Thread: Rover v8 differences

Cranks are the same on 3.5 and 3.9 although I think some late 3.9s had the bigger bearings. There are differences in the nose of the 3.5 and later 3.9 cranks from when the crank driven oil pump was fitted, but this is minor and can be overcome with a spacer.
By definition the 4.0 and 4.6 cranks are different as they have bigger main bearings.
The 4.6 has a longer stroke while all 3.5 3.9 and 4.0 have the same stroke.
Regards Philip A
Tell us what you want to do and perhaps we can help more as there are several changes which I cannot remember all of, but I have a Tuning Rover V8s book that I can refer to

thank philip.

theres a 4L out at the wreckers. seeing as my 4.2 needs a rebuild I was considering using the 4L block with all my parts and new conrods to get a 4.6L. by the sounds of it I'd need the block and the crank as a minimum.

the one at the wreckers has had a small fire so not sure what is usable. chassi rail arced on the starter.

A 4.0 with 4.6 crank, 4.6 conrods and 4.0 low compression pistons will give an engine of just on 9.1:1 compession ratio. Note that you will need 4.6 gudgeon's as they are thicker than 4.0 gudgeons. Same outside diameter but inside diameter is smaller.

If you do not want a dizzy on the engine then the 4.0 cam will be OK (if it is in good nick) however if you want a dizzy you will need your old timing cover and camshaft.

Garry

No, you need crank rods and pistons to get a stock style 4.6. AFAIR the 4.0 and 4.6 pistons are different and it may be the deck height is different. The blocks are the same except the 4.6 blocks were a better spec than the 4.0 blocks. So you can build a 4.6 from a 4.0 block , but have to change all the 4.6 innards.

I gather from other posts here that you can use 4.0 pistons with 4.6 rods but the CR ratio will be higher.
Regards Philip A

Originally Posted by PhilipA

No, you need crank rods and pistons to get a stock style 4.6. AFAIR the 4.0 and 4.6 pistons are different and it may be the deck height is different. The blocks are the same except the 4.6 blocks were a better spec than the 4.0 blocks. So you can build a 4.6 from a 4.0 block , but have to change all the 4.6 innards.

I gather from other posts here that you can use 4.0 pistons with 4.6 rods but the CR ratio will be higher.
Regards Philip A

Yes the 4.0 and 4.6 pistons (comparing low compression with low compression) are different. The deck height is the same but the bowl in the top of the 4.0 has a smaller volume than the 4.6 pistons so if the 4.0s are used in the 4.6 the compression ratio is just a bit lower than a 4.6 with HC pistons - I estimate around 9.1:1 but I have not worked it out accurately.

Garry

I was told using the 4l pistons with a 4.6 crank would get 9.7.
there was some mention of HC pistons though,,

Originally Posted by PhilipA

The blocks are the same except the 4.6 blocks were a better spec than the 4.0 blocks.

what makes the 4.6L block better?

The short story is that 4.6L blocks are less susceptible to having slipped liners. The long story...

Sand casting is not a highly accurate process, so there are dimensional variations from block to block. The dimensionally critical area for Rover V8s (where many fail) is the thickness of the wall between the cylinder liner and the water passage. When they fail, water seeps into the cylinder from behind the liner and gets worse and worse and worse... and the engine gets a slipped liner.

On A 4.0 / 4.6 block with an ideal cast, this wall should have a thickness of no less than 3.0mm at its thinnest point. In practice a block of wall thickness of between 2.1mm to 3.0mm was deemed acceptable by the factory. All 4.0 / 4.6 blocks were ultrasonically tested at the factory and minimum thicknesses recorded next to where the pushrods and lifters are. Blocks with a minimum measured thickness of 2.5mm or more were sent to the 4.6 production line, while the remainder (2.1 to 2.5mm) to the 4.0 line. They later started grading them as blue, yellow and red (corresponding to minimum thicknesses of 2.2, 2.5 and 2.8mm respectively) instead of writing the minimum thicknesses. Red went to the 4.6 line while Blue went to the 4.0 line. Yellows were used for both the 4.0s and 4.6s when the factory ran out of either blues or reds.

Hope that explains it.

Bojan