XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

Jaguar flatheads

 
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Old 05-31-2019, 01:25 PM
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Default Jaguar flatheads

Hi can anyone tell me how much a pair of Jag flat heads in good condition are worth? Are there good or bad flat head casting numbers? Were the early castings with 5/16 threads better then the heads with metric threads? OR are there no metric thread flat heads? Are these flat heads only XKE? or XJS and XJ12 too? Is there a minimum thickness for these heads? What are the correct search terms for these heads?
 
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Old 05-31-2019, 01:31 PM
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They were used on all V12's up to about 1980, so all E Types, the XJS up to 1980 as well as the XJ12. They are usually called pre HE heads, as the HE engine came in for 1981. As far as I know they are all the same, and will not have metric threads.

Can't help you with minimum thickness or value, but I don't think they are worth a lot. The HE is generally more desirable, unless building an engine for racing.
 
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Old 05-31-2019, 02:22 PM
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Scott I got a set of eBay with one dropped valve seat but other wise good for $450. What are you planning to do?
 
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Old 06-02-2019, 01:12 PM
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Default pre he heads

I am trying to get an idea of what these are worth. My heads have 5/16s threads. They are in perfect condition.
 
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Old 06-02-2019, 05:45 PM
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My 89 5.3L has 5/16 but my 93 6.0L is M8
 
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Old 06-03-2019, 01:12 AM
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Originally Posted by scott 409ss View Post
I am trying to get an idea of what these are worth. My heads have 5/16s threads. They are in perfect condition.
They are worth something only if you find someone who wants to build a high-performance high capacity racing engine. For road-going cars, the HE motor makes more power and is far more economical. Best to advertise it in racing circles, eg the JEC racing guys, etc.
Even if someone wants to build a high capacity road engine, they are less likely to start with an HE, as to convert to pre-HE means new pistons, possibly cams, as well as new heads.
Whether the threads are metric or imperial does not affect their value in the least.
 

Last edited by Greg in France; 06-03-2019 at 01:19 AM.
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Old 06-03-2019, 12:05 PM
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Hi Thanks, but I find it hard to believe a H.E. engine can make more power then a flat head.
 
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Old 06-03-2019, 12:42 PM
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Originally Posted by scott 409ss View Post
Hi Thanks, but I find it hard to believe a H.E. engine can make more power then a flat head.
The qualifier was for a road car. Under most part throttle conditions the HE makes more power than a flathead at the same loading.The factory spec for the flatheads has them at less power than the HE in both US and European compression ratios.

Pre HE: 244/285hp (US/UK compression)
HE:262/299hp (US/UK compression)
 
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Old 06-03-2019, 02:32 PM
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hmm I think the flathead compression is 7 to 1?and the H.E. was 11 to 1? not really a fair comparison. Are people not allowed to replace H.E. heads in the the popular racing classes? I have a set in mint condition. I want to make sure I use the correct title words for my add. I really need to know if the correct terms for these heads are Pre H.E. or simply flatheads?
 
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Old 06-03-2019, 03:12 PM
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Common terms now is a Pre-HE engine. Both pistons and heads are different between the Pre HE and HE, so simply swapping heads wouldn't work. If you put a pre HE head on an HE engine you'd end up with something like 18:1 compression. Fine if you intend to run it on diesel fuel....
 
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Old 06-03-2019, 03:35 PM
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value of old Jag V12 engines,, around 10 yrs ago i had accumulated 3 complete take out Pre-HE engines , one a 4 carb V12 , and 2 FI engines V12s!!

had them for for 5yrs before making a move to Texas , so off to the junk/scrape yard!
i got $25. US dollars for each ,total of $75. bucks, and watched almost crying to see them go into the huge shredding machine , along with 3 V12 transmissions!

SO tell me what conceived value is of what just OLD junk really IS?

i think i said something about "times -are-Achanging!!
ron
 
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Old 06-04-2019, 12:48 PM
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lol I scraped 4 he engines and drive trains- kept 1 400 auto trans and a mint crank.. scrapped 4 xjs bodies sold a nice 87 s (cheap)
 
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Old 06-04-2019, 05:33 PM
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YES like said , JUNK IS JUNK ! China will buy all we can get!
penny a pound!
 
