S2 or S3 engine?

Dear All,
I have just bought a XJ6 series 2, automatic 4.2
The cylinders suffer ovalization (twice overheated, two pistons “clogged”-seizure?, according to the seller, who forgot to mention it).
Some specialists suggest that the problems in the Series 2 block was fixed in the Series 3 (improved design, cooling…), so they recommend me to install a S3 rebuilt engine.
Other specialists believe the S2 is even more reliable than the S3 and I should repair mine (machinning and installation of oversized pistons).
Hard decision…
I’ve found this article: http://www.jagtas.org.au/torque/techtorque/fighting-the-flaw-in-the-4-2-engine-block/

Please, feel free to comment.
Thanks and Happy New Year!


Once again I see that a private seller was not completely honest, now you are left to pick up the pieces.

Many cars have had design faults - and by now virtually all have been fully sorted, or the most awful examples have expired and been eliminated. Yours is a wildcard because at best the engine is just a tired but good core. However “twice overheated” would not usually be the basis of an engine I would want to fully rebuild. That said, I cannot image there are very many 1978 4.2 Series 2 engines in your area of Spain. In the last 40 years between my engine projects and my friends engine projects I’m sure that more than 50 engines have been brought back to life. For me it is a simple question of how much machine work is needed. I am okay with a little machining, but if all the pre-machining specs are too far out, then I look for another better used engine to rebuild. For example, I generally do not like to machine more than .030" oversize on the cylinder bores - except in the case of really old blocks that were cast with very thick cylinder walls.

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Thanks for this!
The main issue is whether I should go ahead with the machinning of the block and probably crankpins, plus head, timing… or if a S3 will offer more guarantee for its better design.
New to the Jags and reading and listening to different opinions, I want to make sure I take a good decision.
No problem is I order something from other country. That’s the smallest incovenience now.

No author for the Tasmanian thing. I can imagine why, as there is some nonsense in it. I wouldn’t put my name to it.

“Super cooled” fluid: ooh ohh! Dramatic!
“Sudden thermal shock” at 88/0C (never heard of the bypass plumbing?)
Long studs leading to unpredictable clamping (the opposite is true).

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Hi Peter - I completely agree. That erroneous document is the source of why the 72 XJ6 I’m working on came to me with thermostat diy drilled but with no bypass block-off valve. I found a high-lighted copy of that article in the paperwork. The PO obviously had pulled the head (to replace the gasket), saw the 7L block cracks and then took that semi-nonsense to heart. When I first drove it it took ages to reach operating temperature and then it started to overheat when it finally did cause the bypass was open. Can’t run these motors without the proper thermostat (unless they are modified).

If anyone hasn’t stumbled across that article before, the crazy advice about modifying the thermostat is in the 3rd paragraph from the end.

I will say though, that it helped me get a good deal on the car - so maybe I should thank that anonymous author :slight_smile:

Inaki - If the block is straight just have top-hat liners installed and the crack issue will be fixed for good. might be a little pricey though.

72 XJ6 (x2)
73 XJ6
85 XJ6
84 XJS

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frankly, this is a bit like telling us you have a serious cold with a cryptical diagnosis from your physician and asking whether it is better to wait for recovery or take antibiotics instead. No offence intended, please, hope you get my point:slight_smile:
Without any more concrete information it is hard to give any particular advice. The problem of quality concerns of SII vs. later SIII engines seems largely overstated, as Mike and Pete have explained. Any decently running engine will keep running for quite some time. In its days the XK engine had a good reputation for longevity. Its main enemies are lack of use, lack of antifreeze, long term full throttle and lack of warming up periods (we’re talking almost 9 litres of oil, 12 litres of water and 300 kg of steel and aluminum expanding at different rates). If you avoid them chances are you will not survive a good XK engine, if you run it less than 3,000 km per year.

Apart from this you have to make a decision between rebuilding your engine and buying another one at some point. Both alternatives are fraught with uncertainty: Do you have a rebuilder you can trust? How far is his analysis? Does he give you a list of what he will do and what he quotes for it? Will he stick to the quote he gives you before? On the other hand: which engines are available to you? How much will they cost you including shipment and installation works? How much do you know about them and their quality? Did you see and hear them running? Oil pressure? Compression? Leaks?

Only if you have concrete options it makes sense to compare or opine … wait, one abstract thing maybe: Some people care about originality, so if your “original” engine is truly original it might be a certain value as of itself to keep it.

BTW, did you get my PM?

Good luck


75 XJ6L 4.2 auto (UK spec)

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How much of the eventual damage has been verified, Inaki - and at what degree of dismantling has been done…?

