Fitting a 4.2 into a Mk2

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Fitting a 4.2 into a Mk2

Postby PeteHickson » Wed Dec 12, 2007 3:14 pm

I've picked up a rebuilt 4.2 engine from a Series 3 XJ6 with a view to fitting it to my Mk2 in place of the original 3.8 unit which needs a full rebuild.
This only cost me £150 which is a small fraction of the cost of a full rebuild.
However, I am sure there will be a number of problems to overcome in fitting this to the mk2 shell.
In particular I want to use the original smooth rocker covers, and there may be problems in fitting the rev-counter generator to this engine.

Can anyone offer advice on the best way to do this?

I'd also like to use an alternator, later power steering pump, oil filter etc if at all possible but the XJ6 engine bay is a lot wider so I'll have to make a lot of mods here too I expect.

I have been told the Mk2 engine mounts will fit the later engine, and that any XK bellhousing will fit any XK engine - can anyone confirm this?
My 4.2 engine is auto, but the mk2 is manual.

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Joined: Wed Jun 10, 2009 8:35 am

Postby PeteB » Fri Dec 14, 2007 3:54 pm

It is possible to fit a 4.2 XJ engine into a Mark 2, but - as you suggest - there are problems in doing so.

If you're fitting a complete engine you'll need a replacement inlet manifold (XJ & Mark 2 cylinder heads have a different port arrangement) and thermostat housing that can be mated to the radiator. The sump will be a tight fit (a Mark 2 sump is shaped to give the centre trackrod some room) and a 4.2 XJ water pump won't give you enough room to fit an engine-driven fan.
XJ camshafts have no provision for the rev counter generator drive, so it would be simpler to convert the tacho to electronic and blank off the hole in the back of the head.
Late series 2 XJ's used an upward-pointing, spin on oil filter that will fit a Mark 2, and you'll also need the camshaft oil feed pipe that goes with this.

Think of the bellhousing, flywheel and starter motor on these cars as a matched set - 3.4 & 3.8 are the same, but 4.2 components are different and can't be interchanged, unless you're changing all of them together. Theres nothing to stop you from fitting 3.8 equipment on a 4.2 engine, so long as you're not 'mixing' these parts.



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Joined: Tue Apr 11, 2006 1:00 pm

Postby PeteHickson » Fri Dec 14, 2007 5:08 pm

many thanks Peter, that is all useful stuff.

I've got an inlet manifold from an early XJ series 1 with 2" SUs (HD8 I think) which I believe has the same port spacing as the series3 head.

I was also planning to fit an XJS steering rack which may make it easier to use the XJ sump, although I have heard the rack upgrade is viewed by some as a bad move and may adversely affect the steering and suspension geometry.

I've got a Getrag 5-speed box to fit, from a manual XJ40 but have not yet acquired the adapter plate and not sure if the 3.8 bellhousing and clutch are the best option here. I think a later bellhousing, clutch and starter would probably be best? Maybe from a series3 - I'm assuming the newer design is probably best, but I may be wrong!

Any pointers on where to get the revcounter converted would also be great.


Posts: 48
Joined: Wed Jun 10, 2009 8:35 am

Postby PeteB » Sat Dec 15, 2007 3:34 pm

An early Series 1 XJ inlet manifold is ideal, athough you may still need an alternative thermostat housing (maybe 420?)
Depending what parts you have access to, it might be simpler to delete the bypass hose & remove the thermostat, if its difficult to match the waterpump to the inlet manifold.

For instrument upgrades, try either John Ostick or Speedy Cables

The XJ40 gearbox may not have a speedo take-off to connect a cable to (I think these used a pick-up on the diff) in which case a 'box from an XJS may be a better option. You'll have to get a cable made to suit the Getrag, as the nut and inner cable size are both different to the original Jaguar type. Again, Speedy Cables could help you here.

As for the clutch, you'll need to use the later 9.5" diaphragm type, which - I think - will suit a standard XJS /XJ40 centreplate.
Theres no reason why you can't use the original bellhousing as part of the conversion.

2 things to bear in mind here, depending on the year of your car - early Mark 2 flywheels were only drilled to accept a 10" clutch, so this may need to be changed / modified. Also, bellhousings of the same era had a little 'outcrop' of aluminium beneath the clutch slave cylinder which needs grinding flat. Otherwise, your new clutch cover will foul it.

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