3.8 Vs. 4.2

90sc35thann

Registered User
I am aware that the 4.2 uses split port technology and the entire upper half of that engine is different than ours, but I am curious are the bottom ends the same? I know the 4.2 has a longer stroke and a cast crank but I would like to know if they are interchangeable physically speaking? Would all of our accessories bolt up and all that good stuff?
 
Good news to your question.

There is a company that makes those kits for our car. THey run an 8.5:1 comp ratio and have all the heavy duty goodies that our engines have. They claim you can run over 20lbs of boost with that kit, but you better have a real high flow exhaust and really good heads gaskets. This is the web site, www.moranav6racing.com.

Enjoy!

Later,
Hock
 
The short answer is yes. I'll qualify by saying that my research has been done for replacing a n/a 3.8 with a 4.2.

There still seems to be some "debate" on whether the blocks are *exactly* the same internally but everything external will indeed bolt up to a 4.2 shortblock except there are oil pan (and opickup) clearance problems with some, apparently the girdle and windage tray was done differently with different year 4.2s? Internal vs external balance could be an issue also but I haven't finished my homework there. You already know about the cast crank which seems to be the weak link for SC roots style blowers on the 4.2.... the rods are not forged and the pistons are not hypereutectic like SCs are.
 
Last edited:
Unfortunantly the sc crank has smaller end journals then the n/a motor, ford did this so we couldnt swap weaker parts in a sc motor for those reasons
 
Oops, forgot about that .010 difference in one crank journal... ass-u-med use of the whole 4.2 shortblock.
 
not really sure what that ass-u-med coment is all about.I did a post on this very subject and was told by kevin varnes in this fourm and talked to many other poeple with the same awnsers that a n/a 4.2 crank would not fit in a sc 3.8 for the reasons that i stated .Use the whole 4.2 short block YOU just said that the 4.2 botom end is too weak what would be the point of that?maybe you SHOULD finish your home work.

go to that link and you'll see what im talking about.
http://www.sccoa.com/forums/showthread.php?s=&threadid=7550&perpage=15&pagenumber=2
 
There's a lot of 4.2 swap discussion on V6Power.net;
Several Stang owners have used the 4.2 shortblock
with single-port heads, or swapped the 4.2 crank /rods
/pistons into 3.8 blocks.

The 4.2s and late 3.8s use longer rods, and short pin-height pistons, but are made of soft "powdered" metal,
and the crank is nodular cast iron.
SC or 351w rods would work, with custom forged pistons,
but it's still a cast iron crank.

Some Stang guys are running centrifugal chargers on the 4.2s,
but the Eaton charger is supposed be much harder on the bottom-end, because it makes so much boost so quickly,
and more load on the crank nose, to drive the blower,
so no-one, to my knowledge,
has put a 4.2 shortblock under an SC top end.

It's not that turning .010 off the rear journal is any problem;
it's whether the cast crank would survive.
 
Reason I asked is beause the 4.2 are ridiculous hard to find because they blow up all the time. A buddy of mine has a late model truck with a 4.2 in it and he can't find a rebuildable engine for less than $1,000. Since there are so many 3.8's around and they seem to be more reliable, we want to use the 3.8 lower end (block and all) with his 4.2 upper end and install in his truck. I know there will be a loss in power but he isn't racing this truck and I don't think it would be that noticeable to him.
 
As you said, you'd be giving up some power,
but this swap would work just fine;
The SP heads swap onto the 3.8,
and the motor mount pads and
bellhousing are the same.
[That's for all RWD applications, of course;
the FWD blocks are altogether different.]

If the 3.8 you find is 96-up, use the 4.2 cam.
If its an 89-95 3.8 block,
the cam is the same.
 
Last edited:
Back
Top