The say-so of others must be verified by your own examination and checks. Ovality is a ‘natural’ occurrence, but fitting the pistons the ‘wrong way around’ may enhance ovality wear. But overheating itself is not a normal cause of ovality - adding that with ovality exceeding 0,15 mm reboring is 'recommended…

So the amount of ovality measured is one input for a rebore - but if excessive; cylinder lining, when fitted, replacement is another option…

The redesign of the SIII basically related to better block/head cooling, but earlier versions wasn’t all that bad in that respect. A point here when going to a used SIII engine - that one may have it’s own wear, tear and ill treatment concerns as well. The devil you know…

As for cooling; on all xk engines it is very important that the bypass hose is present and open. It allows the coolant to circulate through the block with the thermostat closed during warm-up. If this fails, serious block overheating may occur - with inadequate temp gauge warning…

xj6 85 Sov Europe (UK/NZ)

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“Rebuilt” by who, I wonder?

I also wonder if the “improved design” is in reference to the slotted blocks of the Series III? If so, I’m not sure that this was the be-all and end-all of improvements…but I’m not an authority on the subject.

The Series III engines themselves were a bit notorious for head gasket problems. Many years ago there was a discussion that this came down to the gasket itself…with Payen redesigning the gasket at some point. I don’t recall if the was speculation or known fact.

Anyhow, I’m not sure that going to a Series III engine, simply because it has some improvements, guarantees happiness.

Repair existing Series II engine (which will end up being tantamount to a full overhaul, I reckon) or install a rebuilt Series III engine?

At the end of the day I speculate that success and reliability isn’t a matter of Series II versus Series III design changes but, rather, will depend very heavily on the quality of the work being performed and the quality of parts/materials being used.


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I sure second that, Doug…

xj6 85 Sov Europe (UK/NZ)

Dear All,

I appreciate very much all your comments and I take every detail into consideration, so thanks a lot.
The idea of replacing my engine came from my own mechanic (who btw, has rebuilt lots of classic and modern engines). He proposed that because he noticed the ovalization in the cylinders and the two brown decolourated areas in the block iwo the cylinders 3-4. I wish I could upload a pic, but not priviledges yet.
His suggestion was based on the idea that the engine could be found “rebuilt as new” at a reasonable price, which now we know, it’s not that easy. Otherwise, he suggested to overhaul it.
I have contacted as many suppliers and workshops as I have been able to identify. Regrettably, I don’t know them personally, so it’s hard for me to choose.
Some specialists suggested me the S2 engine, while other ones insisted on going for a S3, despite the non-originality result.
The main reason for the S3 was that, according to two UK-based workshops, the XJ6 S2 cylinder block was a great concern to Jaguar in period, with lots of warranty claims due to heat expansion and contraction between the bores. I was told that Jaguar overcame the problem on the XJ6 S3 with a cylinder block that had reinforcement and expansion slits between the bores. On the other hand, I was also told about the S3 head gasket troubles, as Doug Dwyer suggested.
I also 100% agree with Frank Andersen with regard to the verification of the damages. The problem is that to verify it in detail we have to fully dismount the heads and crankshaft and we haven’t yet. True, that’s the only way to find out the extension of the overhaul needed. However, I am quite sure that the previous seller, who lied me about the condition of the engine, and later “remembered” he had twice two different cylinders “stuck (?)”, which now we can see in the red-brownish block - neglected the proper repair of the car for years (stopped for long time and cheap repairs, according to a former Jaguar mechanic I’ve managed to contact). The heads had a loudy metal noise, which I feel it can related to the timing of the valves (height adjustment) or tappets.
Also, two cylinders “stuck” (is this called “seizure” in English?) could have damaged the crankshaft, so we’ll have to send it to a specialized workshop to measure the deflections and crankpin condition to see if it’s a matter of skimming or machinning.
With this scenario, I still want to know if we should go ahead with the recondition in our workshop once everything is checked, or if I should look for a reconditioned engine (S2 or S3). Today, this option sounds more appealing to me, depending on the price, and considering a workshop specialized in this type of engines.
I’m waiting for some more quotations, while I’m reading as much as I can to decide if I should install liners (some workshops say no, they turn and make the block weaker even), Mahle pistons, electronic distributor and enhanced pumps.
The main positive outcome of all this is that I’m getting a lot of useful information in short time, which is partially thanks to all of you.
I want to get all the info and, as unfortunately every penny counts for me, make a wise decision that could mean having a car is road condition for many years.
Thank you, All and best regards from Santander!


I think that you have the right idea to ask questions and find out what others have done. I think that you are also discovering that there is not just one sure way to decide - everyone has an opinion based on their own experiences. That is the tricky part for you, you don’t have the experience and your engine has not been completely inspected to verify the extent of damage.

Since your mechanic does not have the precision tools to properly measure the wear and damage that means you will need to use the services of an outside specialist. Your mechanic will be of great use for the engine removal and installation.

