My first post on here, Thought I would write this up and share my experiences as this applies to a lot of BMW’s including the 328i, 528i, 728I and many of the six cylinder cars.
Driving home a few weeks ago the low coolant warning came on my BMW 728i. When I checked, it had lost a little coolant and there were no signs of any external leaks. I topped the water up and checked for bubbles in the water…. Sure enough there was a steady stream of bubbles coming up, a sure sign of a head gasket, or worse… At least the engine hadn’t overheated as I had got it in an early stage. The temptation is to ignore the first signs, which only leads to more serious and difficult to fix problems down the line. Ignore water loss at your peril!
I checked on this site and others and saw several horror stories of cracked heads, stripped threads in the block ect but even so decided that for a couple of hundred pounds it was worth seeing if the old girl could be put back to life. The car has 145k miles on it but it has a very good service history and many of the common jobs had been done.
I set about removing the head, following the procedure in the workshop manual. Easy enough, if a little time consuming with lots to undo and take off. The fire rings on the old gasket showed slight signs of breaching, but only slight…. I feared that the head might be cracked. I took it to a local firm (Cylinder head services, Ash St Bilston – a reputable firm) who crack tested it and pronounced it OK They gave it a light skim as there was a 4 thou bow when I checked it with a straight edge, I could just get a 4 thou feeler gauge under the middle. While I was there he showed me a couple of heads from BMW 6 cylinder cars with cracks in them…
I decided that as it was off I would drop the valves out and re seat them as the car had done 145K, and also replace the valve stem oil seals. To be honest, the valve / head seats were in perfect condition, even at that mileage, so I left them alone. The valve stem oil seals were easy enough to replace, the new seals came with a neat little protection sleeve to prevent damage to the seal by the collet groves (see picture) Getting the collets in place is a little tricky. I used a 6mm wood chisel which I magnetised by stroking it with a magnet, this held the collets at just the right angle to get it into place. I use a dab of grease to hold them in place (see picture)
I cleaned of the block ready to put the head back on, and importantly cleaned out the head bolt threads. This bit is REALLY IMPORTANT. I used one of the old head bolts which I ground a slot in (see picture) and used this to clean the threads, right down to the bottom. I then use some petrol and a small bottle brush to give the threads a final clean. This was finished off with a blast from the air gun to make sure they are fully clean and there is no debris in the bottom of the threads. Can’t stress the importance of this enough…
Before fitting the gasket to the block I gave it a good clean with some Trichoethelene to make sure the surface was completely grease free, along with the head. This again is important as some gaskets have a compound on that bond to mating surface which can be affected by oil /grease deposits. I decided to put the head on without the cams and cam carriers as this made it lighter. Even then it was a 2 man job!
Next is the most important part of the job, torqueing up the head, this seems to be where people strip the block threads… First, make sure you replace the head bolts, and make sure the new ones are the right length! I was supplied in the first instance with bolts that were 10mm to short! Lightly lubricate the thread on the bolts and also the head of the bolt and the washer. These heads have 3 tightening phases. The first phase is to torque them down in the correct sequence to the specified torque. The next two phases are where the bolts are turned through a specified number of degrees. In my case 32 NM followed by 90 degrees and then another 90 degrees.
What is important to do is NUMBER THE BOLTS! This is best done with a felt tip pen, writing the number in the tightening sequence next to the bolt on the head. The reason the bolts need to be numbered is simple, if you don’t, you WILL forget which ones you have already tightened, and end up tightening one or two of them to much! This is the single most cause of stripped threads!
I user to be a fleet engineer with 30 workshops under my control, and I would see this all the time… The mechanic would start to tighten the head, half way through he would forget where he was in the sequence and end up giving one or more bolts an extra 90 degrees. There are 14 bolts on the head! If you are lucky the bolt breaks, if you are unlucky the thread in the block strips… These bolts are stretch bolts and you need to be accurate with the degrees you turn them. Its best to use a big breaker bar and try to get the full degree increment in one smooth turn.
Fitting the cam carriers to the head was a bit of a challenge… the cam buckets drop out! I overcame this by using 12 small magnets to retain the buckets in place. I have also seen people use grease on the cam buckets, but I am not keen on doing this as it can give problems with some engines which have hydraulic tappet adjustment.
From then on, it was a simple re-assemble, following the procedure in the workshop manual. I didn’t need any of the special tools the manual said I needed; just make sure that as the instructions say, engage the VANOS on the first tooth as you turn the exhaust cam!
Hope you guys find this useful if you end up doing a head gasket on a Beemer!