P38 Gems 4.6 from Thor 4.0 conversion

Questions, questions!

I have the opportunity to change my existing sickly ’98 4.6 Gems motor (with slipped liners) for a post ’99 Thor/Bosch 4.0 re-con/manufactured motor with top hat liners and after several hours trawling the forums believe it may be possible to regain the full 4.6 litre capacity, subject to the following adjustments:

Refit the 4.6 front cover for water pump & oil filter mounts.

Refit the 4.6 Crank & Con-rods to maintain the stroke (is sump pan still the same?).

Refit the 4.6 Camshaft gearwheel/sprocket and Position Sensor to maintain timing.

Refit & relocate the 4.6 Crankshaft Position Sensor so it matches the existing Gems reluctor ring, which I understand is critical and the sensor mounting position of the post ’99 4.0 block is appx 6mm different.

Are these actions likely to work with the existing 4.6 management systems and are there any other alterations necessary – or will this literally be a non-starter when it’s all back together? Should the cylinder heads be the same, apart from a minor valve seat angle difference?

Any advice would be gratefully received.

Is the 4.0L Thor a full engine (with inlet manifolds etc)? The Gems / Thor have different manifolds.
I would suspect the management systems are different between Gems / Thor but no idea if these can be re-programmed.
Cylinder heads are the same.

martynv8 said:

Is the 4.0L Thor a full engine (with inlet manifolds etc)? The Gems / Thor have different manifolds.
I would suspect the management systems are different between Gems / Thor but no idea if these can be re-programmed.
Cylinder heads are the same.

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GEMS was a Lucas invention and THOR is Bosch....totally different and incompatible.

Block should be the same for both, but the front covers, ancillaries, intake manifolds, throttle arrangement, ignition system, fueling system are all totally different.

Sounds like youve not missed anything but would it not be easier and possibly cheaper to remove your block and heads and get turners to recon with top hat liners etc?

You can also use the 4.0l pistons on the 4.6 rods ,this will give a higher compression ratio,

dove said:

You can also use the 4.0l pistons on the 4.6 rods ,this will give a higher compression ratio,

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Did just that on my top hat block rebuild. Works an absolute dream on LPG.

Done the same with mine on lpg

Think mine was a 4.0 and now fitted with a 4.6 block

The blocks are the same (gems to gems) (Thor to Thor)only crank ,rods ,pistons are different to make 4.0 or 4.6

Can be done,I've done it a couple of times and mopped up for other garages a couple of times when it has gone wrong.All a long time ago,but one of the main things that the op didn't mention is changing the cam sprocket.The spokes are what the camshaft position sensor reads and are different between Gems and Thor.I seem to remember something about the mounting of the knock sensors too,but that could easily be changed by altering the connectors and keeping the old sensors.
Make sure the crank sensor mount is properly Tig welded into place when its moved,it causes some awkward problems when bodged in place with self tapping screws which is what I remember having to repair on one...

dove said:

The blocks are the same (gems to gems) (Thor to Thor)only crank ,rods ,pistons are different to make 4.0 or 4.6

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I should have said short motor not block.

Saint.V8 said:

GEMS was a Lucas invention and THOR is Bosch....totally different and incompatible.

Block should be the same for both, but the front covers, ancillaries, intake manifolds, throttle arrangement, ignition system, fueling system are all totally different.

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Thanks Saint.V8 and regret my original post was a bit vague but the plan is just to use the post '99 4.0 recon motor (with all my 4.6 Gems ancilliaries) as a swap for the sickly 4.6, but also with the noted alterations so that I still have a 4.6 motor and the Gems system will still work - I appreciate the 2 systems are not interchangeable. So, from your kind reply and the various other replies, it looks like what I'm proposing may be do-able. Tricky job doing all this with your fingers crossed though!

cooltide said:

Sounds like youve not missed anything but would it not be easier and possibly cheaper to remove your block and heads and get turners to recon with top hat liners etc?

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Cheers Cooltide, that'll be Plan B !

martynv8 said:

Did just that on my top hat block rebuild. Works an absolute dream on LPG.

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Thanks martynv8, if all goes well mine should be back on lpg too.

eightinavee said:

Can be done,I've done it a couple of times and mopped up for other garages a couple of times when it has gone wrong.All a long time ago,but one of the main things that the op didn't mention is changing the cam sprocket.The spokes are what the camshaft position sensor reads and are different between Gems and Thor.I seem to remember something about the mounting of the knock sensors too,but that could easily be changed by altering the connectors and keeping the old sensors.
Make sure the crank sensor mount is properly Tig welded into place when its moved,it causes some awkward problems when bodged in place with self tapping screws which is what I remember having to repair on one...

Click to expand...

Cheers eightinavee, looks like this is a goer and thanks for tip on Tig welding the crank sensor mount. If I come across any further pitfalls I'll post them - might be useful to another fellow sufferer.