Main bearing clearance?

[Hugo]

I just came across an article which lists the main bearing clearance for the Jag 4.2 engine. In a footnote it says that when fitted to the E Type, the clearance should be half a thou more on every bearing except the front one.
WTF is that all about?

[Geoff Green]

What article? Author, Title, page, publication number if a magazine. Hope you are not reading dribble.

Factory publications are best for repair information.

[Hugo]

It's actually a CD of a Mitchell manual. The same one that gives the later valve clearance as .012" inlet and .014" exhaust!
This seems such a weird thing to invent though. Not that I'm bothered about it - Formhalls did the machining and they work to their specs anyway - in any case what's half a thou between friends?

[christopher storey]

It sounds like nonsense to me . The main bearing clearance given in the factory manual for both 3.8 and 4.2 for all the main bearings is .0025" to .0042" , so that there is a 1.7 thou tolerance allowed anyway. What I think may have confused the author of the Mitchell manual ( never heard of it incidentally ) is that bearings 1,3 and 5 are dealing with a crankshaft journal diameter of 2.750 to 2.7505" diameter, whereas the intermediate mains, i.e. nos 2&4 , have a diameter of 2.7495 to 2.750" so that there is up to 1 thou difference in the clearance between 1, 3 and 5 and that of 2 and 4. Certainly, there should be no difference between front main and all the rest .

[mgcjag]

Told you already that the Mitchel manuals are poor second to a Haynes and full of inacurate info when you quoted the valve gaps as 12 & 14 ....keep useing it at your peril

[Hugo]

Told you already that the Mitchel manuals are poor second to a Haynes and full of inacurate info when you quoted the valve gaps as 12 & 14 ....keep useing it at your peril

I'm not using it, and thank you for your concern. It's just something I came across - in fact I have it on a CD listed as "Factory workshop manual". This struck me as not the sort of thing you would make up. It must have a basis somewhere.

[Hugo]

It sounds like nonsense to me . The main bearing clearance given in the factory manual for both 3.8 and 4.2 for all the main bearings is .0025" to .0042" , so that there is a 1.7 thou tolerance allowed anyway. What I think may have confused the author of the Mitchell manual ( never heard of it incidentally ) is that bearings 1,3 and 5 are dealing with a crankshaft journal diameter of 2.750 to 2.7505" diameter, whereas the intermediate mains, i.e. nos 2&4 , have a diameter of 2.7495 to 2.750" so that there is up to 1 thou difference in the clearance between 1, 3 and 5 and that of 2 and 4. Certainly, there should be no difference between front main and all the rest .

The manual I have on CD gives the smaller diameter for journals 2, 3, 5 and 6. You say it's 2 & 4 that are smaller. I'm sure your sources are more reliable, but I have a suspicion that you may have mis-quoted them here, and that my numbers might actually be correct?
I'm still puzzled as to why they would specify different journal sizes for different locations - assuming it is even possible to grind a journal to within half a thou.
Also, my manual says this applies to the XKE, without mentioning other variants. Elsewhere it cites slightly different piston-ring gaps for the Mk X, 420 and XJ6. Now that I've turned my attention to the engine, I keep coming across little oddities like this - and the later valve clearances still remain a mystery.
I'm amazed that the pistons only have just over a thou of clearance (.0011" - .0017"). I would have bet money they would seize up when they get warm. Mine are very snug in the bores, but everything seems to go round & round and up & down ok. Let's hope it stays that way

[Geoff Green]

Hugo,

Why keep looking (and finding) information that causes questions? The factory made a manual with all the information for maintaining a fast car that can be driven aggressively without concern something will break. It's only when you delve into the witchcraft of extreme conditions that some of the odd information can be used with the remainder tossed over the hedge.

Changes in main bearing clearance from factory as an example would be done with different amounts on different journals to counter harmonic vibration bending the crank at secondary harmonics due to running at the causal RPM for extended periods - offshore boat racing - or due to excessive load from valve train modifications.

You will never encounter any conditions outside the engineering by the factory. I misplaced all my Mitchel manuals years ago.

