I’m planning to build up a 4.0 to 4.6 using internals salvaged from a used 4.6. I already have all the 4.6 needed internals and everything is still in standard spec. I had the crankshaft polished (probably didn’t need it) and the rods checked/cleaned. I believe the 4.6 had right around 100k. At 100k and still within standard spec, I’d say that is pretty good. My current 4.0 has about 125,000 miles and runs quite well. While I can’t say the cylinder walls in the 4.0 I plan to use will be within standard spec, if they are, this a pretty inexpensive rebuild. If I have to rebore and buy pistons, that does add a good amount of expense. I’m optimistic the cylinders will be ok as it burns next to no oil as far as I can perceive.
My goal is to wait until it “needs” to be replaced. Fuel economy definitely could be better but for reliability, I can’t complain as it still runs great.
I have another D1 that I swapped in a 300tdi and R380. It is very underpowered and pulling a small utility trailer (mountain roads/highways) is out of the question. I will admit, I am using the V8 transfer case (1.2) and I strongly suspect that replacing it with a 1.4 would really help and considering this one will almost never see the interstate, increasing rpm’s would not matter much. I only use 5th gear (with the 1.2 t-case) when going downhill. I just never get going fast enough to need it and keep the engine from lugging. It gets about 23 mpg now but with diesel prices so much more than gas, the increased fuel economy isn’t as much a benefit as it once was. Also, the 300tdi is an old school diesel and it is really smelly.
Frankly, for ease of installation, cost, and cost competitive reliability, I think an early 4.0 built up to 4.6 is about as good as you can get. Are there better options, of course. But for what you will spend either for the engine/trans and adapter parts combined with the labor required, you could go through several Rover V8s.
Rover V8s get a bad rap in my opinion. Up until the slipped liners in the newest 4.6 version, 3.5-4.0 is pretty darn reliable if you provide proper overall maintenance. I will admit, failed head gaskets that became much more prominent in the 4.0 is disappointing. The 3.5s I have had over the years never had head gasket problems.
If you buy a D1 used with a bunch of miles, there is a good chance it hasn’t been properly maintained. My current project D1 needed just about everything but the engine replaced. Radiator was clogged, water pump leaking, original hoses, lots of leaking gaskets and seals, clogged and rattling catalytic converter, as well as a host of other odds and ends that would in short order lead to engine damage. It has 125,,000 miles and is 27 years old. I’d say that is pretty darn good. If previous owners had done normal repairs and maintenance…
All that being said, it’s all about personal wants in this area. If you want to put a VW diesel in it, I’d really enjoy you creating a post that details how you get it all done.
Another thought, if you are interested in diesel, a 300tdi has become a lot less expensive recently as Defender people are jettisoning them for Cummins and LS swaps all the time. 300tdi was all the rage 10ish years ago. I purchased a rebuilt R380 from a Defender guy who removed it and the rebuilt 300tdi after only running it for 5000 miles. He replaced it with an LS and automatic. A 300tdi also is a lot simpler as you will be using parts originally intended for the vehicle. A 300tdi is a very reliable engine. Pair it with a 5 speed and 1.4 transfer case along with realistic performance expectations and it is, IMHO, a nice option. Also, silly I know, if there is an EMP that wipes out electronics, I will still have an operational vehicle. (I am mostly joking about an EMP.)
I was considering doing another 300tdi but have decided one is enough. I have acquired just about everything to do the swap but will just keep it all for spares. That may well be pointless as I wouldn’t be the least bit surprised if the currently installed 300tdi and R380 outlives me. When I get too old to turn wrenches, I will probably take all the spares to a metal recycler as by then nobody will want any of it.