Can anyone tell me the differences between our motor and the 4.2L from a 1993 RRC? If I had the opportunity to pick up one in need of rework (head gaskets), for the price of a really nice steak dinner, would it be worth my while? (ie; as a replacement motor if I should need it, or to convert my 4.0 to 4.2, etc)

 

As far as I know they are the same engine and it should bolt right up BUT it will have a dizzy and you have electronic ignition, so it would be a step backwards as far as engine management goes.
You will need the RRC ECU.

 

So then what are the differences that gives it that extra 0.2 L? Is it one of those things where the crankshaft or piston rods or something or different, and could be cannibalized to give my motor that extra displacement? If that's possible, would it even be worth the effort?

 

I'd say no. If you want more displacement buy new crankshaft don't use old stuff.

 

Most old Rover people and mechanics will tell you the 4.2 is a much more solid engine, better torque and HP, smoother idle and less chance of head gaskets issues. Go for it if you can get a decent price on it, they are few and far between and pretty expensive.

 

It's $75, in need of head gasket repair and Water pump. If it takes the same pump as ours, I've got a spare. Thought it might be a good idea to have that 4.2 on hand to slowly rebuild, and would give me peace of mind in case my motor happened to go south.

 

As long as the block is good, you would be foolish not to buy it.
I nknow a old Rover engineer who told me they used to rebuild them, using a combination old Rover parts and turn out about 275 HP.