I was grazing and found a post that mentioned long vs short studs in connection with a 4.2 engine.
What’s this?
Very broad question. Why not browse some more and then ask about any specific clarification points based on your own learning?
Win-win situation: you take pride in improved awareness and retention, we get to save baby steps and help you with the interesting meaty stuff about how & why, etc.?
Pete
I find the history of the XK engine to be a fascinating tale of development, compromise, lavishness and economy. If there is a book that puts the whole story in words and pictures I have not yet read it – but piecing it together from parts manuals, the shop manual and mostly J-L Forum posts (as Pete seems to be suggesting) has been a great way to learn & well worth the effort.
BTW - I have been told that if you guess at an answer and then learn the facts you will remember the correct answer much better than if you were simply told in the first place (even if you guessed wrong).
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Yea, I know… research. I’ve been on forums for 12+ years and contribute when I know an answer. Often. I owned a 3.8. I now have a 4.2 and just don’t want to some day blunder into chasing down a part that doesn’t fit. OK, head. Coulda been oil pan, water pump, intake, oil pump…
On T-Series MGs, the XPAG engine will have either banana or round water passages, and mixing heads/blocks/gaskets is generally not done, but if one doesn’t know there’s a difference until an expensive, but wrong, part comes in the mail, you might not know to even ask.
Thanks.
Up until sometime in 67 or 68, the 4.2 block changed from short ~6" long head studs that screw into the top of the block, to long ~14" studs that go thru the water jacket into the base of the block.
originally to increase rigidity and head gasket issues, the long studs eventually corrode in the water jacket, and can break before or during removal. This is difficult and expensive to rectify
If you have a 4.2, there was a change of cylinder head and gasket around this time, so the best thing is to check the Parts Manual for your Engine number, and do your research from that point.
Such things as water pumps and thermo housings may change, the Parts Manual makes these things clear
My series II (1970) has the long studs. I found this out the hard way when one broke (rotted through). The long stud version passes through the water jacket to the bottom of the block. The short stud version I believe just attaches to the top of the block. Rumor has it that the long stud version makes for a more rigid block?
Glenn
A pretty good write up of the development of the XK engine can be found here:
Sorry Andrew, I found that article to be very much a ‘curate’s egg’, starting with his opening remarks and extending into what I believe are errors of engineering fact.
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Thanks for that reference (I was not familiar with it before now).
Bishop: “I’m afraid you’ve got a bad egg, Mr Jones”; Curate: “Oh, no, my Lord, I assure you that parts of it are excellent!”
“True Humility” by George du Maurier, originally published in Punch , 9 November 1895.
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Thank YOU George - that is a wonderfully evocative drawing/engraving. You can almost smell the hydrogen sulfide. As a former colorblind cartoonist (and currently still colorblind) I always stuck to black ink line work, of which that is a superb exemplar. “More tea Vicar?”
So Peter which bits of the Tasmanian egg are worth eating, and which left behind?
“At best the 4.2 litre Jaguar/Daimler engine block could be described as suspect…”
I think that’s a bit jaundiced, frankly. It’s a value judgement not a data point. I can think of many better adjectives every bit as justified. A lot of car makers would regard a circa 30-year run as a success or proven track record. Not perfect, of course, but suspect? At best? Almost everything that follows is a result of owner neglect or his misunderstanding, not a fundamental design flaw. Now you could argue (and I would agree) that placing too high a reliance on high standards of coolant maintenance is asking for trouble after, say, ten years, but that’s pretty much all they expected, yet thousands are still running after 50.
" These studs are also twice as long as the pre-1968 blocks, allowing more stretch and therefore more variation in clamping pressure against what has been set with the torque wrench and this makes it more prone to head gasket problems. "
I will defer to our engineers, but in my understanding this is almost diametrically opposite to the truth. A longer stud gives more stable clamping pressure under variable conditions and is a key benefit both in clamping force terms and in force distribution terms through the block.
" The water pump outlet is positioned so that when the thermostat first opens, super cooled fluid is pumped mainly down the exhaust side of the block. The inlet side with its much less efficient circulation, remains closer to the thermostat temperature. This means that you could possibly have 0 degrees C fluid entering the exhaust side of the block and 88 degrees C fluid (fuel injected cars) leaving the inlet side. "
I have to go to bullets as this will take too long:
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by super cooled I assume he means ambient. since with neither evaporation nor refrigeration going on, there’s no means of getting the radiator contents below ambient.
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The cooling system is designed around pressure differentials and engineered to exploit them. This is not ‘inefficiency’.
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The bypass principle ensures water circulation and even-ish engine temps pre thermostat opening.
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The thermostat is not a 100% on/off binary switch. Apart from the bleed hole leakage and gradual opening, neither the main orifice nor bypass passage are often hermetically sealed, especially with the sleeve design. So there is gradual change and even when you disregard the first 4.2s bypassing directly into the cool low-pressure radiator tank (super-cooled my arse) the other versions still don’t suffer from the posited 0-88C thermal shock.
