XK6 Engine Education

Thanks to the kindness of strangers, I have a 1977 series 2 XK6 engine sitting in my garage. I’m slowly stripping it of unneeded parts for my eventual kit car build, and learning as I go. I thought I’d start my own thread to post questions as they arise.

First question. Looks like there was a hose and duct type system for emission controls. I removed a rail off the top of the motor and there are now exposed threaded holes. Do these holes need to be plugged? I don’t know where they go, and I don’t want to damage the engine when I start cleaning it. I currently have 3/4 length 7/16 bolts in them to be on the safe side. Do I need to plug these up permanently with a shorter bolt and thread lock?

Plug them…they seem to go either right into the combustion chamber or are routed toward the exhaust valve area as an air injection system…quite a common system on smog engines.

What you show in the photo are fittings that unscrew from the head. You can then plug the remaining tapped holes for a cleaner look. The threads are 1/8 BSP (British Standard Pipe). The best looking plugs are 1/8 inch BSPT with recessed hex heads–they can go in so that they wind up nearly flush with the head surface. I like brass. These are very close to 1/8 inch NPT by the way, but not the same. IMHO.

1 Like

That pipe and those 6 fittings in tbe head are part of the Air Injection System that uses an Air Pump to pump air into the exhaust stream to assist with burning off excess fuel right after the engine is started. Shortly after start the Air Pump air is diverted from the air rail and not used this way during normal engine operation.
Attached are two pictures that show what I did to cap off the six threaded holes in a Series III XJ6 cylinder head that I currently have on the 4.2L XK engine in my 1969 E-Type Fixed Head Coupe.

I removed those small air rail fitting and took them into a local hardware store where I found the small brass pipe fittings that I used in the plumbing section. They were a common pipe thread size and easy to find. You must block off those tapped holes in the head that the air rail fittings connect to otherwise hot exhaust gases will come out of them when the engine is running. There are many ways to do this and this is what I did.


1 Like

I agree. As Robert says, these are British pipe thread, not ANSI. ANSI NPT is 27 tpi and BSPT is 28 tpi.

A source for hex socket BSPT plugs.

Thank you all. Looking at it again, it should have been obvious. Though I’m hesitant to start yanking and pulling on crusty parts that I’m not sure are supposed to move, so I though it best to inquire.

On that note… I’m attempting to remove the intake manifold. It’s a 1977 engine that has two Strombergs with a complex water choke system. I was hoping to use the intake manifold for dual SU carbs with manual choke. Is there any reason I couldn’t use it? I would hope I could circumvent or not connect any unneeded coolant passages.

These long studs that I have pictured… are they actually studs that can be removed or are they permanent? Can these coolant box things not be used if I’m using a manual choke?

Greetings All,

I may be under the impression that the “water choke”, could be useful since even my triples has a similar device. I don’t think its removal will help anything.

You might need a transition piece made to allow the SU’s to fit the opening where the Strombergs used to reside? Otherwise you’ll need to source another intake.

I bought a ‘for parts’ SU off eBay for really cheap just to take it apart and study it. The four bolt pattern matches and it seems to fit. The water choke system looks overly complicated and ready to fail… it’s more than I want to mess with, and I definitely don’t want to make it work with SUs. Also, this block won’t be going into a stock XJ6 or similar. I’m not restoring a Jag. This is for a potential build of a replica D Type, so simplicity is the goal. (I failed to mention that in my original post- sorry)

I hope I can use the stock intake, at least for a while. If I can’t, what ones should I look for on eBay to fit a 4.2? I’ve seen many for an XK150.

Greetings All,

That carb, won’t work as is.

You apparently have bought a carb for an Austin Healey, like a100/4 or 100/6 or possibly a 3000.
The angled bowl is a dead giveaway.

If simplicity is what you are after, the SU has four moving parts. D-Types typically had Webers, I own some, long learning curve and to set up, you’ll need a fair bit of cash.

You need to get more familar with Jaguars. A 4.2 intake is not similar to the 3.4/3.8 unit that appeared on the XK150. No 150 ever came with a 4.2, nor did the D, in case you are thinking about authenticity, plus you have that whole tilted engine thing to deal with to keep the bonnet height low.

Good luck.

Oh yeah, I know it wouldn’t work. It’s off a Volvo engine, actually. It’s a ‘for parts’ carb that I bought for cheap on eBay so I could take it apart and study it. You can only learn so much by staring at online pictures after all. No intention of using it with the XK6.

I am indeed trying to get more familiar with Jaguars, hence the title of the thread I started. I’m not going for complete authenticity nor am I keeping it in the garage until it’s 100% perfect. My goal is to get something running so I can get to driving. A D-Type reproduction is a car I would keep for the rest of my life, and I would add and tweak and learn and modify as the years went on.

@PeterCrespin would be a good source of info, and good luck!

1 Like

Here is a straight-port manifold from a 4.2L North American E-type, originally fitted with twin Stromberg carbs. The two bottom holes for each carb have been redrilled accommodate the bolt pattern for 2" H8 SU’s. Photo of original E-type installation shown for reference.

Here is a link to 1/8" BSPT socket head, nickel plated, brass plugs at McMaster-Carr.


Greetings All,

Doug, this is what I thought you could do.

The removal of that other casting between carb and intake is superflous in my opinion. This set-up is reminiscent of like the MKVII.

It also makes thing somewhat less complicated plumbing wise.

took the fitting out as well and plugged with brass 1/8 NPT allen headed plugs.

Thank you for the pictures and info. I ordered the recommended plugs and they arrived just after Christmas. They look like they’ll do the job.

Question about the top of the engine. It’s filthy and needs cleaned, especially in the little divot areas around the spark plugs. Any suggestions on cleaning that area without getting crud down into the cylinder? I looked for a threaded plug to screw into the spark plug hole but couldn’t locate any.

Also, I did my first compression test on the engine. Results are as follows…

1 : 110
2 : 90
3 : 110
4 : 100
5 : 100
6 : 105

Seems low, but I recall reading that it’s less about the pressure and more about the uniformity between all six cylinders. Any flaw in that logic?

1 Like

Hi Doug,
The best thing to use to fill the holes would be the old spark plugs themselves. Go crazy and shout yourself a new set for Christmas… :smiley:

The compressions do look low but of course this is on a cold engine, they should be within 10% so looks like the second cylinder could have broken rings or valve leaking. Try doing another test with a teaspoon of oil in each cylinder.