MG TA Cylinder Head

One of the most common problems with the MG TA (MPJG) engine is the sudden appearance of oil in the cooling system. This is not a drastic problem until the amount of oil becomes excessive. I have been running my TA with just such a problem for the past three years, and have covered around 10K miles during this time. Fortunately it is only oil in the water, not water in the oil, this condition is far more serious! The most likely cause of oil in the water is that the cylinder head has developed a crack, usually around the siamesed exhaust port area on cylinders 2 and 3. This part of the cylinder head gets extremely hot, and as a consequence is very prone to cracking, probably made worse by the higher burn temperatures of modern fuels. Have a look at photo 1 to see where this particular head had cracked, also note that there is no paint left on the centre exhaust port, it is all burnt off. This head had cracked before and had been repaired about 15 years ago.

Photo 1 showing the crack – the second time this particular head had developed a crack.

The MPJG engine has a non-pressurised cooling system, and due to the design of the rocker gear has an abundance of oil at the top of the engine, just lying in pools below the rocker shaft before flowing back down to the sump. This oil gets sucked into the cooling system and eventually ends up floating on top of the water in the radiator. I regularly clean out the gunge from the top of the rad, and a couple of times a year drain out the coolant and refill with fresh water. I am told that putting a dishwasher tablet in the rad for a couple of days before draining helps to clear out the rubbish, but I have not tried that yet!

I decided this Winter I would solve my problem by replacing the cylinder head with one of my spare heads. So I selected one of the three spare cylinder heads that I have, stripped it down and took it across to my local engine repair shop, Knight Engine Services, run by Dave Knight, for testing. He very quickly cleaned it and thought he could see a crack, which after a quick dose of Ardrox soon proved that it was quite badly cracked, see photo 2. I quickly beat a retreat, embarrassed that I had not spotted it!

Photo 2 shows the crack in one of Brian’s spare cylinder heads.

A couple of days later I returned to see Dave with another of my spare heads, this time cleaned and checked thoroughly by me. The plan of attack by Dave was as follows:

(1) Thoroughly clean the head in a ‘hot bath’ to remove all dirt and grease to get to clean metal.
(2) Pressure test the cylinder head whilst submerged in water to ensure there are no cracks.
(3) Bore out the existing valve guides and insert K- Line phosphor bronze guide inserts, and ream the inserts to the correct size for the new inlet and exhaust valves.
(4) Machine and fit 4 new exhaust valve seats with shallow unleaded valve seat inserts.
(5) Repeat the pressure test as in step 2 to ensure that the head does still not leak.
(6) Re-cut any of the valve seats necessary to suit the new valves.
(7) Skim the cylinder head a minimal amount to ensure it is completely flat.
(8) Skim the manifold faces to ensure that they are flat.

I had supplied Dave with a new set of valves, but the final lapping in of the valves, re-assembling the head and painting etc. would be done by me. I had a new set of standard valve guides, but after a chat with Dave he recommended that we use the K-Line guide inserts rather than press out the old guides and press new ones in. This would be less traumatic to the cylinder head particularly on the area around the valves for cylinders 2 and 3.

So how did it go? The head was thoroughly cleaned and then put into the receptacle to be pressure tested (see photo 3). Plates are attached to the underside and each end of the head, an air- line is attached to one end plate and then it is submerged under water. Air is pumped into the cylinder head up to about 25 psi, and then one looks for bubbles! We had no bubbles and the head retained the pressure, so everything was OK with this cylinder head, thank goodness.

Photo 3 showing the head about to be pressure tested.

The next step was to fit the K-Line phosphor bronze guide inserts, so the old valve guides were bored out to accept the new inserts and they are then pressed in (see photo 4). Unfortunately this did not go to plan, with the first two inserts breaking whilst being inserted. So plan B was put into action, the K-Line inserts were abandoned and the old guides were pressed out hydraulically. The new set of standard valve guides were then hydraulically pressed into place (see photo 5). Fortunately this time it all went well. By the way you must always press out, and press in new guides, NOT use a punch and big hammer as the shock will probably crack the head!

Photo 4 shows the K-Line phosphor bronze guide inserts ready to be pressed in…… except that this was not a success

With the new guides in place work could then commence on fitting the new shallow hardened exhaust valves seats needed for un-leaded petrol. The old seats were machined out and the new seat inserts were pressed into place. We used shallow seats as we did not want to inadvertently cut into a waterway and ruin another precious cylinder head. With the new exhaust valve seats in place the head was then pressure tested for a second time, again everything was fine. The new seats were then cut to ensure they suited the new exhaust valves I had supplied, and the inlet seats were given a quick cut also.

Photo 5 shows the new set of standard valve guides ready to be hydraulically pressed in.

The final tasks were to skim the face of the cylinder head a minimal amount to ensure that it was perfectly flat. The manifold faces were also skimmed to ensure that they were flat, as they do distort, particularly the centre exhaust port. Once this had been done the head was pressure tested for a third time (again OK) before being dried off and painted with red oxide paint (see photo 6). The standard thickness of an MPJG cylinder head was 3.469 inches, this head is now down to 3.440 inches being skimmed 29 thou. I intend to paint the inner oil facing surfaces of the head with Glyptal paint, this is a very hard paint, impervious to oil and fuel, and that also seals any porosity and improves oil flow back to the sump. The only downside is that it is expensive paint! Dave had done a fantastic job for me, now it remains to be re-assembled and fitted onto my TA engine in the car.

Photo 6 shows skimmed and pressure tested (for the third time!) head

Knight Engine Services can undertake full engine re-conditioning and their address is: New Factory Unit, Furnace Lane, Nether Heyford, Northants, NN7 3LB, tel. 01327-340900. They are about 3 miles from Junction 16 of the M1, and about 3 miles south of Weedon just off the A5.

Brian Rainbow