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Old 06-04-2019, 11:39 PM
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lol I am sure they are worth more then that
 
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Old 06-05-2019, 05:01 AM
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Originally Posted by scott 409ss View Post
lol I am sure they are worth more then that
If you can find someone who wants them.

While HE heads will not in any form (NA) make more than 450hp, primarily due to Intake valve size which is limited to about 1.71" a pre-HE will take a much larger intake valve. So yes pre-HE will make more top end power at full throttle running, but it will not necessarily make more power under 5250rpm, read torque. Part throttle running in the HE will be far better this is why a road car benefits more from a HE head.
 
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Old 06-05-2019, 11:54 AM
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I am dumbfounded the H.E. heads can do anything better( or as good) - compression and cams being the same. The stock flathead pistons are wrong and the stock H.E. heads are wrong. I would be shocked to hear a H.E engine can make 400 hp. oh well. so the correct head line to sell my valuable classic parts lol would be XKE/XJS/ XJ12 Flatheads/ pre H.E. for sale? or Pre H.E./ Flatheads for sail. Used on XKE / XJS /XJ12 74-80? Or should I simply list them as XKE/XJS Flatheads 4 sale?
 
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Old 06-05-2019, 08:31 PM
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Scott , TWR raced many Jaguar V12 cars with all types of engine modifictions, and they said the best mod would be to increase stroke as much money as you can afford!

altho for an everyday street Jaguar , a change of the rear end gear ratio is the cheapest way for respectable performance !
ron
 
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Old 06-06-2019, 11:40 AM
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Makes sense. The ports were originally designed for a 7 liter engine. When they downsized the engine they did not downsize the port size.
 
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Old 06-06-2019, 01:51 PM
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Originally Posted by scott 409ss View Post
Makes sense. The ports were originally designed for a 7 liter engine. When they downsized the engine they did not downsize the port size.
Where did you get this piece of info from? I am not saying it is untrue; but AFAIK the original Jaguar twin cam V12 prototype was not 7 litres, neither was the first production version (the carburettor flathead) and the HE heads were an entirely new casting.
 
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Old 06-06-2019, 06:02 PM
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First as I was told 40? yrs ago the xke jag v12 was built to be a Ferrai killer.The v12 was originally built as a 7 liter.Then the gas crunch came and our gov's wonderful fuel economy fuel standards. Jaguar got scared. They made it a 5.3. They did so by destroking it. The rest they say is history. I had both heads analyzed 30? yrs ago. I had both heads tested on a flow bench. The cylinder head designer( died) told me the intake port volume is correct for a 7 liter engine. The H.E. exhaust port is a joke. The flat head ex.port can be cleaned up to be ok. The ex. ports real problem is it's length before it gets to the flange. That prevents it from having the correct taper. The ports shape it limited by the distance it has to go through the cyl. head to get to the flange. The ex. ports short turn can be worked. The long side is a problem. It is difficult to keep the rate of area change correct. While not too hard to fill it is still difficult to get the long turn/ aka roof shape correct. The port really needs to be taller- with less turn. What else is new? The stock stroke is 2.75. Increasing it to only 3.5 is 415 cubic inches. with todays' rod and piston tech you could easily go to a 4 inch stoke on that 9 inch tall deck. that is 475 cubic inches. IF I remember we looked at dishing (cnc program) the flathead combustion chamber to be the same cc's as the HE head.I THINK that would have sunk both seats .090? to get the volume?. (Sorry it has been too long).But .090 shorter valves are simple to order. We were also going to make a tapered sleeve to fit in the intake port. This was an easy way to reduce the port volume. Changing the heads would have been an improvement- BUT I also needed to change the cams At that point it was cheaper to put a 454 in it. the 454 weighed 100? lbs less.IF it only made 450 hp I was way ahead of the game..Any how I have these mint condition flatheads I want to sell. I am not sure what the correct key words need to be for the ebay add.and that is why I made this post. scott
 
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