Fiat Uno Manual

Cylinder head - dismantling and decarbonising (903 cc engine)
Engine / Cylinder head - dismantling and decarbonising (903 cc engine)


1 The exhaust manifold and rocker gear will have been removed from the cylinder head during removal (see Section 7).

2 The valves should now be removed using a universal valve spring compressor.

3 Compress the first valve spring and extract the split cotters.

4 Gently release the compressor, take off the spring retaining cap, the valve spring and the spring seat. Remove the valve. Keep the valve with its associated components together and in numbered sequence so that they can be returned to their original positions.

5 A small box with divisions is useful for this purpose. Remove and discard the valve stem oil seals.

6 Remove the other valves in a similar way.

7 Bearing in mind that the cylinder head is of light alloy construction and is easily damaged use a blunt scraper or rotary wire brush to clean all traces of carbon deposits from the combustion spaces and the ports. The valve head stems and valve guides should also be freed from any carbon deposits. Wash the combustion spaces and ports down with paraffin and scrape the cylinder head surface free of any foreign matter with the side of a steel rule, or a similar article.

8 If the engine is installed in the car, clean the pistons and the top of the cylinder bores. If the pistons are still in the block, then it is essential that great care is taken to ensure that no carbon gets into the cylinder bores as this could scratch the cylinder walls or cause damage to the piston and rings. To ensure this does not happen, first turn the crankshaft so that two of the pistons are at the top of their bores. Stuff rag into the other two bores or seal them off with paper and masking tape.

The waterways should also be covered with small pieces of masking tape to prevent particles of carbon entering the cooling system and damaging the coolant pump.

9 With a blunt scraper carefully scrape away the carbon from the piston crown, taking care not to scratch the aluminium. Also scrape away the carbon from the surrounding lip of the cylinder wall. When all carbon has been removed, scrape away the grease which will now be contaminated with carbon particles, taking care not to press any into the bores. To assist prevention of carbon build-up the piston crown can be polished with a metal polish. Remove the rags or masking tape from the other two cylinders and turn the crankshaft so that the two pistons which were at the bottom are now at the top. Place rag in the cylinders which have been decarbonised, and proceed as just described.

10 Examine the head of the valves for pitting and burning, especially the heads of the exhaust valves. The valve seatings should be examined at the same time. If the pitting on the valve and seat is very slight, the marks can be removed by grinding the seats and valves together with coarse, and then fine, valve grinding paste.

11 Where bad pitting has occurred to the valve seats it will be necessary to recut them and fit new valves. This latter job should be entrusted to the local agent or engineering works. In practice it is very seldom that the seats are so badly worn. Normally it is the valve that is too badly worn for refitting, and the owner can easily purchase a new set of valves and match them to the seats by valve grinding.

12 Valve grinding is carried out as follows.

Smear a trace of coarse carborundum paste on the seat face and apply a suction grinder tool to the valve head. With a semi-rotary motion, grind the valve head to its seat, lifting the valve occasionally to redistribute the grinding paste. When a dull matt even surface is produced on both the valve seat and the valve, wipe off the paste and repeat the process with fine carborundum paste, lifting and turning the valve to redistribute the paste as before. A light spring placed under the valve head will greatly ease this operation.

When a smooth unbroken ring of light grey matt finish is produced, on both valve and valve seat faces, the grinding operation is complete. Carefully clean away every trace of grinding compound, take great care to leave none in the ports or in the valve guides. Clean the valve seats with a paraffin soaked rag, then with a clean rag, and finally, if an air line is available, blow the valves, valve guides and valve ports clean.

13 Check that all valve springs are intact. If any one is broken, all should be renewed.

Check the free height of the springs against new ones. If some springs are not within specifications, replace them all. Springs suffer from fatigue and it is a good idea to renew them even if they look serviceable.

14 Check that the oil supply holes in the rocker arms are clear.

15 The cylinder head can be checked for warping either by placing it on a piece of plate glass or using a straight-edge and feeler blades. If there is any doubt or if its block face is corroded, have it re-faced by your dealer or motor engineering works.

16 Test the valves in their guides for side to side rock. If this is any more than almost imperceptible, new guides must be fitted.

Again this is a job for your dealer as a special tool is required to ensure the correct installation depth and the cylinder head must be warmed to 80ºC (176ºF) before fitting the guides.

17 Commence reassembly by oiling the stem of the first valve and pushing it into its guide which should have been fitted with a new oil seal (photos).

17.17A Valve stem oil seal
17.17A Valve stem oil seal

17.17B Inserting a valve into its guide
17.17B Inserting a valve into its guide

18 Fit the spring seat. Fit the valve spring so that the closer coils are towards the cylinder head and then fit the spring retaining cap.

19 Compress the valve spring and locate the split cotters in the valve stem cut-out (photo).

17.19 Fitting split collets
17.19 Fitting split collets

20 Gently release the compressor, checking to see that the collets are not displaced.

21 Fit the remaining valves in the same way.

22 Tap the end of each valve stem with a plastic or copper-faced hammer to settle the components.

23 The cylinder head is now ready for refitting as described in Section 7.


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