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Decoke your Imp

Decarbonising a Cylinder Head

With the cylinder head on the bench

  1. Remove most of the carbon deposits from the combustion areas and valve heads by using a soft non-scratching tool, like the round end of a brass door key. This will ensure that the cylinder head does not receive accidental damage.

    universal valve spring compressor

    At this stage it is best to leave both sparking plugs and valves in position, as the plug threads and valve seatings could be damaged during the removal of the hard carbon deposits and old gasket material.

  2. Carefully use a blunt screwdriver (or something) for more stubborn deposits.

  3. Brush away all the loose carbon

    To remove the valves a universal valve spring compressor will be the only tool required (generally speaking).

  4. Place the cylinder head face down on wooden blocks on the bench.

  5. Remove the collets
    Use the spring compressor as shown (fig.3)
    You will find that the spring retaining cap will compress the valve spring with the collets, which are then easily removed. (Sometimes a slight tap with a hammer will be needed on the valve stem end to break the grip between the collets and the spring retaining cap) (fig.6).

    collets -yellow
  6. Slacken the spring compressor.

  7. Removed and number the spring retainer, spring, valve and where applicable valve stem seal.
    They should be numbered for later replacement in the same valve guide.
    This must be done until all valves etc. are removed from the head.

  8. Inspect each valve and seat for cracks, burns or anything other than minor imperfections.
    If there is any doubt at all as to the condition of any item, remove it. It would be false economy to do anything other than renew it.

  9. Turn cylinder head over so that the valve seatings are uppermost.

  10. Apply three small dabs of lapping paste to the valve seat and a spot of oil to the valve stem.

  11. Slide the valve stem into its correct guide.

    Hand suction tool
  12. Slightly moisten the rubber sucker on the lapping stick and apply it to the valve head as centrally as possible. By rolling the stick clockwise and anticlockwise between the palms of both hands and with slight downward pressure, we can start to lap the valve into its seat, lifting occasionally to add more paste, until both mating faces, when wiped clean, show up as an unbroken, matt grey, smooth finish. (fig.5).

  13. Repeat this procedure on all valves and seats.
    Use paraffin and if available compressed air to remove all traces of paste and carbon from in, on and around valves, guides and seatings.

When completed, the cylinder head will be ready for reassembly.


Decarbonising the Piston Crowns

As we do not want any bits of carbon etc., falling into oilways, waterways or bores during the 'Decoke', the use of a vacuum cleaner would be helpful.
If no vacuum cleaner is available: Turn the crankshaft nut until a piston is at the top of its bore, stuff balls of oil dampened newspaper into adjacent bores so that any bits which do drop into them tend to stick to the paper.

Carefully remove the old cylinder head gasket.

Cover all the oil and waterways with masking (or similar) adhesive tape.

Remove all the carbon from the top of the pistons and cylinders.
Some form of scraper must be used for this such as the round end of a brass door key or an old blunt table knife.
Be careful not to damage either the pistons or bores, not to mention yourself!

Remove the carbon from the top of the uppermost piston.

Polish with metal polish (to slow down the future build up of carbon).

Brush away the resultant debris with special attention given to clearing the gap around the top between the cylinder wall and piston, of any carbon which may otherwise lodge there.

Having completed one cylinder and piston, mask it over to exclude contamination from the remaining pistons.

Again turn the crankshaft until the pistons push out the balls of newspaper and are at the top of their cylinders, in turn repeat the scraping, polishing and brushing exercise until all pistons-cylinders are carbon free.

Remove masking tape and double check that nothing has been overlooked.

When the top of engine is clean, the new cylinder head gasket can then be carefully positioned, on being sure that the top/front marks are correctly placed.

Refit cylinder head and valve gear as described (Section 5) and torque down bolt-nuts as described.

It is very important to follow the 3 stages of tightening before doing the final retorque, which will be after the engine has been gently heated up to normal operating temperature. (20 minutes at tickover). Any other method of retorque could result in a blown cylinder head gasket, or a warped cylinder head.

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      see also the part on cylinder heads on the page on engines
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