S2 engine into S3: issues?

I’ve been asking a lot of questions lately, I hope I haven’t used up my fair share.

A friend has a 1978 XJ6 with fuel injection, and in a run of bad luck, had the engine start knocking badly just after paying large to have his BW65 rebuilt.

I have a good S3 engine I offered him, but I’m not sure about a couple of things.

Can I leave the intake complete from the S3 and use it in his car? The fuel rail is a different shape, and maybe the ECU won’t like it? If I have to put his intake on the later engine, that’s not too big a job when the engine’s out, but it would be faster just to swap it whole.

The transmission shift cable linkage appears the same. The fuel return line is a different shape, but that’s not a big problem.

Does anyone see an issue doing this swap?

As an aside, this friend works for a company that maintains lake freighters that run up the Great Lakes near my home. He got me a tour of a ship while in winter dock, when they do annual work. Pictures attached.



I would use the more modern fuel injection pipes. The big LOOP fuel system was replaced by the later one and is easier to work on as well as fewer potential leaks.


The Ser III engine might have the later style cooling system plumbing: different water pump, thermostat housing (or perhaps entire water rail?), some hose differences, different coolant tank, etc.

I don’t think it matters too much if you choose early or late style…but you’d probably want to all newer or all older rather than trying to meld the two types together


If the complete S3 engine is transplanted, Robert, there should be no issues. It’s swapping parts between engines that may be an issue…

I assume he will be using his original ECU to run the S3 engine - which it will. As far as I know, the relevant sensors are the same and there should be no problems connecting the other electrics, like S2 injectors to the S3 engine…?

The BW66 in the S3 is externally more or less identical to the S2 BW65, and the controls are the same - so the latter should bolt directly to the S3 engine.

xj6 85 Sov Europe (UK/NZ)

Someone mentioned that the AFM location is different with the idle air adjustment more accessible on the S2. Is that correct?
The manifold should be the same, so that won’t be a problem hopefully.


your friend’s 78 FI SII car is an unknown beast in this part of the world, so I know even less about the FI used on these cars than about the FI used on SIII cars.

Maybe that’s why I find Doug’s advice so convincing - if I don’t really know what’s ahead of me, I’d rather stick with what I know works well. And that is either the entire old system that has been running well except for that bearing damage or the entire new system. The latter choice is probably even better as the SII head has smaller valves and maybe the SII FI was adjusted to the smaller valves while the younger SIII FI was designed together with the new head.

Good luck - and nice pictures! Thanks for sharing


75 XJ6L 4.2 auto (UK spec)

Thank you all for responding.

We have my wife’s Dad here from Wales - he’s an HVAC guy - and I conned him into helping me pull the good motor.

There are a lot more connections than on the carb models I’m used to.

Here’s a picture of a piston from the freighter

More to come…



The piston is larger than the whole XK engine. Very impressive!

Hello folks,

I’m half way through swapping a S3 XJ6 engine into a S2 car.
The S2 car had the big heavy cast flywheel/ring gear, and it has tooth damage, so I don’t want to use it.

Can I use a S3 flexplate? It seems like it would fit, everything I can measure appears right.
The parts book says the heavy one for was for BW12 and 65. I will be retaining the 65 from the S2 as it was rebuilt.
Am I missing something? Why did Jag change anyway?
I see Moss Motors lists one light weight flexplate for all series.

I’d like some assurance, as it would mean engine out to correct if I get it wrong.

I do not have a specific answer. But, things to consider.

  1. Tooth count. to match the pinion on the starter.

  2. Are the engines internaly or externaly balanced.


Carl has a very valid point, Robert; your picture seems to show different pitch (and diameter?) on the two ring gears - which would mismate with the starter pinion unless properly paired off…

The flexplate is part of the inertial mass in a ‘flywheel’ action. Generally; lightweight items are fitted for quicker engine response - at some possible cost to engine smoothness at low revs.

Whether Jaguar changed items you mention for this or other reasons is not known (I think!). But the S3 engine itself should mate with the BW 65 without problems - there is little difference between the 65 and the BW 66 usually fitted to the S3 engine…

xj6 85 Sov Europe (UK/NZ)