Main Bearing cap came loose

XK-Engine
1

Main Bearing cap loose!!!

Just had a very strange occurrence in a customers car.

The car is from undetermined lineage and history. Nice overall condition, but overheating and running like a dog (Car came in on a flatbed) Customer got a great deal, but suspected it needed some work.

Basic checks showed a poor/non existent spark.
Compressions were OK - not spectacular, but OK, a little down on one cylinder, but nothing to get dizzy about.

Distributor was so worn that we couldn’t realistically set the points.
So, new (electronic) distributor fitted, and she started up instantly.
Timing set, we gave the carbs a quick checkover (balance checked and mixture set.).

Car sounded really good. The valve clearances need to be done - a little noisy on the top end, and a little lumpy running (I suspect some worn carbs).

Oil pressure was a bit low on the gauge, but the oil light wasn’t on (but does function) - all the instrumentation is flakey on this car, and is going to be checked through.

So we go out for a test drive.
Car drove really really well - pulled like a train, and was very smooth.
Overall, I thought, this wouldn’t need much work at all!!!
Thermostat has been removed (temp not coming up)

Then, a tinkling sound - sounded like a pulley coming loose.
Pulled over in a garage, engine idling, and “clunk - tinkle” - it spat out number 4 (I think) main bearing cap through the side of the sump, complete with the Bearing cap bolts.

What the hell?

Bearing cap bolts haven’t pulled out - the witness marks on them look like that have been coming undone for a long time, until the crank has caught the cap and spat it out, and left a slight gall mark on the very last thread of one of the cap bolts.
No signs of overheating on the cap (but very obvious signs of the collision that pushed it out the side)

I have never seen this before - ever.

Has anyone out there seen it.

I initially thought that the lock tabs had been omitted (last jag engine I built was a 3.4 in an XK120, and they had lock tabs originally). But it seems like the S1 4.2 didn’t have lock tabs, am I right?

The car is otherwise sound - but the rear of the engine is suspiciously dry - I suspect that a rear oil seal conversion may have been done, and this is perhaps when the cap wasn’t tightened properly.

I’m pretty sure I can fix it - but this failure is a new one on me.

Anyone else seen it?

I’ll post up some heartbreaking pics when I get it up on the ramp…

2

S1 E-type? I think all the E’s had them. I think the XJs had them also although they lacked cotter pins on the big end bearing caps. If they were missing here and they’re still original bolts it sounds like they’ve been removed before and put back without tab washers. The aftermarket ARP bolts don’t use lock tabs. I think they use dished washers.

3

Sorry, my bad, I should have said, It’s an XJ6 S1 4.2.

Definitely not ARP bolts - these look original.
So, it should have tabs fitted?
Mmmmm definitely no tabs in here.
(The bolts were still in the main cap when it exited the engine!)

This car has had lots of the right kind of mods fitted.
Larger fan, matching shroud and a viscous coupling.
Manual choke conversion on the SU’s.
So someone has been loving it.
but the classic Madeira mod of removing the thermostat hints of a less than capable mechanic being in there.

The lack of ANY oil leaks leads me to think that the rear seal has been upgraded.
I’ve never seen an original straight six XK jag engine with a rope seal that was completely and utterly dry. So it’s possible / likely that this is when the problem was caused.

Still - it’s something new for me - never seen it before.

4

I’ve never done the measurements but I wonder if there is actually enough room for them to fall before being caught by the pan baffle? One bolt might have been quietly resting there for short while until the second came loose and the cap then fell and got snagged by a lobe. I’m sure it made quite the mess.

I’ll have to let some else chime in on the existence of tab washers as I’ve never owned one of those. It sounds like it might have been a rebuild error.

5

Oh yeah, I forgot about the baffle. This sounds about right.
TBH, it’s been a really hectic and strange day!

First thing this morning, we met up with the recovery lorry to pick up a customers land rover (for full rebuild after a hefty crash), and it’s an utter nightmare - the shell is like lace curtains - we need another body shell!

Another customers VW that we were driving to bed in the freshly rebuilt engine started smelling strongly of petrol, and we can’t for the life of us find the leak!

