Stripping and Rebuilding an M20 Cylinder Head

In this guide, we will show you the correct procedure for stripping and rebuilding an M20 Cylinder Head. This includes 2.0, 2.3, 2.5 litre engines. For the benefit of the guide, we will be using an M20 2.5 head.

• Required Tools
• Female Torx Socket
• Old Timing Belt
• Lockable Grips
• 8mm Spanner
• 10mm Spanner
• 3mm Allen Key
• 11mm Deep 1/4" Drive Socket
• Short 1/4" Drive Extension Bar
• Feeler Gauges
• Valve Spring Compressor
• Valve Grinding Stick & Paste
• Suitable drift (1/2" Drive Extension)
• Hammer
• Magnetic Ended Screw Driver
• Spark Plug Wrench

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Rebuilding

Preparing the Cylinder Head for Re-Assembly

Before you actually rebuild the head, it's a good idea to carry out a few routine checks and maintenance to check that the head is in good order.

Firstly take the cylinder head to a machine shop and request that they check the tolerances of the valve guides (take your valves with you), check the face of the head for warps, have it pressure tested for cracks. If all is okay then request for it to be "chemically dipped" and "refaced". A chemical dip removes all carbon deposits an generally cleans the head up to a new finish again. You'll probably notice that your current cylinder head is stained brown. A reface is where a very thin surface is removed from the face of the cylinder head giving you a nice smooth finish ready for you to mount your gasket against.

When you get the head back from the machine shop you should re-grind the valve seats. If you look around the rim of the valve where it contacts the head when it opens and closes, you will notice that it's most probably very shiny which is bad for a good seal. Anyone can regrind valve faces but you will need a grinding kit which consists of grinding paste and a grinding stick. These can be bought very cheaply from any motor factors.

You can see from the below picture the difference between a valve which has been freshly re-ground and an untouched valve. Even new valves need grinding in to obtain the correct seal against the cylinder head. If you look at the below picture, you will see that the left hand side of this valve has a smooth dull grey finish, this is what the entire rim of the valve should look like after a re-grind. There should be no spots or shiny places.

To regrind a valve, the procedure is actually very simple. Take a small dab of the fine grinding paste and cover the rim of the valve with it. Next place the valve in the correct port on the upturned cylinder head and stick the grinding stick to the valve. Now simply rub the stick back and forth between your hands to grind the two surfaces together. Keep checking and add more paste if necessary. Stop once you see both surfaces (the valve and the head) have a smooth dull grey finish. Repeat for all 12 valves.

NOTE: Ensure that all grinding paste is thoroughly cleaned away before re-assembly of the head.

It's important to install new Valve Stem Oil Seals before replacing the valves. There is a special tool available for installing these seals but a deep 1/4" Drive 11mm socket fits perfectly. Begin by wetting the inside of the valve stem oil seal with some new oil to allow easier fitting. Place the seal on the end of the 11mm socket and then use a firm force to push the seal into place making sure that you keep the seal straight.

Reassembling the Cylinder Head

Reassembly of the cylinder head is very much the reverse procedure for stripping it. Begin by reinstalling all valves and springs in the correct order. The worst part is refitting the valve collets and getting them to stay in place and I've found that dipping them in some clean oil helps them stick to the valve stem. You can also re-install the camshaft at this point, smother all moving surfaces with Engine Assembly Lube and slide the camshaft back in place. It's a good idea to replace the camshaft end seal and o-ring as these are a common source of leaks and are easy to change while the head is off and in this state. At this stage, the head should look like this.

Now is a good time to check that the re-ground valves are sealing properly. Place the head on a bench and then tip a spoonful of fuel down each port in turn and check for any leaks underneath. If you've done it correctly the fuel will not leak through. If you have any which are leaking then it's advisable to remove and regrind again until you reach a satisfactory seal.

The next job is to replace the rockers and rocker shafts. We will start with the exhaust shaft first. Turn the cam until the lobe for exhaust rocker 6 is at full lift, you should now be able to slide the shaft back through (from the right hand side) slipping on the first five rockers as you go. Stop before you get to number six. Now make sure that all rockers are sitting over their respective cam lobes and turn the camshaft so that number one rocker is at full lift. Once again use a suitable drift to knock the shaft all the way in and slide on rocker number 6 at the same time.

Repeat this procedure with the inlet side rocker shaft and rockers. Make sure that when you install the rocker shafts, the notches for the retaining plate are lined up. Re-install the retaining plate, rocker retaining clips and rocker shaft end seals. Also replace the oil spray bar and refit the top pulley.

Now is an ideal time to adjust your valve clearances, please see the tech article for the procedure for doing this. It makes life much easier to do this while the head is off the car.