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4.0 to 4.6 conversion
Posted: Sun Oct 01, 2017 4:47 pm
by gd302
Ive read this is possible with a 4.6 crank and rods retaining the 4.0 pistons for a slightly higher cr as long as the blocks are the same.
Is it as straightforward as just bolting them in with new bearings, rings etc or will the bottom end require balancing correctly?
Im interested in doing this at home on a budget and wondered is it better to start off with the complete 4.6 short engine, or using my known good quality well seasoned 4.0 block.
Any advice would be great.
Posted: Sun Oct 01, 2017 8:29 pm
by ChrisJC
You just put the 4.0L pistons on 4.6L rods & 4.6L crank.
Each time I have done this, I have had both pistons/rods balanced, and then the rotating assembly. Quite a lot of material came off the crankshaft.
I don't know how bad it would be if you didn't balance it.
Chris.
Posted: Mon Oct 02, 2017 1:08 am
by garrycol
As indicated use 4.6 crank and rods - installation is the same as any other rebuild ie balance as necessary.
Use 4.6 pistons if you want standard comp ratios or use 4.0 pistons if you want a higher ratio. BUT be aware (and not many people know this) the standard 4.6 piston gudgeon pins are stronger than the 4.0 piston gudgeons as they have thicker walls (same outside diameter).
So if you want to use 4.0 pistons I would press out their gudgeons and put in 4.6 gudgeons - not an engine stopper but be aware of it.
I found this out when I recently did exactly what you are proposing.
Posted: Wed Oct 04, 2017 8:29 am
by gd302
Seems my engine is a 37A which looks to me as if its listed as intermediate with small journal crank etc.
Posted: Wed Oct 04, 2017 12:16 pm
by garrycol
gd302 wrote:Seems my engine is a 37A which looks to me as if its listed as intermediate with small journal crank etc.
So it is a 3.9 block not a 4.0 block. The 4.0 block is the same as a 4.6 block so has the same size mains. Is also cross bolted like the 4.6 where a standard 3.9 is not cross bolted.
Posted: Sun Oct 15, 2017 11:07 am
by DEVONMAN
Options for capacity increase for this block would be:-
4.2 crank/rods /pistons.
5.0 TVR crank/rods/pistons.(externally balanced so either re-balance or obtain with matching front pulley and flywheel).
Aftermarket stroker kit.
BUT. Probably cheaper to get a complete 4.6 block/crank rods/pistons.
Posted: Sun Oct 15, 2017 7:20 pm
by gd302
Thanks, im waiting on a rebuilt 4.6 bottom end coming my way.
Much better than trying to modify what i have.