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How to Decoke Classic Engines

Want To Be A Carbon Neutral? Published: 27th Jun 2011 - 0 Comments - Be the first, contribute now!

How to Decoke Classic Engines
How to Decoke Classic Engines Keep all the parts for the particular cylinder together – don’t mix ‘em up
How to Decoke Classic Engines This is really all you need and most autojumbles sell sets, cheaply enough
How to Decoke Classic Engines If you want to remove the valves safely, you’ll need a proper spring compressor
How to Decoke Classic Engines Patience with a normal drill with rotary brushes and polishers reaps rewards
How to Decoke Classic Engines As removed (inset) and the fi nished job takes just a leisurely two days and few of tools. A good decoke will put the buzz back in your classic
How to Decoke Classic Engines
How to Decoke Classic Engines A head skim isn’t essential but a cheap pep up as it raises compression ratio
How to Decoke Classic Engines After 500 miles running in, check head bolts again and reset the tappets
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A clear head on your classic will mean more power, more economy plus less emissions. What’s more a simple decoke can provide it all for £20!

Before you wake your classic from its winter slumber why not treat it to a TLC tune up – by ripping off the engine’s sleepy head? We’re talking about a good old fashioned decoke (or top end overhaul if you want to be posh) and for around £20, or even less, you can put back all the missing bhp from your classic – and even add a couple of horses at the same time. Before the mid 1970s, decokes were commonplace every 20,000 miles thanks to inferior fuels, oils and engine designs; today a modern engine can cover 100,000 miles and the head is still clean as a whistle. But a decoke on a typical classic engine will reap big rewards for a day’s graft doing nothing more sophisticated than scraping away the carbon deposits on the head. Apart from curing ills such as poor performance and starting, damaging pre-detonation, running on or overheating, some deft work with nothing more elaborate than an electric drill and rotary wire brushes can give it a pep up for just pennies! When decokes were all the rage practical car magazines were stuffed with features on how you can rejuvenate your Cortina/Mini/MG for around £2. Well 40 years on you can do the same job for £20 max and perhaps just a tenner if you scout the auto jumbles for a cheap decoke gasket set. And that’s really all you need! If you’ve never tackled a decoke before then relax as it’s one of the easiest nut and bolt jobs you’ll encounter – especially on an overhead valve engine. Overhead camshaft engines are trickier as the valve timing has to be disturbed but it’s fairly straightforward if you follow the workshop manual. Apart from a head gasket set (don’t attempt the job without one) and perhaps some gasket joining cement, you’ll need a torque wrench to tighten the head down correctly; this is most important. If you don’t mind a grind, just a blunt knife will suffice to scrape away the carbon deposits although an electric drill with accessories sure make a better job if it. If you want to remove the valves you’re going to need a spring compressor and a valve grinding stick and grinding paste. But penny to a pound you’ll know somebodywho can lend you theirs (they do gather dust in the garage)! If you can lay hands on one, a compression tester is useful to check thecylinder compressions beforehand. Your manual will give you the relevantpressures but even more important is how uniform they are.  A variation of more than 15/20lbs across the cylinders suggests that a decoke is wise. Our pick strip shows you the essentials – now get out their and rip your engine’s head off, in the kindest possible way.

A stage further

If you want you can go further than carry just a simple decoke and tune the head to a ‘Stage 1’ upgrade with just a little more effort. Essentially it’s the same principle but whereas before you were removing carbon now you’re also deleting some metal. Look at any old mass produced cylinderhead and you’ll be dismayed at the poor fi nish and roughness and by removing any high sports and irregularities a better gas fl ow is achieved. Much of it is common sense although bear in mind that a degree of unevenness on the inlet ports helps fuel/ air mixing but smoothing the exhaust side aids scavenging.Apart from rotary drill brushes you need special grinding and burnishing tips, all which are fairly inexpensive. You can go further with cutters to dramatically alter the shape of the combustion chambers and ports but this is more involved. Other head work includes skimming which raises the compression ratio and thus more power; typically up to 60 thou is acceptable. A typical engineering machine shop will charge around £25 for this job and the same again for crack testing – the later which is a good idea on some designs to see if the head is up to it. If any valve seats need replacing then you may as well go for hardened inserts to make the engine better suited for unleaded fuel.

    Dozen DIY decoke directives

     

  • As there’s a fair bit of dismantling involved get organised with lots of little boxes/cartons to put the separate nuts, bolts etc and label them so you can’t get them muddled up. Valves, pushrods, rockers etc should be retained for their cylinder as they have worn in to suit that bore
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  • Always have a workshop manual handy. Some engines have some crafty touches that may fl ummox you. For example on certain Vauxhall engines an inlet manifold bolt is hidden inside the ‘top end’ and only accessible by removing the carburettor or the thermostat!
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  • When releasing the head bolts, do it in a sequence as you would torquing it down, gradually so avoid any chance of distorting the head. It may be stuck and need shocking free
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  • Once the head is removed, place some rags down the galleys and bores to avoid the chance of a dropped nut, bolt or debris entering the engine
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  • The state of the head and bores tell you a lot about the engine’s health. Look for burnt valves, valve seat recession, and worn guides; the latter checked by half removing a valve and feeling for slack in the guide as well as oil burning marks
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  • Check the bores; there will be a ridge at the top but it should not be excessive. Try to move the pistons sideways – an undue amount suggests ring wear; Anything above a 0.004in feeler gauge which can be slid around the piston circumference points to a rebuild
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  • The carbon deposits will be tough to remove, but don’t use a chisel or screwdriver as you’ll scratch the surfaces. A blunt knife or hacksaw blade is ideal. Rotary brushes are best for the valve throats, ports etc
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  • Check the valve seats. A burnt valve or hammered head seat will need replacing and this is a job for an engineering shop. Otherwise ‘lap’ them in (see box out)
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  • Cleanliness is essential when carrying out a decoke so have plenty of petrol or white spirit to sluice old debris, and grinding paste away; any left in the head will promote rapid engine wear
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  • Valve springs last a lot longer then they used to and may not need replacing. You can measure them against each other by the data in your workshop manual. That said, they are cheap to renew. And while they are off…
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  • Place the cylinder head on the block with care, remembering to fi t the new gasket correctly (‘dry’). Torque down as per manufacturers instructions (starting at half the recommended poundage) and in the right sequence
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  • After setting the tappets and running theengine to check for leaks and any air locks in the cooling system, run the engine for around 500 miles, gently before rechecking head bolt tightness and the valve clearances. And that’s all there is to it!


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