The V8 Owners Forum -

Hi,

I have a 3.9 small journal engine, can i fit the later style 4.6 pistons to my rods? or with any modification??? Ive been told they are a much better piston to use...

Thanks

Neal

They have a different size gudgeon pin so no.

is it not possible to modify the rod at all ? or the pin?

Rods tend to have quite limited material on the small-ends, so I'd not risk drilling them out to acept bigger pins. But if the 4.6 pins are smaller then the only way would be to use the bigger pins and drill out the holes in the pistons.
I'd not trust either way as the speeds and forces they're subjected to are horrendous, so eroding the safety-margins is just asking for a nice big hole in the side of your block IMO, unless snyone else has already tried it with (or without!) success.

There is another option, get a 4.0 rotating assembly and have the mains turned down so that the crank fits in your block and use the 4.0 crank, rods and pistonswhich are the same strength as the 4.6 stuff other than dimensions.

You got a better rod ratio too as the 4.0 rods are alot longer than 3.9 rods (the gudgeon pin is higher up in the piston) so a mroe reliable, stronger setup, or so I've been told.

I just so happen to have some mint looking 4.0 rods, pistons and crank. . . .

that's another reason you cant use 4.6 pistons, the rods are a diff length so the pistons have a diff crown hight.