1977.
You also need to know the correct clearances for both the inlet valves and exhaust valves, sometimes they differ, for your car. This information is usually given in a car’s handbook, however skimpy.
Finally you need to know whether the valve clearances of your car should be checked with the engine hot or cold.
A ‘cold’ engine means that the engine has been standing idle, ie not been run, for a set period-preferably all night. Checking the clearances on a ‘hot’ engine requires a warming-up run of eight km (five miles).
On some cars one set of valves are checked on a hot engine and the other when the engine is cold. Alternatively some cars can have their valve clearances checked with the engine either hot or cold and two sets of figures are given in the handbook.
As an added complication some manufacturers (Vauxhall are one example) recommend that the valve clearances of several of their models should be checked while the engine is ticking over. But unless the manufacturer of your car specifies otherwise always check valve clearances with the engine stationary.
The tools you are likely to need will include either a ring spanner or a screwdriver to remove the rocker box, a set of feeler gauges, and another ring spanner and screwdriver to adjust the valve clearance. You will also need a spare rocker box gasket.
Remove the Rocker Box cover
To adjust the valve clearances you may have to remove the air-cleaner unit before you can undo the rocker box retaining screws or studs.
Next a careful check around the rocker box is essential to spot any pipes, wires or other connections bolted to it. With these off and the retainers free, the rocker box can be lifted off fairly easily.
With the rocker box removed, the valve springs and the rocker arms will be exposed. Unless the valve clearance is the same for both inlet and exhaust valves, you will need to establish which are the inlet valves and which are the exhaust valves.
The simplest way of doing this is to look at the two manifolds. The inlet valves will line up with the inlet manifold while the exhaust valves will line up with the exhaust manifold.
To adjust the valve clearances, you must be able to turn the engine over by hand.
What sequence to adjust Valve Clearances
On a four-cylinder in-line engine whose valves are Siamesed (arranged exhaust, inlet, inlet, exhaust, and so on) the most straightforward method of valve clearance adjustment is to use the rule of nine.
The Rule of Nine
To do this you number each of the eight valves starting from either end of the engine. Then turn the engine over until, say, No 1 valve is fully open (when the rocker arm has pushed the valve down as far as it will go).
Next subtract 1 from 9, which leaves 8. This means that No 8 valve is fully closed, and that is the one you adjust.
The Rule of Thirteen
The rule of thirteen is identical in principle to the rule of nine.
It applies only to in-line six-cylinder engines whose valves are arranged in the same way as that of the engine above (inlet, exhaust, exhaust, inlet and so on).
As an example, if No 4 valve is fully open, subtract 4 from 13, which means you check valve No 9.
You will notice that more than one valve will be open at one time so it makes sense to check two valves ( one inlet, one exhaust) together to save turning the engine over unnecessarily.
When using this system, make a mark with a piece of chalk or crayon on the rocker arms because it is easy to forget which clearances have been checked.
Note that the rule of nine and rule of thirteen work only on engines with Siamesed ports, and do not work on engines whose valves alternate (inlet, exhaust, inlet, exhaust, etc) Neither do they work on V or flat engines.
Adjusting Valve Clearances ‘on the rock’
Engines whose valve ports are not Siamesed are often adjusted with the valves rocking. This means you turn the engine until the valves of one cylinder begin to rock (when one of the rocker arms is just beginning to move while the other arm has just closed).
With the engine in this position, you check both the inlet and the exhaust valves of the ‘opposite’ cylinder in the firing order. On this chart below (for a Fiat 127) the numbers refer to the cylinders not to the valves. This being an extract from the Basic Engine Data guides shown earlier in this post.
Adjusting Valve Clearances by TDC
Yet another method, favoured by many, is to adjust the valve clearances by bringing each piston in turn to TDC ( what is TDC ? is cover in the post below).
First you bring No 1 to TDC on the compression stroke and adjust both its inlet and its exhaust valves. Then you turn the engine over by 180°.
This brings to TDC the next cylinder in the firing order, so now you adjust both its valves. This procedure is continued until all the valves have been adjusted.
As an example the Chrysler Avenger fires 1, 3, 4, 2, and that is the order in which you adjust the valve clearances.
Note however that in this case too, you are adjusting by the numbers of the cylinders not by the numbers of the valves.
When valves are side by side – Cross Flow Engines
On engines where the inlet manifold is located on one side of the cylinder head and the exhaust manifold on the other side (cross-flow engines) the valves are sometimes arranged side by side. The best way to adjust the valve clearances is with the aid of a chart, such as this one for the Peugeot 504
Should your engine be of the cross-flow type with the valves arranged in pairs then it is possible that you will find a chart such as this in your handbook.
