Xu9j4_bottom_end

Citroën BX – Bottom End Teardown

7 min read

While testing the cylinder head for pressure leaks, I noticed a small bleed past the cylinder rings into the oil sump and out of the dipstick. Not a big leak, in fact, I could only hear it and not feel it. I could probably get away with leaving the bottom end alone. But having seen the mistakes previously made in building the head, I’d best do a bottom end teardown and at least whip the crank out.

The other reason for the bottom end teardown

One of the ‘unique’ features in the Mi16 XU9J4 engine is the aluminium cylinder block. Okay, it’s not revolutionary, nor the first time the technology was used by Citroen, but compared to other XU engines, it’s pretty special. It is made up of the aluminium block for weight reduction, coupled with steel wet liner inserts. Loosely put, the liners contain the explosions in the cylinders while sitting in a bath of coolant.

MI16 XUJ94 bottom end with liner retainers in place
MI16 XUJ94 bottom end with liner retainers in place

And here lies a bit of a problem. The liners pass through the bath of water so the upper section is sat in coolant but the bottom half is sat in oil. And the seal between the block and the liner? It’s a small, easily disturbed rubber o-ring, which by this point in the engines life is probably bonded to block and liner. Despite the liner retainers shown in the picture above, they’re guaranteed to be disturbed. Which will mean oil will flow into the coolant, or worse, coolant with the flow into the oil! It will be better in the long run to teardown the bottom end and replace these seals.

Off with the sump

The first task in tearing down the bottom end is to get the sump off, and man was that hard work. I’ve mentioned previously when tearing down the head just how much sealant there was, and the sump was bonded on in the same way. Too much sealant is definitely a bad thing, as we will see.

Sludge in the sump has the consistency of warm Nutella
Sludge in the sump has the consistency of warm Nutella

After much cutting and splitting of the multiple layers of sealant, I finally got the sump off. You can see just how much excess was used. It is, however, confirmation that this is not the first time the bottom end has been apart. The remaining ‘oil’ in the sump is free from metal, a strong magnet helps here, which is always positive. Unfortunately, it has the consistency of Nutella (chocolate spread) that’s been left out in the sun for a while. Sucking that into the oil pump would mean instant engine failure!

Sealant shards found in the oil pump
Sealant shards found in the oil pump

And with the sump off, the very first thing I noticed was a big piece of sealant inside the oil pump pick up. It was big enough to almost completely block off the ~13mm hole below the strainer. More than enough to severely restrict the flow of oil to the head, and more worryingly to the bottom end bearings, and I think that’s exactly what has happened, as we will see.

Removing the crank shaft from the bottom end

With the sump out and sitting in degreaser, the oil pump could be removed and the crank accessed.

Mi16 XU9J4 crankshaft with oil pump removed
Mi16 XU9J4 crankshaft with oil pump removed

And on first appearances, it’s looking pretty good. I’m starting to question if the bottom end has been apart, or if the excess sealant is just because the sump has been replaced. They are terribly brittle. There is a bit of varnish build-up in places, but otherwise, the crank is unmarked and free from any more sealant. After much head-scratching looking for the two hidden bolts in the central main retainer, and I had the crank out.

Crank shaft out of the bottom end, with both good and bad results.
Crank shaft out of the bottom end, with both good and bad results.

The state of the crank was one of my biggest fears about the bottom end teardown. It was a running engine, so it couldn’t be too bad and if I didn’t look then I wouldn’t know how bad it was. Tearing it down meant I would absolutely have to deal with the problems! I think on balance, it’s not too bad. The surfaces of the main bearings (the five down the centre line of the shaft) are in really good condition. Only one is particularly marked and I’m pretty sure some wet and dry with cure that. The bearing shells, however . . .

Bottom end teardown reveals heavily worn main bearings
Bottom end teardown reveals heavily worn main bearings

The main bearing shells are wrecked, number 5 looks like it’s been driven over and probably hit with a hammer. In places the material looks like it has been torn apart, I’d best describe it as spalling. The picture above (shown with the big end bearings) doesn’t really do justice to how bad the finish is. It’s also marked as an SK bearing which is a quality bearing, but not original to the Citroen engine. The lack of matching damage on the crank is a testament to just how hard the crankshaft material is!

Finishing the teardown

In for a penny, in for a pound. At this point, replacing shells and putting the bottom end is pretty straight forward. But it seems daft now to dig so far into the engine and not finish it off. Completing the teardown of the bottom end will mean honing the liner bores, cleaning the block and prepping the liner mating faces and probably an extra 8 hours of work. But as @BXNerd points out, it’s a saleable addition.

Mi16 XU9J4 bottom end teardown complete
Mi16 XU9J4 bottom end teardown complete

And there is it, the entire bottom end torn down and laid out bare on the table. So far it hasn’t been too painful, except to the wallet knowing how expensive engine parts are. I suspect the painful bit will be trying to shove it all back in the block without damaging anything. But before I get to that stage, are the removed parts serviceable?

Cleaning out the bottom end and measuring up

When I was last over at Engine Machining Services, they offered to clean up and measure the crank. So I headed back down with a car full of parts to make use of their industrial parts washer, and save a few quid on yet another set of micrometres.

XUJ94 bottom end cleaned up
XUJ94 bottom end cleaned up

Amazing what a good clean up can do for aluminium parts. While the block came out considerably cleaner than I’d managed by hand, it did need a bit of finishing off by hand, especially around the wet liner mating surfaces. The old wire brush on a drill has done wonders, and I happen to have one that fits beautifully into the liner holes.

Measuring the crankshaft surfaces
Measuring the crankshaft surfaces

The news was a bit mixed on the crankshaft. The mains are all perfectly within size for taking a set of standard bearings. And the big ends were in spec too. Except for one. Just one. I could cry. The spec is 50.00mm nominal, with a minimum of 49.984mm allowed. Before the damage is polished out, this single big end journal measures 49.89. It’s not a crank that needs throwing out, but it’s going to need professional precision grinding to enable oversized big end bearings to be used. And that’s going to be expensive, but perhaps there is another option.

Sealing the aluminium parts

With the bigger parts having been through the industrial cleaner, they are looking truly stunning, but they won’t stay that way for long. Oxidation will set in pretty quickly unless something is done about it. So back out with the wire wheel on the drill to remove some of the more stubborn corrosion.

Aluminium parts cleaned, finished and sealed with Comma wax seal
Aluminium parts cleaned, finished and sealed with Comma wax seal

Once they’re as clean as I could manage, the cleaned and shiny parts get a coat of engine wax. I’m using Comma Wax Seal which sprays a fine mist of way over the parts, holding out the air and preventing immediate oxidisation. While the mist is fine, it’s inconsistent and in places, the wax builds up leaving a yellow residue. Not a problem as I can spread this out later. For now, these cleaned, restored and protected parts can be wrapped and put to one side until I’m ready to rebuild.

Decisions on what to do with the bottom end

Where I go next is a bit up in the air. The crankshaft is perfectly useable and can be ground back into a spec for oversize shells. But is that the right way to go? I’m repeatedly told that a turbo-diesel crank in the bottom of the Mi16 is a significant improvement. It is reportedly the same stroke length so drops straight in while being 4kg lighter. The DW10 crank will be forged (stronger) and have a better oil feed to the No2 big end bearing, and it looks like I could do with that!

I guess I need to weigh up the cost of a new crank with getting the original restored. I’m just glad I haven’t bought the big end bearing set yet!

M

PREV – Cylinder Head Overhaul

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