Kerbside Care

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Decokes may be a dreary job but you’ll see a good return for a day’s hard labour on your classic’s engine – and all for a few quid, too

Chances are that with your classic safely tucked up for the winter and you’re so bored you could rip your head off – well why not do it to your classic’s instead and feel better?

If there’s one single job that can really put the pep back in your old classic’s engine it’s a good old fashioned de-carbonising and valve grinding – or top end overhaul to give the job its correct term. And best of all, such a pep up costs a pittance in materials – say £20; a gasket set usually suffices but if you really want to do a job it’s worth spending some money. The real cost is time as it’s a tedious task – but what else have you got to do – house decorating?

Decokes mostly died a death in the 1970s once better fuels and engine designs were developed, but back in the 1950s and 60s, it was a regular maintenance job that owners would carry out every 20,000 miles or even less. Some engine specialists say it is making a return due to emission controls.

Even if you think your engine is running fine, a decoke is good, preventative maintenance as you can rectify any problems – such as an imminent head gasket failure – before the problems lead to bigger things; MGF owners take note!

PRESSURE’S ON

Even if you intend to rip your engine’s head off, it’s not a bad idea to carry out a compression test before hand. These aren’t expensive tools (starting from £20) or you can hire one. Essentially, you insert the tester where a spark plug sits and turn the engine over (don’t start it). The gauge will register like a tyre pressure gauge.

Make a note of readings and compare them to the factory recommendations you’ll find in a workshop manual. Lower than prescribed readings are nothing to worry about so long as they are pretty uniform but a variation of 20lb or more suggests a decoke is wise. Any major imbalances suggest gasket failure, valve wear or perhaps engine bore deterioration.

A ‘wet’ test will help prove this. You simply introduce a little oil in the bores and retake the readings. If they substantially improve it points to engine wear.

OFF YOUR HEAD

A decoke is a straightforward job rather akin to oven cleaning and a doddle on overhead valve engines where the camshaft and valve timing remain untouched. Any average bod who was wielded a spanner before can tackle it with a basic toolkit although you will need a valve spring compressor (starting from £20) and a torque wrench, which can be begged, borrowed, stolen or even hired. Actually you can sometimes get away without a spring compressor by way of chains and bolts, but a compressor is the safest method and lessens the risk of bending a valve. As for cleaning the cylinder head, an electric drill with rotary brushes makes the job easy, otherwise a blunt knife or hacksaw blade suffices.

In terms of replacement materials, the job costs pennies.  A new cylinder head gasket is a must – never use the old one – but unless you disturb a joint you can usually get away with existing ones although it’s better to renew them.

Unless the valves and their guides are worn, you really only need to look at renewing the valve springs and valve stem oil seals.

HEAD EXAMINATION

Take a good look at the head as you dismantle it. Are the valves okay and the seats have no missing chunks – was the gasket in one piece without signs of localised ‘blow’ indications? Is the head,well coked up, suggesting fuel mixture ills?

Remove the valves carefully and keep all collets, springs and so on together. Check for guide wear by pulling the valve out by an inch or so and ‘rocking’ it. Any undue movement and you may need new guides pressed in. Check the valve for stem wear for another sign.

Inspect the valve seats for undue pitting and cracking (as can happen with tuned engines). A flat metal rule or glass plate is ideal for checking the head’s trueness.

HIGH ON COKE?

The actual decoking process is simple enough – you need to scrape away the carbon build up either manually or by use of an electric drill, which speeds things up considerably. The toughest deposits are on the exhaust side due to heat. Be more gentle on alloy heads as they scratch easier and, on all heads, take care not to damage the valve seats.

Valves are best cleaned by putting them in a drill chuck and finishing with emery paper to make them smooth and shiny to aid gas flow. Clean the valve throats and along with the combustion chamber, take time to remove any rough or high spots. Do the same to the inlet and exhaust ports but be careful not to grind away the venturi itself. Polish the exhaust side but don’t do this on the inlet ports as a rough finish helps the air and fuel mix better.

When you have completed the decoking, wash the head thoroughly of debris. As you can see, it’s not a difficult job, just tedious but there is a lazy alternative and that’s to have the head bead-blasted, which brings it up like new.

Don’t forget the pistons. These can be shone like silver but leave the carbon seal around the cylinder bore as it aids compression on high mileage engines.

SKIM HEAD!

Fancy a bit more pep? Then have a head shave! Most oldies run on low compression ratios to compensate for the poor fuels which were around back in the 50s and 60s.  By having the cylinder head skimmed to raise the compression ratio (say to around 9.0:1) you can gain a fair bit more power quite simply.

How much you need to shave off to achieve this depends upon the engine so consult a marque expert, but typically between 50-60 thousands of an inch is the norm – unless your head has been previously shaved, of course!

It’s always best to slightly skim an alloy head anyway as they are prone to distort. Generally, it costs around £50 – and that much again if you fancy having the head crack tested at the same time; many workshops advise this.

Don’t go too mad on a skim because too high a compression ratio will promote pre-ignition. Also it can drastically alter the operating angles of the rockers – again seek advice if you are unsure.

HARD HEADED

With the cylinder head off, now is an ideal time to have hardened valve seats installed so you can safely run on unleaded petrol for ever more. We were quoted £15 per seat plus fitting (it’s not a DIY job) so you’re looking at between £150-200 on, say, an MGB.

But do you need to have it done? Common sense says yes, but if the valve seats on your head look okay then there may not be a need – it depends on your driving. Obviously, if the valve seats need replacing anyway it’s a no brainer.

LAST LAP OF A DAILY GRIND

Grinding, or lapping in the valves is a real grind but it’s worth sticking with it, as a good valve seal will put the power back into your engine much more than scraping away the carbon. Even if the valves look fine, a light lapping with fine valve grinding paste is a must.

A tin of grinding paste comes with two ‘ends’, coarse or fine. If the valves are okay, then usually a light lap is sufficient – if they are pitted but salvageable, then you start with the coarse before finishing with the fine stuff. On some engines, the valves are specially coated and only the lightest lapping can be done. If it’s heavy then coarse grinding paste using an old valve is best to start off with. What you’re aiming for is a slim, unbroken seal of around 1/16in thick – anything broader will promote a burnt seat.

Here’s how to do it, place the valve on the sucker and place a smear of grinding paste on the seat.

Now ‘rub’ the stick in your hands briskly, turning the valve a quarter turn every so often to even the process out. When finished, thoroughly wash all residue from the valves and seats and oil the valve stems before refitting the springs. It’s a labour of love but at least you can do it in the kitchen watching the tele!

REBUILD RULES

It’s best to renew the valve springs although you can get away with using the old ones. Measure them against each other, and by the data in your workshop manual. Don’t be tempted to fit stronger strings as you’ll gain no benefits with a standard tune engine. Refitting the collets can be fiddly.

Place the cylinder head on the block with care, remembering to fit the new gasket correctly – check with your manual whether it needs to be fitted ‘dry’ or requires a sealing compound. Torque down as per manufacturers’ instructions (starting at half the recommended poundage) and in the right sequence printed in your manual.

After setting the tappets and running the engine to check for leaks and any air locks in the cooling system, run the unit for around 500 miles before rechecking head bolt tightness and the valve clearances.

And the job’s done!