Disassembly and assembly of the cylinder head ZMZ-402

Dismantling the ZMZ-402 cylinder head

We are dismantling the cylinder head for subsequent repair or for replacement.

When dismantling, the cylinder head parts should be laid out in order, bearing in mind their further use.

Removal of the cylinder head was covered in the article ‘How to dismantle the engine’.

Cylinder head with thermostat housing and rear cover
We start from this state: the cylinder head is still assembled, with the thermostat housing and rear cover in place

Using a 13 mm spanner, slacken the nuts securing the thermostat housing.

Open-ended spanner on the thermostat housing securing nut
First we release the thermostat housing — it is removed before the other assemblies

We remove the thermostat housing from the cylinder head.

Thermostat housing separated from the cylinder head
The thermostat housing is removed — access to the rear cover of the cylinder head is now clear

Using a 12 mm spanner, slacken the four bolts securing the rear cover of the cylinder head.

Spanner on a bolt of the cylinder head rear cover
The rear cover is held by four bolts; we slacken them one after another

We remove the rear cover of the cylinder head.

Cylinder head rear cover removed
The rear cover is separated; its gasket is fitted with sealant during reassembly

We remove the valves using a valve spring compressor. This compressor has a stop for the valve.

Valve compressor with stop mounted on the cylinder head
The compressor stop presses the valve from below so that the spring can be compressed

If no such compressor is available, place a suitable stop under the valve — a thick rubber washer or a wooden block will do.

The valves must be marked or laid out in order so that they can be refitted in their original positions during reassembly.

We fit a special tool for compressing the valve springs onto the cylinder head.

Spring compressor tool secured on the cylinder head
The tool is positioned on the head so that it grips the spring from above

We compress the valve spring with the tool. If the valve retainer sticks, tap the tool stop with a hammer.

Compressed valve spring under the tool
The spring is compressed to the position where the collets come out of the valve groove

Holding the springs compressed, we extract the two collets with tweezers or a thin screwdriver.

Then, gradually releasing the pressure on the tool handle, we fully release the valve springs.

Tweezers extracting a collet from the valve retainer
The collets hold the retainer on the valve — they are extracted before the spring is released

We remove the tool and take off the valve retainer.

Valve retainer removed from the spring
Once the retainer is removed, access to the springs is clear

We extract the two springs.

Two valve springs taken out of the cylinder head
The outer and inner springs are removed together; on reassembly they go back in their original positions

With pliers, carefully remove the valve stem seal from the guide.

When removing, twist the seal slightly to make it easier to take off. Careful: the guide is very fragile!

Pliers on the valve stem seal of the guide
The seal is removed with a twisting motion so as not to damage the fragile guide

We remove the support washer.

We turn the cylinder head over.

Cylinder head turned combustion chamber upwards
In this position the valves are extracted from the combustion chamber side

We withdraw the valve from the cylinder head.

Valve drawn out of the guide
The valve comes out of the guide freely; if it sticks, check the stem for distortion

If the guide wear is large and exceeds 0.25 mm, prise out the circlip with a screwdriver and remove it.

Screwdriver under the guide circlip
The circlip retains the guide in the head; it is removed only when replacing the guide

Using hammer blows through a drift, we drive out the guide.

Drift and hammer when driving out the guide
The guide is driven out from the combustion chamber side, using a drift of the correct diameter

Fitting a new guide: cool the new guide in dry ice or in the freezer, heat the cylinder head to 150–190 °С and press the guide in.

The guide must protrude 20 mm above the cylinder head.

After pressing in the guides, they must be reamed with a special reamer to the required size.

After that, the valves must be ground into their seats. To do this:

Grinding paste on the valve face
The paste is applied in a thin layer over the whole working face — the quality of the seat depends on it

We apply grinding paste to the valve face and insert the valve into the corresponding guide in the head.

We attach a special grinding tool to the valve stem.

Grinding tool fitted on the valve stem
The tool transmits a back-and-forth rotary motion to the valve

We press the valve against its seat and turn the valve in both directions.

Valve turned with the tool on its seat
Grinding is carried out alternately in both directions, lifting the valve periodically

We continue grinding until the valve face becomes matt and clean over its entire length and width.

We wipe the valves free of paste residue.

Drift with a new seal on the guide
The new seal is fitted over a drift so that its lip is not damaged when seated

We fit the new valve stem seals using a drift.

Reassembling the cylinder head

We reassemble the cylinder head in the reverse order of dismantling.

The cylinder head is reassembled in the reverse order of dismantling.

Before fitting, lubricate the stem seals, valve stems and rocker shaft with engine oil.

The valves are fitted in accordance with the marks made during dismantling.

The thermostat housing gasket and the rear cover gasket are to be fitted with «Hermesil» type sealant.

A more detailed fault inspection is covered in the article ‘Inspection and fault diagnosis of the cylinder head’.