There was other stuff I think, but I lost interest. I suppose he forgot that pointing his finger at a suspect component left three fingers pointing back at himself…
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Precisely. When I read that, I was ready to respond before I read further to see that you did. The longer the stud, the smaller the influence of length change upon tension. Also true for thinner versus thicker studs. We went through all of this in a thread with Alex Cannera (sp?) about 10 years ago.
I’ve been doing a lot of runs form cold to fans on recently while I fine tuned my otter switch fans-on-temperature and the length of time the fans run after the ignition is turned off (variable time relay). What you say is entirely true. The small tube to the radiator pushed a lot of water into the radiator and consequently a lot of water out of the radiator into the block. I’ve observed the temperature at the inlet water neck of the radiator continuously from ambient temp to 190F and I’ve seen no huge spike in temperature hat would occur if the T-stat just “suddenly” opened. Also, at the point my T-stat opens the lower water neck is at about 118F.
That’s a long way of saying “Your Right”.
Thanks Pete that’s very helpful.
Can you shed any light on why Jaguar changed to the long stud block/
‘Why?’ questions are potentially endless, as parents of inquisitive infants know. I can guess at reasons the same as anyone else, but only for fun because it’s a hobby forum, not a professional society…
By process of elimination we can probably rule out reasons like:
Refinement (no change)
Performance (no effect)
Styling/appearance (invisible)
Economy (no effect)
Emissions (no effecf)
Cost (probably negative in short term)
There remain perhaps two other parameters where one could envisage a need that was addressed by the change: Durability and Reliability. These are related but different and of course can affect or be affected by some of the above criteria, such as performance, emissions or cost (if only indirect warranty costs).
We know the trend at the time was towards emissions reduction and that would suggest leaner running, catalysts, retarded ignition and higher thermal loads.
We also know that even though Jags were not really sold in the US on price, there were stirrings around fuel economy, eventually surfacing as CAFE regulations. Add high sales in hot states where A/C was always fitted and you probably start to get heat-related reliability Issues, manifesting as moving tappet guides in the XK and dropped valve seats in the V12 - neither of which were serious issues in temperate markets.
Dowel stud(s) notwithstanding, an alloy head/iron block combo will always stress the head joint and its fasteners in two planes: horizontally by fretting and vertically by stretching. The higher the temp the greater the differential expansion effect. After enough cycles of sufficient amplitude I’d guess some loss of joint integrity would be inevitable. Long studs would provide improved resilience against gasket failure and block burning/ cracking.
The original 4.2s had twin slots cut horizontally between adjacent bores prior to liner fitment, thereby providing some coolant flow at the hottest part of the bores near the top. Once chunks of rust or mineral deposits began to form, these narrow passages would be where deposits would have the most severe narrowing effect. A passage 1cm wide is hardly troubled by a deposit layer 1mm thick, whereas the same deposit each side of a narrow 2mm slit would block it completely (being a gas man you’ll recognise the analogy with respiratory epithelium and the turbinates). The long studs also gave scope for more generous opening up around the top of the bores without distorting or compromising the previous highly-stressed block deck stud attachments. The crank saddle at the base of the block, by contrast, is both the heaviest, most rigid part of the casting and long studs put the rest in compression - which suits cast iron better than tension.
Enough blather - it’s time for a coffee.
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is it known if the short-stud versions had a major head gasket issue?
did the long stud versions prove less prone…back in the day ?
they would not have been considering what would happen 20yrs down the track
I have heard that nowadays, engine builders prefer the short stud version
of course the alloy head/ iron block junction is an issue
most XJ motors I have dismantled also had bore wear and considerable lip on especially the rear cylinder
Iron motors I am familiar with, such as AMC/Jeep I6, seen them with 300,000miles, never been apart, no bore ridges at all, only light honing needed
presumably those are the sort of issue refered to as “engineering flaws” in the mentioned article
even that 4.0 motor which is reknowned for reliability is prone to overheat, as the cooling system capacity is a bit marginal, and the later heads crack
It’s always nice to involve the elderly in conversations. Makes them feel wanted.
WRGT (with regard to) the article about 4.2 cylinder block cracks, what are the particular errors?
We find that once the sleeves are removed cracks spreading up from the water passages to the deck are pretty common, and so we routinely install top hat liners.
I agree the cracks happen. From memory I saw one once but don’t recall whose engine. It was a hidden-slot block.
I’ve done only 4.2 engines since, of both types, and have not yet seen a crack again. I’ve done two more this year - both short stud - no cracks and have two long stud ones left to do, of which my 4.7 slotted block is fine. I have had to do one snapped stud so far, although I haven’t taken the core plugs out of one remaining early long stud block yet. Who knows what might lurk in there? To me, the biggest PITA is the occasional stuck head on long stud engines, but I didn’t notice he mentioned that, at least as far as I read…
I’ve found that once sleeves are removed on hidden-slot engines, the two small passageways are invariably crudded up. I think that’s the reason for any eventual cracking - poor maintenance, not a fundamental design error (although we could get into semantics about designed-in tolerance to abuse/neglect etc.) I agree flanged liners are the fix, although the base of the block overbore should ideally be very slightly deeper at the outer edge, to make sure the flange is gripped hard up against the sleeve. I have heard of flanges cracking if the recess is done badly, allowing the flange t be griped at the outer edge, which encourages bending/cracking under load.