4.2 engine block variations

Hi ,

I’m a bit confused regarding 1960s 4.2 engines , there’s a lot of expertise on here ,so hopefully some clarity !

I know that early ones had short head studs ,like 3.8 etc.& their cylinder heads were slightly shorter than later .

What engine numbers did the longer studs come in ? & are there visible external features on an engine that will tell you ?

Did the 2 extra rear water holes come in later than the long studs or same time ?

I have seen a May 1967 420 sedan engine ( 7F 78** ) which has the slight extension on the rear of the head & has a large plate with 7 screws on the rear of the block.

I have seen a straight port head which has the extension at the rear , but no extra water holes. Possibly from a 240 or 340 ? Guess all 240/340 had this ?

I have seen the following engine numbers as being when the extra water holes came in , not sure how accurate.
420G 7D 58882
420 7F 11173
E type 7R 1915
E type 7R 35389
XJ6 7L 1176

Presumably at any one time , the 4.2 blocks in all the models , 420G,420, E type , Xj6 were the same ?

Interested to get some feedback , thanks in advance !

All long stud blocks have five core plugs along the exhaust side for a start, and access to studs on the opposite side too.

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I have only seen this head fitted on late 240s not 340s, 240s were in production up to
march 69 so well into XJ production. The early production 68 XJs might have been fitted with short stud blocks but not sure . Same year models same blocks I would venture…
The block casting numbers are more informative than engine numbers.
The May `67 420 engine you mention also has the central engine mount bosses cast in,
but not drilled and tapped I would guess.
Peter B

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Thanks guys , interesting info , yes , the May 67 420 engine has the bosses ,& neither drilled or tapped. It only has 3 large core plugs + 1 threaded bung at inlet side of block.

So did early 240/340 heads not have the small extension at the rear end ?

Interesting to see what other info turns up on older 4.2 !

Hi to All :+1: - I am new to Jag lovers membership and really enjoying reading posts - I am also interested in the short stud long stud debate- I have heard that the short stud blocks are sort after for race engines? Why would they be preferred if later long stud blocks we’re improved?
My car is 1968 so I believe could have been fitted with either at this time?
Any feedback would be appreciated
Thx Iain :uk:

I assume racing eligibility varies by organizer, but I’d look at the relative merits the other way round: later is better, so is prohibited in some cases.

The long studs give better clamping because they stretch more and feed the loads directly into the crank saddle. This keeps the cast iron in compression (its best mode) instead of tension. The long stud blocks also have better cooling passage around the bores and rear of block and head.

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Do the short stud 4.2s also crack in between the bores?

Yes but with less incidence than the
early long stud variety

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the long stud ones can give a lot of trouble if the studs corrode at the base, which is very common. Short studs dont have that issue

If you state engine serial number (and vehicle type) Parts Manuals will identify what yours is

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As @awg said, since the studs run down through the water jacket, stud corrosion can be an issue. If the vehicle was operated for any length of time with inadequate antifreeze or corrosion inhibitors, that is a possibility. If you snap off a stud down inside the water jacket it can be very difficult to remove.

The long-stud blocks and heads are also slightly longer to accommodate two extra coolant passages at the rear, so if you are swapping parts around, you need to be mindful that some combinations will require those passages to be plugged.

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Thx for reply Tony - Yes I am aware of this issue having just drilled out and re tapped a series 2 Block ! I can’t believe that is the only reason for demand for short stud blocks for racing?
Thx Iain

I have also read that in one of my books, but I cant think of any other reasons, as other respondents have mentioned the long stud motors have supposedly better cooling and clamping pressure stress distribution, which would suit a racing motor

Plus less bore distortion, but are the short stud plugs stronger since the forces are no longer through the block, the deck is likely thicker for the studs?

Corrosion is really only an issue if the coolant wasn’t refreshed regularly and on a racing block they wouln‘t be in bad condition (besides, I spoke to someone who is racing a 4.2 that literally blew out a stud and they just glued it down so the water stays in, and it works)