Citroën BX – Cylinder Head Overhaul

6 min read

From the cylinder head leak testing previously carried out, we know that we’re losing compression, largely in cylinder 3. Given how much the valves are leaking, this could be where all of the engines missing 20+ bhp are escaping. There is nothing in the engines service history to suggest head work has been carried out, but clearly, something is a miss. Can we find the reason for all this leaking pressure and overhaul the cylinder head?

One of these valves is not like the others

Something I didn’t notice when I first took the head off was the ‘odd’ looking valves over cylinder 3. Unfortunately, I didn’t get a better picture before the head went to the machine shop, but hopefully, this close-up shows the difference well enough.

Cylinder 3 valves are not like the rest in the head

The valves on the left are Cylinder3 valves, with cylinder 2 on the right. With a bit of cleaning is was immediately clear that the materials are different. The valves over cylinder 3 are a yellow, almost brass colour. The dimple on the bottom of the cylinder 3 valve is very different and the inlet valves are missing a relief angle. My suspicion is that this engine has dropped a belt and had new valves, but can I prove that?

Removing the Cylinder Head Camshafts

To get to the valves, I need to remove the cams. This is a fairly straight forward process, well documented in the good old 0908 Haynes Manual. The only thing that worries me is warping the camshaft covers, so I’m pretty meticulous about taking the screws out in the right order. All 24 of them.

D6C cylinder head laid out on the'bench'
D6C cylinder head laid out on the ‘bench’

No power tools here either. These are just the sort of bolts that will turn to cheese when faced with 18v of Makita brutality. Every bolt was carefully undone by hand then the camshaft caps removed. The final result is pretty satisfying and very reminiscent of the film ‘Short Circuit’

A note on valve spring compressors

Something that caused me a headache when I first took an Mi16 head apart was finding a suitable valve spring compressor. My old faithful Machine Mart spring compressor used on countless XUD engines was too small in every way. It took a few purchases to find the right spring compressor.

Mi16 spring compressor. It took a long while to find the right one.
Mi16 spring compressor. It took a long while to find the right one.

The spring compressor above is the one I found that does the trick. The problem is how far the springs are from the edge of the head, largely because of the angle. This means you need a wide opening to clear the head and the long through to get down the tappet hole. This is set to 200mm max open which gives about 60mm minimum closed position, compared to the XUs 130mm max opening, this is a big difference. The Machine Mart website doesn’t seem to list this one anymore (the Laser 2609 looks similar) but the Sealey VS154 spring compressor might do the job.

Non-original valves

With the hydraulic tappets pulled out, a strong magnet is ideal, and the valve spring removed, I could finally release the valve. And immediately something stood out.

'V537686' stamped into the after market BGA valve
‘V537686’ stamped into the after market BGA valve

On top of the valve, a part number is stamped in ‘V537686’. I’ve never seen a genuine Citroen/Peugeot valve with a part number stamped in so this is a pretty obvious sign these valves aren’t original. Looking up the part number, this is an aftermarket BGA part. And it doesn’t like it was ever ground or lapped in. Could the missing BHP simply be caused by poor engine building after a valve has bent?

Bent Cylinder Valve From Cylinder 3

While the aftermarket inlet valves moved freely in the valve guides, I figured I best throw them into the lathe and see if they were straight. Before setting up with the DTI, I thought I better just spin it up incase it was visually out.

A bent valve from a running engine. Didn’t really need a dial on this one.

And oh boy was it out. I’m not sure the video above does justice to how comically out the valve head is. The shaft its self measured out by ~0.4mm which is a fair old bend, but that was nothing compared to the run out of the valve seat. My suspicion is that someone tried to put a corner, bought aftermarket valves that weren’t ground, and didn’t lap them in. Then the valve has stuck out just a bit too far and caught the piston. Instant compression loss, but not enough to notice to the non-enthusiastic driver? Regardless, this head needs help!

Cylinder head machining at EMS

After asking around some local petrol head friends, I was put in touch with the guys at Engine Machine Services in Crewe. I’m pretty sure these guys sweat oil. Proper petrol heads that carry that faint aroma of 10w40 where ever they go. It’s always good to turn up at a machine shop to see a bench of recently ground heads, and shelves of parts waiting for some treatment.

Blatant advert for Engine Machine Services in Crewe
Blatant advert for Engine Machine Services in Crewe

The service received was fantastic, and quick. I’m pretty sure if I’d needed it they could have done the work the next day. The head was degreased in what is best described as a massive dishwasher before the remaining valves where removed. Exhaust and inlet mating surfaces were cleaned up before the cylinder head was ground. It took 16x 1000ths of an inch to remove the corrosion around the water inlets, that’s a fair chunk of metal. Hopefully, a +2 head gasket should make up for the lost compression, though it’s not obvious how much has been taken off in the past.

XPOs refurbished cylinder head on the left, and the spare on the right
XPOs refurbished cylinder head on the left, and the spare on the right

The valves where polished, to remove any imperfections on the shaft, then ground and replaced. Many years ago I picked up a reasonably cheap head from a fellow BX owner which provided replacement genuine valves of cylinder3. The guys at EMS actually walked me through the process and showed me the vac down test they do on each valve. I’m very confident they won’t leak any more. I’ve just got to get the rest of the head build right now!

Hydraulic Tappets

One of the features of the Mi16 that allows it to run as well as it goes is the use of hydraulic tappets. Tappets keep the camshaft lobes in contact with the valves on an overhead cam engine. There are a few ways to do this the XUD, for example, has a solid spaced under the bucket. The cam presses the bucket, which presses the shim, which presses down on the valve. Hydraulic tappets use oil pressure behind a floating sprung valve to take up the gap between the camshaft lobe and spring. Getting this valve out is a bit brutal.

When the engine is as old as this one, these valves can become jammed. A careful ear will hear a sticking tappet, and they are a reasonably easy job to tackle. I’m heeding the advice of Taylor Engineering who have some great detail on building the Mi16 head, all the things the Haynes manual leaves out!

Gummed up and'cleaned' hydraulic tappet valves
Gummed up and ‘cleaned’ hydraulic tappet valves

Once the valve is removed from the bucket, it’s clear that they need a good refurbishment. The sliding parts are well gummed up and the oil in the tappets is brown and just a little sludgy. The valve on the left is as it came out of the head. Its not just hydraulic pressure holding the plunger in. The valve on the right has had some of the dirt removed and filthy oil cleaned out and shows just how much the valve can move. These will go into the ultrasonic bath and be scrubbed and reassembled.

Rebuilding the cylinder head

The next step in the head rebuild will be to clean all this stuff up.

More parts of the cylinder head to overhaul
More parts of the cylinder head to overhaul

Once all these parts are cleaned, and the hydraulic tappets refurbished, I can start to build the head up. Then I just need to play catch up with the bottom end and I can put the whole engine together. Still a little way off driving the car though!


NEXT – Bottom End Teardown
PREV – Cylinder Head Leak Testing

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