If this is not the case, then you can use the rule of five (which is very similar in operation to the rule of nine or rule of thirteen). This will work for any four cylinder in-line cross-flow engine whose valves are arranged in this way.
Using this rule, you can adjust only one set of valves at a time.
Start with the inlet valves. Number each inlet valve only, starting from either end of the engine. Then turn the engine until No 1 valve is fully open. Subtract 1 from 5- the result means you check inlet valve No 4. When that particular valve clearance is set, carry on and check the rest of the inlet valves in exactly the same way. Then repeat the procedure for the exhaust valves.
Valve clearance adjustment on V Engines and Flat Engines
With V engines and flat engines there is no rule for working out the order to adjust the valve clearances, you need a chart showing the correct order.
For example this is the chart for the Ford V-four engine:
For the valve clearance order of a V6 or V8 engine you need a table, as for a V4. Here is a chart to cover the British Ford 3-litre V6:
Note the way the valves are numbered in this case:
Not all V engines number in this way. But since the valve numbering always follows the same order as the cylinder numbering (and there are two valves per cylinder) you can easily calculate one from the other. No 1 cylinder has valves 1 and 2, No 2 has valves 3 and 4, and so on.
Adjusting Valve Clearance
To check the clearance of a particular valve, insert a feeler gauge of the correct thickness between the rocker arm and the valve stem.
If the blade of the feeler gauge will not enter the gap, or if it is a sloppy fit, then the valve clearance will have to be altered.
With the screw and lock-nut type of fitting, loosen the nut and turn the adjustment screw until the feeler gauge is a reasonably tight fit between the rocker arm and the valve stem.
Then hold the adjustment screw firmly in place while you tighten the lock-nut. Finally re-check the gap.
On engines whose rockers have a self-locking nut fitting, the procedure for checking the valve clearances is easier. Simply turn the lock-nut with a good quality ring or socket spanner until the clearance is correct.
Then remove the spanner and re-check the clearance.
Once this is done forget about the valve concerned. If you re-check later after doing some other valve you may find that the previously adjusted is now slightly out. There is nothing you can do about this.
Adjusting the valve clearances with the engine running is slightly more difficult. Long strips of metal of the correct thickness, obtainable from a specialists supplier are needed because the-hammering of the rocker against the valve stem tends to flatten feeler gauge blades.
Set the engine at its lowest possible idling speed by adjusting the idling screw on the carburettor. Then slide the gauge-strip in and out of the gap. After a short while you can see when the gap is momentarily at its widest and adjustment can then be made accordingly.
Checking for wear on Valve Rockers
One possible wear point with push-rod-operated ohv valve gear is at the valve stem end of the rocker.
The end of the rocker is case-hardened but after a considerable mileage this hardened part of the rocker may become pierced.
This means a small crater will be set up in the rocker arm and inevitably the valve stem will reach inside to the bottom of this crater. A feeler gauge slid between the two faces will tend to rest on the lip of the crater and give a false reading.
When an engine persistently rattles on one cylinder, always check for this fault. A replacement rocker is the only answer.
How to use Valve Adjustment Tools
Instead of using a screwdriver and a spanner to adjust the valve clearances you could use a special all-in-one valve adjustment tool. A popular at the time tool is the SPQR, which is available from most branches of Halfords and other motor accessory stores. This tool is a combined wrench and screwdriver and works by measuring the travel of the valve-adjustment screw.
Here are a few adjusters and feeler gauges currently available on eBay
If you decide to buy one, use the tool to undo the valve lock-nut, then keep turning the adjustment screw until all the clearance is taken up and the rocker arm rests on top of the valve stem. (A ratchet arrangement inside the tool will prevent you from over-tightening the screw.)
Next turn the thimble on the tool back the required number of ‘clicks’-each click representing so many hundredths of a millimetre or thousandths of an inch.
Then finally, with the tool in position, hold the thimble and tighten the lock-nut with the wrench (see below).
Apart from accurately setting up the valve clearances, manufacturers often their tool will take into account any wear of the valve gear. But as a final check it is a good idea to check that the clearance is correct by briefly running the correct feeler gauge between the rocker arm and the valve stem.
Replace the Rocker Box cover
Once all the necessary valve clearance adjustments have been made, all that remains is to make sure that the rocker box cover or covers are correctly re-fitted.
Do not forget also to ensure that all the appropriate cables, wires or fittings return to their proper place.
If you have fitted a new rocker cover gasket, it is a good idea to make one further check of the retaining screws or bolts a week or so later. This will allow you to compensate for any bedding-down of the new gasket.
About this site
Visit SCOTTYS Supplier Library for Classic Car Parts Suppliers.
Visit SCOTTYS Technical Library for Guides and Parts Manuals.
Visit SCOTTYS Artisan Library to find a specialist company.