thanks for letting me join your group
i have a 1999 range rover p38 and I'm in the proses of rebuilding the 4.6 lump that was in it as head gasket went on it so stiped down and was venting exhaust gases into atmosphere as the head was off fined out it has a top hat liner engine. so the big job had started so thought as I'm in there I would swop out the 4.6 pistons for the 4.0 and swap the camshaft for a Kent camshaft H180
in doing the swap with the pistons I find out that the 4.0l pistons are 20grams les than the 4.6l so less mass to move witch should be better for the mass of the bottom end. so I started to clean the ports to give good air flow thru the engine (wished I hadn't started) but work on them so I can feel the benefit later so no pain no gain.
I have spent a lot of time cleaning the engine as it had a fair bit of oil all over it it was looked after very well by the first owner but the next two did it no favours as it must of had head gasket done but must have been done by a monkey as head boults were so tight had to use breaker bar and an extension bar all in all a bar three feet long bar on some boults and some were lose they had managed to brake some of the rockers as not given the right gap settings so its been a bit of a challenge to make it right but so far the bottom end is back together and sorting the heads out

Welcome to the pub, must be your round? If it's got top hats the blown head gasket will be down to the bolts being either cheapo stretch bolts that stretch too much, or a mix of old and new. They do put up a fight getting them off, ARP studs are a much better way to go, not cheap but worth every penny. Top hats and studs mean you won't have any head gasket problems for the foreseeable future (mine has done 131,000 since being top hatted and put together with studs). Wasn't aware that 4.0 litre and 4.6 pistons were interchangeable, but I know when mine was rebuilt I was told by V8 Developments that 4.0 pistons are difficult to get hold of and a lot more expensive that 4.6 ones.

Sounds like you've got one much like most of the others, well looked after initially then neglected and/or abused after that when they got cheap. Setting the rocker preload is fairly simple, see http://www.v8developments.co.uk/technical/valve_train/index.shtml.

Interesting conversion there unimog: Knew the 4.0/4.6 difference was stroke not bore but are the conrods also interchangable too ? Wondering about the ECU map differences now too....

I think you need to replace connecting rods as well if you are replacing pistons.
A 3 foot breaker bar probably means they were torqued correctly. If you use new stretch bolts instead of the ARP studs you’ll find out why. Torque to a value (115 ft lb or so from my poor memory) then another quarter turn, then another quarter turn.

4.0 litre con rods are longer than 4.6 (155mm centre to centre rather than 149mm) so with the longer stroke, unless the pistons are a lot shorter, then they will hit the heads.

Thanks for the info had to use the 4.6l conrods as people say the 4.0l conrods are longer all I replaced was the piston it just puts the compression up to about 10.5 to 1 instead of 9.35 to 1 so should be good for bottom end tork that's the reason I have done the conversion
I have new head bolts for it and timing chain but will see if the ecu can cope if not will be sent of for a remapping
Thanks for all the information

If it needs remapping, or even if it doesn't, the man to speak to is Mark Adams at Tornado Systems (https://www.tornadosystems.com/product-category/rover-v8/), he is the man for mapping on these. Didn't think it would be possible to use the 4.0 litre rods but without a pair of pistons sitting side by side it isn't really possible to tell.

It's the opposite to the engine in a Corvette C2 that a mate's son imported from the States. He was told the engine, a 350cu small block, had recently been rebuilt but it didn't sound right and was idling on 7. A compression check showed that all cylinders seemed a bit low and one was very low. Turned out one had a broken valve spring so the heads were pulled off to sort that. Mate had the block on an engine stand and was turning the engine over to get it to TDC and found that none of the pistons reached the top. No idea what combination of crank, rods and pistons had been fitted but they definitely weren't the correct ones.

(https://drive.google.com/file/d/1nIvcSNCjqCxX4DulFl8VjblJDpgbEemN/view?usp=drivesdk "pic of 4.0l pistons in a 4.6l")The pistons on the 4.6l have a deeper dish than the 4.0l that's the only difference rover did it to keep the compression the same so putting the 4.0l in increase compression the diameter is the same and hight of pistons are to
But I did find that the 4.0l pistons are lighter by 20 grams as I made shor the balance was good over the hole crankshaft

I’ve not heard of people going from 4.6-4.0, but I’m pretty sure when people make a 4.6 out of their 4.0 they need to change pistons and rods.

You just have to get the 4.0l piston remove conrod and replace with the 4.6l rods and then off you go with higher compression

Harv wrote:

I’ve not heard of people going from 4.6-4.0, but I’m pretty sure when people make a 4.6 out of their 4.0 they need to change pistons and rods.

That's if you are going to use the 4.6 crank too as it is that that dictates the increased stroke over the 4.0. So if using a 4.6 crank, you also need the matching rods but it seems you don't need the 4.6 pistons.

@Unimog, just tried to edit your post so the picture appears but it has permissions set so can't be seen by anyone but you.

Did try to attach pic have now made public

The link you embedded goes to the whole drive so there's all of the pictures there rather than the one individual picture.

Yes quite right Richard. I forgot about the obvious need to replace the crank.

be careful changing pistons and rods as it will interfere with the throw of the crank and the piston, important part of engine operation .when they stroke engines they lengthen the stroke by offsetting the crank journals and the gudgen pin in the pistons and use different length rods if the 4.0l piston is higher and your fitting a hi lift cam make shore you have the valve clearance

y is it I post what I'm doing to my engine and people say I'm putting 40l parts in a 4.6l
all I'm doing is changing ( THE PISTONS ) not conrods or crankshaft as its a 4.6l engine in the first place
please read my first comment properly and it would make sense. I do no what I'm doing been messing with engines for 30 odd years
sorry if it feels like I'm annoyed but people need to read comments properly first before commenting

I agree, it did go off at a bit of a tangent but I knew what you were doing. It's made me start thinking of going the other way. Keeping my 4.0 litre pistons (in my top hatted 4.0 litre block) and putting a 4.6 crank and rods in. Running on LPG the higher compression could be used without any danger of pre-ignition.

The 4.6 has a stroke of 82mm and rods that are 149.7mm so from crank centreline to TDC is 190.7mm, whereas the 4.0 litre has a stroke of 71mm and rods that are 155.2mm which, as you would expect, gives a crank centreline to TDC figure of 190.7mm so the blocks are identical and obviously the piston pin to top of the piston has to be the same. The difference is that the dish in the top of the piston is smaller on the 4.0 litre so the compression will be higher.

Hi Unimog996 (great cars, too btw),

interesting approach, following with interest. Please keep us updated!

I guess You have a Thor / Bosch injection? With higher octane fuel and the knock sensors it might have a nice effect making use of the higher compression.

Don‘t know much about the motronic, though.

Shouldn't matter, GEMS and Bosch Motronic both have knock sensors. Might be worth running it on 98/99 Octane petrol (or 112 Octane LPG) to get the best out of it.....

Thanks for commenting I am running the car on lpg thats one reason I'm doing it as you say would have to run on higher octane fuel so should be good will keep people updated when I finish porting the heads and the engine goes back in to the car and also will make a good tow car to