This means that you will either be shipping your existing engine off for inspection and possible repair, or you will be just ordering in a rebuilt engine.

I would suggest that S2 or S3 will not matter much for long term reliability, because either one when done properly should offer more than 100,000 miles (160,000 kilometers) of trouble free motoring. You may be over thinking the whole process. On the one hand I see that you talk about quality parts and upgrades - but I also notice that you say every penny counts.

If you are honestly thinking of keeping and frequently driving this car for many years to come, then spending more time and money will be well rewarded. If you just want to have fun with this car for a little while then choose the lowest cost way to get a running engine back into this car.

One of the great truths of the old car hobby is that it will become as expensive as you let it. Also true is that spending a lot of money to get the best parts and repairs will not bring you more money upon resale. Nobody cares to pay you back all the money you have spent to keep the old car running, since most buyers of classics do not intend to do much driving. So choose wisely according to how long you expect to own and drive this car.

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Based on what I know—admittedly getting a bit long in the tooth—I concur.

This is the shipwrights disease, Inaki - inevitably leading to a full engine overhaul…:slight_smile:

The initial, and easy checks on an engine misbehaving is compression checks and verifying ign and valve timing. Noises are harder to diagnose - but may be pursued to some extent without excessive work. Only then, with no cause identified, are detailed inspections warranted - unless you want perfection…or you cannot live with the doubt…

Such detailed inspections inevitably requires strip-downs - at which stage the cost of parts replacement is ‘small’ compared to the work involved. Professionals are required to do the ‘right thing’ - cost is not an issue…

There is no way of assessing competence and quality except by the results - which only will show over time. Certainly, other customers feedback is valuable, when available - but cost estimates, or any other proposals, does not necessarily reflect quality of work…

As an aside; if pistons seize the engine stops turning and serious damage usually occurs - the cause is ‘interesting’ simply to ensure against recurrence after repairs. Usually, seizing is a consequence of serious overheating - lack of coolant or oil is typical. Discoloration may be rust or overheating - neither a good sign…

xj6 85 Sov Europe (UK/NZ)

There is also another option, which at the end of the day might be no more expensive, fitting an AJ6 3.6 litre with later 4-speed auto transmission. This does involve plumbing for EFI and some fabrication, but does give a more modern (my, hasn’t time passed?) engine and much much better transmission and lower fuel consumption.


I have. Both a series 2 and a series 3 im not an XK engine expert, but what i do know about the two engines is the series 3 has bigger intake valves. Between the two cars, the series III is more reliable. in my case it has to do with the fuel injection, and the fact the its is an unmolested example 54,000 miles. The series II Is just the opposite. Its a culmination of several cars the series II Is also carbureted, and originally had points, in a completely warn out distributor.
That being said, my post is pretty much worthless.

If it were me i would put the money (spent on a series III XK) into the numbers matching engine. And make the car right.

I find this discussion interesting. Many judgments about reliability and robustness were made whilst the cars were in regular use by the mechanically ignorant with poor and unsophisticated service providers. As perhaps for elsewhere our early owners opted for corner garage maintenance in many instances - we’ve had our Series 1 for 30 years and its log book reflects service at the corner BP for many years. These types were much more familiar with our pushrod Holdens and Fords than DOHC Jaguars and I think en masse the cars suffered. Now we (all of us!) baby them. We’re well informed, alert to the slightest noise or performance wrinkle and often carry out our own work, cognisant of often long histories. The aftermarket has produced some sound improvements and performance initiatives- notably electronic ignition. Our S1 does multiple country runs and usually one trip of several thousand kilometres per year - reliably and comfortably! It has a wonderful engine rebuilt at 141,000 miles, now at 148,000 miles. Agree with Jochen - they must cover considerable ground each year for their own good. Build whichever - SII or SIII- either will be fine - you will ensure that yourself. Paul


Good point, well-made:,even in their day, Jaguars, properly maintained, were every bit as reliable, and in many cases MORE reliable than other makes.

That was the question that got me here to begin with. What would be the best Jaguar engine to put in an S2 XJ6 you don’t want to lump.

The S2/S3 engines were the most developed of all the XK series engines. I don’t exactly know what the commonalities are between the S2 and S3, but the S3 is generally still somewhat plentiful.

generally, the best running one you can find in a rusted out or damaged vehicle
This often means buying the whole vehicle, if you have space, this may be handy for spares, if not, you remove the engine and GB and have the carcass taken to the metal recyclers

also contact your nearest Jag club. There are almost always members who have engines, and sometimes they need to move them on

You do not say if you have any remains of a busted engine, but in general it would be easier to mount a carb S2 engine in a S2 vehicle rather than a S3, as you would or would/may need to add either carbs , or all EFI controls/harness

if possible, do a compression test. At least make sure it rotates, even by hand