So, after catching some fresh breath and moving onto the XJ6, we got it running beautifully - very happy, was going so well, then clunk… and another trip on the ruddy recovery lorry (with a 90 minute wait in the baking sun!)

I think I’ll go to bed now!

6

Not in all my years: one time I did find it, the owner torqued the caps (TR4) to 105…inch-pounds

:roll_eyes:

1 Like
7

That is a definite possibility here. Everyone is a “mechanic…”

I was building up one of my TD5’s for a customer - we tend to sell to customers who go heavy offroad (and with big turbos etc,.) or where the engines see heavy duty use (local fire brigade, for example). So I take extra care to check the clearances, torques, fastener quality etc,.

A local mechanic came in looking for a part ( a connector I think - I always seem to keep weird connectors to hand), and he couldn’t believe that we had the workshop manual out, and were torquing up the mains and rods to spec, and putting a little dot of paint to signify we had tightened it (or to keep track of the number of times we had tightened a bolt by 90 degrees, for example. It was completely alien to him - he just did them up until they felt right…

At the same time, I had a little 4 pot VW engine apart on the bench for full rebuild - the block was cleaned, honed and ready, and I had been checking and balancing the old rods and new pistons. He thought I was insane when I was measuring and balancing the weights of a bottom end. He didnt think it mattered. (in this case, someone had been there before me and substituted a piston and rod from another engine) fixing the 4 gram imbalance made a huge difference to the engine when we started it up yesterday.

Ahh, island life!

8

This page from the spare parts catalogue would seem to indicate that, no, they did not have locking tabs on the main bearing cap bolts, just the C28082 washers.

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Interesting to note that the C2262 bolt is the same as used in the original XK120 and Mark VII XK-type engines.

9

Yeah, that’s confusing.
The early XJ6’s use the XK120 bolts, but (according to the parts list) with no lock washer.
A specialist friend in the UK said that the early XJ’s should have lock tabs unless you use different (higher tensile) bolts.
I suspect the parts list might be wrong (wouldn’t be the first time!)
I’ll probably fit ARP bolts - we will see what I find when I break it down.

10

I’ve never seen a properly torqued main cap come loose or otherwise fail on anything. They are soooo lightly loaded in a seven main bearing Jag 6 there’s no reason to ever see one to creep or stretch to plastic deformation. 7 and 8 litre Chevrolet big blocks use bolts virtually the same size and get by with just 10 of them.

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11

Lock tabs would not have prevented this. Unless the crankshaft was bent (hard to tell at this point) these bolts were improperly installed. I can come up with better engineering arguments for lock tabs as an origin of failure than prevention.

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12

Lock tabs are mild steel and the set-to torque crushes it - but Jaguar used them for years. You should get a set of washers (not locking) from ARP and use those. They don’t crush. As said the whole purpose of torquing to a specific value is to prevent the bolts from loosening.

13

I agree about improper installation - I’ve asked around, and the only cases of this happening are incorrect installation - I’ve never come across it before.

As an old mechanic, Marsden, told me “Do what it says in the book, and you can’t really go wrong. Unless the book is wrong”.
For this reason, I tend to stick to what the manufacturer recommended.
If Jag used Tabs, I’ll use them with the standard bolts.
These old 6 cylinders don’t have these bolts coming undone as a norm.
Interesting that the early XJ’s used the same bolts, but don’t list the tabs.

1 Like
14

There is a curious advantage to the lock tabs, that is visual inspection. The factory line inspector could look at them and know that the installer up the line had tightened them before he bent the tabs.
But you should only use them once, because they become too weak from rebending, and should be thrown out and use new.
One may presume that the factory went away from them as an assembly time savings, perhaps when they got automatic tightening tools, reducing the chance of an error with just a flat washer.

15

I’ve seen to many cotter keys in the sump, have even seen a couple in the filter housing, to ever use them in any engine that I rebuilt, and besides that how can you get an ACCURATE torque setting on the nuts?

1 Like
16

Sorry, I was not intending to start up an old argument. The subject was mains without locking tabs, and in the case at hand, somebody didn’t use the tabs, and somebody didn’t torque the bolts right, and somebody failed to notice that the bolts weren’t torqued right and somebody put the sump on and somebody said the car was all set to go.

17

Occam’s Razor. Likely what happened.