offset grinding a 4.6 crank to reduce stroke?

[Gasket]

Hi all. In parallel to my crossbolted 3.5 engine build, I'm quietly gathering bits to build up a nice strong crossbolted 4.2 Rover V8 for a period install in my '93 Range Rover Classic (hence not going to 4.6).

I could buy a complete 4.2 if one pops up at a sensible price, I could base it off an interim 3.9, fit a 4.2 crank if I can find one and have it crossbolted - or... I could start with a P38A bottom end if I could have a 4.6 crank offset ground to reduce the stroke from 82mm to 77mm while reducing the big end diameter from 55.5mm (4.6spec) to 50.8mm (3.9/4.2 spec).

Pistons/rods seem to be the same as the 3.9 and I think I have a 4.2 branded plenum and most of the other bits seem to be common to bog-standard Rv8s.

Offset grinding of cranks makes my brain hurt. I know it could be done to increase capacity and logically an opposite offset would reduce capacity.

With P38A bits being very cheap and commonly available this could be the quickest and easiest way to get the parts I need (4.6 parts being much more common than 4.2) plus I'd get the benefit of the 38A big main journals and it already being crossbolted.

I'm having two engines machined so will have a good relationship with the machine shop and I'd be having the crank checked and probably ground in any scenario so having a grinding operation and the nose turned down would give me a 'big main journaled 4.2 crank' with little effort. add 3.9 interim ancillaries and I'm there.

(or it might be a complete non-starter in which case I would go for a 4.6 with a 4.2 plenum - but I do like a challenge and prefer period engine capacities)

[ChrisJC]

Are you sure that 3.9 and 4.2 rods are the same? If they are, then I guess the pistons must be different?

Chris.

[Gasket]

Well spotted Chris.

Just double checked my books and I was wrong.

Rods are the same (143.8mm long with 50.8 and 22.2mm ends) but the pistons are indeed different, 3.9s are 49.5mm high with 22.5cc dish whereas 4.2s are 45mm high with the same 22.5mm dish.

Plus the cam is different too, so that’s plenum, crank, pistons and c that are all unique… starting to think I’d better just start looking for a complete 4.2

[ChrisJC]

One comment I can make about 4.2's is that they can break the crankshaft!
I can't remember the details, but there was a guy on this forum who had worked for Land Rover 'back in the day'. In fact, the post was probably 10 years ago at least!

The gist was that you could snap the 4.2 crank fairly easily. The snag is that I can't remember how!!

That could be one reason they increased the journal sizes for the later engines.

I'd stick a 4.2 plenum on a 4.6, and enjoy the extra torque!

Chris.

[Gasket]

Think you're right Chris, - 4.6 seems like the easier way to go.

Shame, as I really like period tech, but I'd be cutting my nose off to spite my face.

Ah well. Guess I'll just have to enjoy the extra horses

[garrycol]

If you cannot get a 4.6 go a 4.0 - same block etc - usable power difference between a good 4.6 and a 4.0 is not a lot.

[Gasket]

Thanks Garry.

The vehicle is originally a 3.9 - so just the same as a 4.0 in terms of power assuming I keep the factory ancilliaries. But the block isn't the original so I lose nothing by switching to a 4.0 (plus I get big mains, crossbolting, etc) and can keep the same ancilliaries, so the 4.0 is certainly the backup.

I'd just fancied doing a little bit of a hot factory-special build and the 4.2 was available in period - though only in LWB and some autobiography SWB RR Classics. But I do want to incorporate the P38 bottom end improvements.

I'll have a weigh up, have plenty of time on this one - the crossbolted 3.5 RV8 project is the priority - though it'd be nice to have the machining done on both engines at the same time.

[unstable load]

Send me your 4.6 crank and rods and I'll send you my 4.0 crank and rods...
Should work out for you, and I'll get my 4.6 that I want....

[paul c]

I bought a 4.2 crank from Turners a few years ago but ended up selling it on as pistons were not going to be easy, it was easier to go to a 4.6 and enjoy the benefits. I don't know if Turners still have any cranks left.