offset grinding a 4.6 crank to reduce stroke?
Posted: Wed Mar 15, 2023 11:41 am
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)
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)