Valve clearance: why it drifts, and what it sounds like when it has

The gap between a valve and whatever pushes it open is not slop. It is a deliberate allowance for metal that has not got hot yet, and the engine only works correctly once thermal expansion has used most of it up.

The original explains that part well. What it never says is the thing that decides everything else:

Too loose is a noise. Too tight destroys a valve — and makes no noise at all while it does it.

That asymmetry is the whole subject. It tells you which way to err, why the check exists as a service item, and why the most dangerous engine is the quiet one.

What each direction actually does

Too loose. The cam or rocker arrives at the valve with a running start and hits it. You get the classic light metallic tick from the top of the engine, fastest at idle when you can hear it, rising in frequency with engine speed and loudest on a cold start before expansion has closed the gap. The valve also opens slightly late, closes slightly early and does not lift as far, so the engine breathes a little worse — measurably, not dramatically. Left alone it accelerates wear on the cam and the valve tip.

Too tight. Nothing. No noise, no warning light, nothing to hear on a cold start. As the engine reaches temperature the parts expand into a gap that is no longer there, and the valve is held very slightly off its seat.

That is the failure, and the mechanism is worth understanding because it is not intuitive. An exhaust valve has no coolant near it and it sits in the path of combustion gas on its way out. The only way it sheds heat is by pressing against its seat, which is in the cylinder head, which is water-cooled. A valve held open by two hundredths of a millimetre has lost that contact for part of every cycle, so it gets hotter, and hot metal expands, which holds it further off the seat. It runs away from there. The valve edge burns, the seal goes, and now you have a cylinder with no compression, a misfire — P0301 and its neighbours — and a cylinder head off the engine.

So the reader's takeaway is a rule of thumb the original does not offer: if you must be out, be out on the loose side. A tick is a nuisance. A tight exhaust valve is a repair bill.

Which way it drifts, and why that is the wrong way

Here is the part that makes the check non-optional, and the original omits it entirely.

Two kinds of wear act on the clearance and they act in opposite directions. Wear on the cam lobe, the rocker face or the valve tip opens the gap up. But wear on the valve face and its seat closes it, because the valve sinks a little deeper into the head each time, which pushes its stem up toward whatever is meant to be clear of it.

Seat recession usually wins. Which means that on most engines the clearance does not stay put and does not drift randomly — it tightens, steadily, toward the direction that burns valves. An engine that was quiet, got a bit ticky, and then went quiet again has not healed.

And on gas the effect is much faster. Petrol wets the valve and seat and carries some heat away as it evaporates; CNG and LPG are dry and burn hotter, so the exhaust valve and its seat wear against each other with nothing between them. Fleets running on gas commonly halve the inspection interval for exactly this reason. Given how much of the fleet this archive was written for runs on CNG, that is the single most useful sentence in this article: if your car is on gas, check the clearances at half the interval in the book, and expect them to have closed rather than opened.

Hot or cold is not a detail

The original gives four guides copied from four sources. Three of them specify clearances and never mention engine temperature at all. The fourth says the Pride must be done at operating temperature, per the manufacturer, and describes waiting for the fan to cut in.

A clearance figure without a temperature is not a specification. The difference between hot and cold on the same engine is commonly several hundredths of a millimetre — a large fraction of the clearance itself. Set a hot figure on a cold engine and you have deliberately built in the tight condition described above.

So: find the figure and the condition for your engine, and reproduce the condition. If the book says cold, cold means genuinely cold — overnight, not twenty minutes after switching off. If it says hot, run it to temperature and work quickly, which is unpleasant and is why the book says so only when it matters.

Finding which cylinder to measure

The method the original describes is sound and is worth stating cleanly, because it is stated three times in three different ways.

Turn the engine over by hand until one cylinder is at valve overlap — its exhaust valve has just closed and its intake is just starting to open. This is the position the original calls scissors, and it is the point at which that cylinder is finishing its exhaust stroke. Its partner cylinder is therefore at the top of its compression stroke with both valves fully shut and both clearances at their maximum, which is where they can be measured.

The pairing follows one rule: the two cylinder numbers add up to the number of cylinders plus one.

So four positions of the crank cover a four-cylinder, or two if you check both valves of one cylinder and one of each of two others as the original's second guide describes.

Inline engines only. On a V engine the banks are offset and this pairing does not hold — the original says so, and it is right to.

