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XJ6 & XJ12 Series I, II & III 1968-1992

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Old 01-02-2012, 10:08 AM
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Default Questions about the 4.2 XK inline 6 engine

Morning all.... I've a few questions about the 4.2 XK 6 cyl engine

I have a 69 E-type, I'm going to rebuild the originial motor, in the interim, I'm looking for a later model XK engine to drop in so I can drive it....

1) what is the best year engine to use that is
a) plug and play (i have an automatic)
b) can be hotrod's later after I take it out....

Are some years better than others? bigger valves? better flowing heads?

Are all 4.2 6's bellhousing mounts the same? can I use a later automatic in my 69 E-type?

If I wanted to use this motor full time, is there an advantage over a later one than my original 69 motor?
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Old 01-02-2012, 01:54 PM
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Quote:
If I wanted to use this motor full time, is there an advantage over a later one than my original 69 motor?
I read this as asking if a later XK engine to your '69 one gives any advantage. Regrettably I have to say - Not Much !!

In 1969, the E-type was only a couple of years away from it getting the V12 engine. The 4.2 engine carried on for many, (too many !!), years in the saloons, but a crucial change was made by Jaguar in the 70s which was, frankly, a complete disaster, (I had to suffer its consuquences in the late 80s).

Intended to "strengthen" the engine, (Ha Ha !!), the change involved the lengthening of the cylinder head studs so they screwed into the bottom of the engine block, instead of screwing into tapped holes in the top of the block, as per your original engine and how it had been since 1948.

This modification essentially ruined a fine engine by making it prone to cracking between the bores (1-2, 2-3, 4-5, and 5-6) and was not put right until certain mods were done to eliminate block cracking in the the 80s. Way, Way too late - Jaguar were almost bankrupt at that point.

The main problem was the Jaguar Car Company was run on an absolute shoe string. Even the machinery used to make the XK engine was secondhand when it was introduced in 1948 in the XK120, and was in use until the 80s !! So the engineers could be aware of the correct solution to a problem, but there was no money for anything but a spatchcock.

Things were reasonably OK quality-wise until the 70s when Great Britain entered its decade of complete madness. Believe me, this is true; I was there at the time. Quality control and build quality plummeted under the British Leyland regime. Essentially the unions ran the whole country, (badly). Anyway, I digress.

Getting back to your question......

Remember that Jaguar won several Le Mans victories with basically the same engine as in your car, (3.8 rather than 4.2), but with one crucial difference, the valves on the racers were larger. These larger valves were eventually introduced to production cars in the XJ Series 3 saloons. So, if you want to improve power, you need to try and get hold of an XK Series 3 cylinder head. From that point you can start on all the tuning options to increase power. These involve camshafts, carbs (or fuel injection), etc etc. With a few minor items, the Series 3 head is backwardly compatible with your '69 4.2 engine block. Do check head depth for minimum value when buying as these heads could have been skimmed a few times. Of course, you can apply tuning to your existing head, but the brething will always be more restricted.

If you want a reliable plug-n-play interim engine with an auto box, the very latest XK engine you can get is best as it will have the final block mods. These will be from Series 3 saloons from about 1983 onwards. Keep the transmission, as it is the Borg-Warner 66, which is pretty reliable and long lived. However if you want a later box with more gears, you will need a new bellhousing. There are plenty of suppliers of this sort of thing on the internet, but they are not usually cheap. Other alternative is to find and engine and gearbox from a 60s saloon, but I suspect there are not many of these around now.

BTW, if you want to fit the V12, you will have to change an awful lot of stuff, even the front engine cage is different. A lot of mods were necessary to fit the V12. Of course, if you have the money..............
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Old 04-17-2012, 05:06 PM
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Default Engine block question

How interesting. Do you know what engine number is the beginning of the disastrous block modification you mentioned. Now you've got me worried about my '70 XKE. I converted to triple SU's a long time ago and it's always been a good runner. Have I just been lucky ? Thanks !
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Old 04-18-2012, 11:51 AM
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Does your block number end in "7L" or "8L". This may be part of the engine number, but I can't remember as it's too long ago. The 7L blocks are less prone to cracking. If you haven't had any head gasket failures then your engine is either a short stud engine, or the block was correctly aged before machining, so has not cracked.

Has it got a row of core plugs along each side of the block - that normally would denote a long stud engine too, the stud, (about 12" long) is visible once the core plug is removed.

Good news is your engine is just before the 1970s chaos started !! You have done the right modification by converting to SUs, the engine was never much good with the Strombergs, and emissions plumbing. Strangled
more like !

If you want a good read, try this: -

The Jaguar XK Engine / AJ6 Engineering
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Old 04-20-2012, 11:01 PM
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Default '70 Willow Green XKE Roadster

Excuse the delay in responding.

Many thanks for the insight. My engine number is 7R 9071-9 so as you suggested it looks like I'm okay. The switch to the triple SU's was a night and day difference in performance. Getting them optimally balanced was easily an afternoon's work.

I've had more than one jag aficionados (read Brits) comment on how strong the engine is. I'm wondering if it has to do with the fact the engine had to be line bored. The boring work was done at a class A machine shop.

I bought the car when I was a wee 17 from a guy that wanted nothing more to do with it. Evidently an independent shop was doing routine service and advised him of a leaking rear main seal. When he picked up the car he was a bit shocked at the cost of replacing the seal and things only got worse. Twenty miles down the road the rear main bearing cap came off rather dramatically. The shop denied responsibility and he denied payment and insisted they fix it. The car sat is the shop's backyard for over a year accruing storage charges. Through a friend of a friend I learned what the storage cost amounted to and made an offer never expecting anything to come of it. It was dirt cheap but it was all the money I had in the world at the time. Imagine a 17 year old buying his all time favorite car. Long before I got it running again I would just sit in it in pure ecstasy. Don't ask how long ago that was - fire was still a mystery.

Thanks again for the insight and that great link. Cheers mate !
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Old 04-21-2012, 09:29 AM
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Inyeresting story Steve, just one question, what fire?
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Old 04-21-2012, 10:03 AM
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The ultimate incarnation of the XK 4.2 is the very last, Fraser has already covered this.

The cracking between bores was solved with a fillet or slot between each so expansion could take place, even stresses in between the bores.

The cylinder heads are big valve.

I'd suggest hunting for a series 3 XJ 4.2 head and block, and dress it up as the 7R unit by using the sump/pickup and cam covers (if plain, not stripey) Although by 69 Jaguar used stripey cam covers similar to the XJ so no worries there. Also the HD8's.
It'd be a major improvement if you thought about going manual while the engine is out, maybe hunt for a manual gearbox, or do you prefer the auto setup?
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Old 04-22-2012, 03:46 AM
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Interesting story Steve, just one question, what fire?


Its discovery. Tying pointed rocks on the end of sticks was our tech bubble.
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Old 04-22-2012, 09:37 AM
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Old 04-22-2012, 09:37 AM
 
 
 
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1968, 42, block, compatibility, cyl, earlier, engine, fit, foru, heads, jaguar, jagur, type, xj6, xke



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