M40 Head Gasket Change

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Loss of coolant? Cloudy exhaust? Mayonnaise in your oil cap? Oh dear, looks like your M40 head gasket has gone...


This article documents the replacement of an M40 head gasket. This will require the removal of the head, as well as the replacement of the timing belt. Some special tools are required and you need to be physically strong enough to lift the head off the engine safely. If in any doubt, consult a professional.

You will also need to know how to fit the timing belt to complete this job. Learn more about replacing the timing belt.


  • Decent socket set with extensions (various)
  • Female Torx set
  • Few screwdrivers (mainly to remove plastic trim bits)
  • Spanners (the usual sizes including an 11mm)
  • Mole grips come in useful.
  • If you're doing it the proper way you need a viscous coupling locking tool and a crank and cam locking tool. It is possible to make your own.


  • New gasket set
  • New waterpump
  • New cam belt and tensioners


First thing to do is get the bonnet up. Take the heat shield off from above the fan and radiator. Also the plastic scoop that's at the base of the windscreen. They both just unscrew/unbolt. No tricks to removal.

Next remove the top half of the inlet manifold. This first needs you to unhook the throttle cable. 2 Bolts and the plastic part just unhooks from the throttle body.

Then there's a couple of tubes and a cable. Then a few bolts, 11mm iirc. Once they're all loose, the whole top half should just lift off, with the throttle body still attached.


Then unbolt the lower manifold. Again it's a few bolts. You also have to take off the fuel hoses. There are two. Ideally you need to de pressurise the system by pulling the fuse.


It's quite fiddly removing the lower manifold around the petrol hoses, but with a bit of manoeuvring it can be done. Keep a record of the hoses and electrical connectors that get undone on this stage, because they can be a bit fiddly later.

Next step I made was to undo the bolts that hold the rocker cover on. Think there are 8 bolts on the top of the cover. This bit comes off quite easily.

In my case it was very obvious at this point that it was a head gasket failure


You can also lift up the plastic cam cover to show the cam

Don't remove the cam or cam caps! BMW cams are hollow and are extremely difficult to remove safely. You don't need to remove it, so leave it where it is.


In this photo I've also removed the distributor cap and spark plug caps and HT leads. Dizzy cap is just 3 10mm bolts. Make a record of which HT lead is for which cylinder! There's also a plug that plugs in underneath where the inlet manifold was. Unplug that. There's one of the plug caps that plugs into the coil to the left of the engine.

From here the job is made a lot easier if you remove the fan and housing. To remove the fan you need to unscrew it from the water pump which also spins. Requiring the locking tool. In my case I clamped a set of mole grips onto the pulley which as I spun it locked against the belt. Not professional, but it worked and was free. Note that the nut is left hand thread, so you turn to the right to undo it. It's a huge size as well, think it was 37mm. While it's locked you may as well remove the pulley from the water pump. There are 4 bolts. It's easier if you remove the belt first which is slackened by moving the alternator. Which is a 13mm locking nut on the back, then twist a mounting bolt on the front.


Then you can remove the rotor from the dizzy cap, that's 3 bolts, believe it's a 3 or 4mm allen key. Can be renewed if you see fit. Then remove the various bolts that hold on the upper cam belt cover. Think they're 12mm and there are a few of them. Once you've got them all, the belt cover should just come off with no trouble.

From there you can remove the water pump. 4 bolts hold it in. If it's a tight fit you can screw 2 M6 bolts at the top and bottom. There are threaded holes in the pump and the bolts should push against the block, pushing the pump out. What is most likely to happen is that the lugs on the water pump will just snap off, leaving you to remove the pump with gentle strokes of the hammer.


In order to remove the lower belt cover the crank pulley needs to be unbolted. There are five 13mm bolts, but you can't really see them. There's also a 22mm bolt on the end of the crank, you may need to hold that still in order to remove the pulley bolts. Undo the 13mm bolts and then both pulleys can be pulled off. Note there's a locating pin so it goes back in the right place. The belt can be slackened and removed by adjusting the power steering pump (on my model anyway, it changes if you have aircon).

Now you should have something resembling this:


A few more bolts on the lower belt cover and that can be removed too. Pay attention to the routing of the cable and where the cable clips are situated. Personally I label the bolts/nuts as I remove them so I know where they go.

Now is the point where you can lock the crank and cam. You need to find TDC on piston 1. This is done by slotting a locking pin (or 8mm drill bit/allen key) into a hole in the block. The hole sits underneath the starter motor, and is covered by an orange blanking plug. Find the plug and remove it:


And offer your locking pin into the hole. Now gently turn the crank by hand until you can push the pin in deeper and lock the crank, like so:


Once that's done you can place the cam locking tool onto the cam. It just sits over the cam and rests on the head.


Next step is to loosen the 2 cam belt tensioners. Both are 13mm bolts. One on the left is an actual tensioner, loosen it and it pivots. There's also one on the right that's more of a runner. Loosen both and slide the cam belt off.

Next up is the head bolts. There are 10! I only found 8 first time and wondered why the head didn't come off. You'll need an E14 female torx socket for this.

Undo the head bolts and get under the car. There are 3 bolts that attach the exhaust manifold to the down section. Undo those, it can be fiddly. From there the head can be lifted off, it shouldn't need any wiggling or hammering. Mine just lifted clear off.


Head off


Bare block, just lift off the old gasket, and put the new one on.


My gasket kit came with a new exhaust heat shield. It also came with valve stem seals but I didn't use them.

At this point you should check the head for any visible warping. If the head is warped it needs to be skimmed by an engineering firm, it's not particularly expensive but if it needs it and you don't do it, it'll just ruin the new head gasket instantly. Any warping most likely won't be visible so may need to also be checked by an engineering firm.

I gave it a visual check and decided not to bother, I ran a risk and so far it seems to have paid off.

Pop the head back on, torque it down according to the torque settings with new head bolts, following the correct bolt sequence.

7 5 1 3 9
<-- Rear Front -->
10 4 2 6 8


It's recommended that you also check/clean out the head bolt holes as you can cause damage and/or get false torque readings.

As always, the reassembly is the reverse of disassembly! You can now begin the process of replacing the timing belt.

Things to watch out for is the locating pin on the crank pulley. Make sure that slots into the pulley otherwise it'll run off centre and the belt will come off and also hit the fan.

Also the cables that run across the centre of upper and lower belt covers, make sure they're securely tied out of the way. New water pump can be fitted, and everything back together. This is where it pays off by labelling cables/pipes because you can work out where they all go on reassembly.

Job done, all back together, turn it over by hand a couple of turns to make sure the timing isn't so far out that valves will hit pistons.

Then give it a go! Fire her up!