Turn the engine with a socket on the crankshaft pulley bolt, in the normal direction of rotation. The original's alternative — jack a driven wheel, engage a high gear and turn the tyre — works and is genuinely convenient on a car whose crank pulley is hard to reach. Use an axle stand, not the jack.

The claim that the method fails on an injected engine

The fourth guide asserts that the overlap method "is wrong on injected Prides, and the reason is injector timing", and offers a different sequence in its place.

Injection timing has nothing to do with it. Valve clearance is a purely mechanical relationship between a camshaft lobe and a valve stem. Which valves are shut at a given crank position is decided by the camshaft and the firing order, and by nothing else. A carburettor and an injector deliver fuel to the same engine with the same cam. The sequence that guide gives is simply a different valid ordering of the same positions — which is why it also works, and why its author concluded the first method must have been wrong.

The part of that guide worth keeping is the temperature instruction, which is the one thing the other three lack.

Three valvetrains, and only two of them adjust like this

The original says that engines without rocker arms "also have an adjusting screw anyway". They do not, and a reader will spend an hour looking for one.

Rocker or finger followers with a screw and locknut. Slacken the nut, set the blade drag with the screw, hold the screw and tighten the nut, then re-check — tightening the locknut moves the screw, and re-checking after it is what separates a job done from a job attempted.

Direct-acting bucket with shims. The cam presses on a bucket sitting over the valve, and the clearance is set by the thickness of a hardened shim. There is nothing to turn. You measure every clearance, write them all down, then work out for each one: new shim = old shim + measured clearance − specified clearance, and buy that size. This is why the job on a shim engine is measure-once, order parts, come back — and why the original's own video list includes shimming while its text does not.

Hydraulic lifters. No adjustment exists and none is wanted; the lifter takes up the clearance itself using oil pressure. A ticking hydraulic lifter is an oil problem, not a clearance problem — wrong grade, low level, overdue change, a collapsed lifter or a blocked feed. Nobody should be reaching for feeler gauges on these, and the tick that lasts a few seconds on a cold start and then vanishes is usually a lifter bleeding down overnight rather than anything wrong at all.

Using the gauge

A feeler set is a stack of ground steel blades, typically from 0.05 mm upward. Slide the specified blade into the gap and judge the drag — the original's phrase for it is knife through cheese, which is a better description than most workshop manuals manage.

• If the blade will not enter, the clearance is too tight.
• If it slides in with no resistance at all, too loose — confirm by trying the next size up, which should not go in.
• The blade should come out clean. If it comes out scored, it was too tight and you have been measuring the marks.

Use one blade rather than stacking two, keep them clean and lightly oiled, and check each clearance again after you have locked it.

The numbers, and why the ones here are not usable

The original quotes specifications freely, and it disagrees with itself. For the Peykan and Peugeot RD it gives 0.25 intake / 0.30 exhaust in one place and 0.20 / 0.40 in another. For the Pride it offers 0.25 / 0.35, then 0.25 / 0.25, then 0.25 / 0.30 — three answers in two sentences.

That is what four stitched sources look like, and it is why none of the figures in this article are reproduced here as advice. Get the clearance from the manual for your engine, with its temperature condition, and treat any remembered number as a prompt to go and look it up.

One rule of thumb from the original does need qualifying rather than repeating. It says "the intake clearance is always less than the exhaust". Usually, yes — the exhaust valve runs far hotter and expands more, so it is given more room. But not always: plenty of engines specify the same figure for both, and the original itself says so a few paragraphs later. Treat it as a sanity check on a figure you have looked up, not as a substitute for looking it up.

What to listen for

• A light, fast tick from the top of the engine, present at idle, keeping time with engine speed at half crankshaft frequency, loudest cold and quieter once warm. That is clearance, and it is the benign direction.
• A tick that appears only when cold and disappears within seconds on an engine with hydraulic lifters is a lifter bleeding down, not clearance.
• A deeper knock that gets worse under load is not valve clearance at all. That is bottom-end, and it is a different and more urgent conversation.
• Nothing — on an engine that has lost power, become hard to start, or developed a misfire on one cylinder with no ignition or fuel fault to explain it. Check compression before assuming the clearances are fine because the engine is quiet. Misfire counts per cylinder, read live as described in reading live data, will tell you which one to look at; P0300 says only that it is happening.

Video guides

Video, for the parts of this that are easier watched than read.