Best regards,

Geoff

[christopher storey]

Hugo, you are quite correct : the larger journal diameter applies to 1,4 & 7, and the smaller to 2,3,5, and 6. ( For some reason I was thinking of a 4 cylinder engine!! ) However, the principle of what I stated is still the same - there is nothing that applies to the front bearing only, which is what your manual apparently states

[Hugo]

Hugo,

Why keep looking (and finding) information that causes questions? The factory made a manual with all the information for maintaining a fast car that can be driven aggressively without concern something will break. It's only when you delve into the witchcraft of extreme conditions that some of the odd information can be used with the remainder tossed over the hedge.
Changes in main bearing clearance from factory as an example would be done with different amounts on different journals to counter harmonic vibration bending the crank at secondary harmonics due to running at the causal RPM for extended periods - offshore boat racing - or due to excessive load from valve train modifications.
You will never encounter any conditions outside the engineering by the factory. I misplaced all my Mitchel manuals years ago.
Best regards,
Geoff

I wasn't actually looking for anything! Actually I was trying to find out which way round the con-rods went (yes I know it was mentioned on here recently, but this site chose that precise moment to go down). (I put them all in there with the keyways on the exhaust side - now tell me that's wrong!)
I have a CD that describes itself as a factory workshop manual, but which turns out to be a Mitchell manual (which I'd never even heard of till the other day). I've had it for a while but never actually used it till now. It was there that I stumbled across this detail about the main bearings having clearances specific to the XKE, and about the piston rings having different gaps for different applications.
Having found that info, I'm intrigued by it.
I don't buy your theory about harmonic vibration - we're talking half a thou here. And why would the engine vibrate in an XKE but not in any other Jaguar?
I suppose I should get myself a proper factory manual one of these days.

[Hugo]

Hugo, you are quite correct : the larger journal diameter applies to 1,4 & 7, and the smaller to 2,3,5, and 6. ( For some reason I was thinking of a 4 cylinder engine!! ) However, the principle of what I stated is still the same - there is nothing that applies to the front bearing only, which is what your manual apparently states

Just to clear this up - the Mitchell manual does actually state, in its own rather confused way, that it is the wider bearings (1 4 and 7) that have the tighter clearance, and the narower ones in between that have an extra half a thou clearance, for what that is worth. You have to read it a few times to decipher that this is what they mean. They only apply this to E Types, saying nothing about the other variants, but I suspect that may be just an omission on their part.

[Geoff Green]

Hugo,

The factory manual is on site: ***The E-Type Knowledge Base ***

Some times you go off into left field and spend time there. I know I have the choice to read or not. You do bring good info also like the head studs.

Keep us informed on your rebuild.

Best regards,

Geoff

[Hugo]

Haha 'left field' - I like it. I just don't like things that don't make sense - it'll bug me night and day! In this particular case, it seems the info I was quoting was actually correct (that the journals are different sizes depending on position). Still no idea why though!
Believe it or not, I didn't even know that the factory manual was available on here! It never occurred to me to look. I've practically got my engine back together now apart from the head, so I'll take a peek to see if I've got anything wrong!
I must admit I did find a few odd things about the engine assembly - like you can't fit the rear oil seal housing once the crank is in place, and the distributor drive won't go in the hole without fouling the front main bearing cap, but apart from that it's all going well. Except I don't know why there's only one dowel stud instead of two to locate the head?
Pistons are 9:1 +.020", crank -.010", Rob Beere uprated oil pump, new valves, tappet guide hold-down kit - did you know SNGB only supply half a set, i.e. three plates, in their kit? That reminds me I must call them to get the other half.
As regards the rest of the car, I have FINALLY got the bonnet back on (after a complete restoration of the engine bay - including changing from the original horrid cow-sh!t colour to bright red!)). All I have to do now is get it to fit & get the gaps right! My instinct is to sit the engine in the frame before doing the final fitting, in case it causes the alignment to shift.
I would post some pics but I have no clue how to do it - I'm not very good with computers.
And I'm off to Florida tomorrow so not much is going to happen for the next few weeks. Any car thieves reading this and getting ideas - my E Type is not at my home address

[PeterCrespin]

...did you know SNGB only supply half a set, i.e. three plates, in their kit? That reminds me I must call them to get the other half.

It's a full set for all normal purposes. To the extent that UK-climate E-Types ever risk a tappet coming loose (low probability), it almost always affects the exhaust side. Hence only really those three plates are necessary.

The problem is mostly an XJ 'hot state' American phenomenon from the late 70s-1980s, when they started running hotter with retard distributors, weak mixtures etc. to pass emissions. Sometimes it happens after an overheating episode due to poor maintenance or bad luck with a hose burst etc.

[Hugo]

Thank you - that did cross my mind as a pssoible reason. Other suppliers include six plates however. This is not a mod I would normally bother with, but since the head is already off and dismantled .....