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3.8 stroker with a 4.2 crank?

Discussion in '1994-2004 V6 Mustang Tech' started by StangRacing99, Feb 1, 2008.

  1. StangRacing99

    StangRacing99 StangRacing99

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    Is it possible to use just the crank out of the 4.2 to stroke the 3.8 without changing the rods and pistons. I want to stroke the 3.8 without changing the bore size.
     
  2. taylor0987

    taylor0987 New Member

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    the bore on stock production 3.8 and 4.2 engines are the same.

    however the 4.2 pistons are different even at the same bore.

    if you reuse your 3.8 pistons they will collide with the valves. your engine will run for only a few minutes before destroying itself.

    if you are reusing your block then at a minimum you will need to have it honed. often the block needs to be bored to clear up wear. if you donot clear up the wear then it is likely that you will have problems. this is not because you are changing to a 4.2, rather it is something true of engines of different makes, displacement, number of cylinders, etc.
     
  3. No Nube

    No Nube New Member

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    So does the 4.2 use a different length rod as well?
     
  4. taylor0987

    taylor0987 New Member

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    stock 4.2 rods are the same length as stock 96+ 3.8 rods.

    so you can take your 3.8 block, drop in a stock 4.2 crank, your existing 3.8 rods, and stock replacement 4.2 pistons and you will have a 4.2.


    this kind of modification is rebuilding the engine, and as with any stock or performance rebuild, the block will need some preparation for it to be done properly. boring may be required depending on the condition of your cylinder walls. stock replacement pistons usually are available in standard, .010, .020, .030 and sometimes .040 over. you will need all of the bearings and gaskets that are usually needed when rebuilding an engine.

    if you are doing a performance build then there are some reasons to change the length of the rods. when you change the length of the rod, you would also need custom rather than stock replacement pistons.
     
  5. shoope1

    shoope1 New Member

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    So if you have the correct block, all I need to get is basically a rebuild kit for a 4.2? How about oil pan is their any issues with this?
     
  6. taylor0987

    taylor0987 New Member

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    Hi Shoope1 and welcome to the board!

    yep a 4.2 rebuild kit that includes pistons plus a 4.2 crankshaft. The 4.2 crank is now available new for $135!! http://www.trademotion.com/partlocator/index.cfm?siteid=214259 (just look up 2006 F150)

    You can use your stock oil pan if you reuse the Mustang block.

    The oil pan discussion for 4.2 swaps was for people swapping truck blocks into Mustangs. The truck oil pan hits the k-member on some cars. The 94-00 RWD car oil pan interfered with the girdle/main studs on the truck block. However the 01-04 Mustang oil pan clears both the girdle and the Mustang K-member. (because the 01-04 Mustang engines got the girdle too)
     
  7. V6procharged

    V6procharged Moderator Staff Member

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    it doesnt show any kind of rebuild kit.... what do you go to?
     
  8. taylor0987

    taylor0987 New Member

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    The link I gave was where to purchase the crankshaft.


    that site doesn't sell a "rebuild kit" rather you can buy the components separately.
     
  9. V6procharged

    V6procharged Moderator Staff Member

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    do you have a list made up of what all is needed for the complete kit? rods, pistons, rings, lox, pins.etc.
     
  10. taylor0987

    taylor0987 New Member

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    again, if you are doing a stock rebuild then again you do not need to replace the rods. the 3.8 and 4.2 rods are the same length.



    pistons
    rings
    rod bearings
    main bearings
    cam bearings
    valve grind gasket kit (this kit includes head gaskets and you should get it from Ford. on that web site part # 2L3Z-6079-BA)
    oil pan gasket
    rear main seal
    timing cover gasket set
    freeze plugs
    head bolts or head studs (head bolts ARE NOT reusable on these engines. failure to use new head bolts will result in head gasket failure)
    pilot bearing if your car is a 5 speed




    depending on your mileage and budget:
    oil pump
    timing set
    lifters (the 3.8 and 4.2 lifters are the same)

    assembly lube
    lock-tite
    RTV
    motor oil
    spark plugs
    other fluids
    shop manual with instructions and torque specs

    and probably other things I have missed
     
  11. mustangvsix

    mustangvsix 4 point 6

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    just curious but what kind of power gain does doing a 4.2 rebuild on the 3.8 show?
     
  12. V6procharged

    V6procharged Moderator Staff Member

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    thats a long list lol. id like to know the approximate gains as well.
     
  13. taylor0987

    taylor0987 New Member

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    2001 F150 4.2
    202 HP @ 4800 RPM
    252 Ft-Lbs @ 3400 RPM

    2001 Mustang 3.8
    193 HP @ 5500 RPM
    225 Ft-lbs @ 2800 RPM

    the f150 upper intake is more torque-oriented
    the f150 exhaust system is probably more restrictive
    and there are other differences
    so you aren't necessarily getting the best comparison.

    going from a 3.9 to a 4.2 in a freestar, you get a gain of 8 HP and 23 ft-lbs of torque.

    remember that those are only peak numbers and you should be getting more torque over a wide range of RPM's with the larger displacement engine.

    if you are building a 4.2 then since you would already have the engine apart, it would be a huge missed opportunity if you did not put a performance cam into it. so you would make more HP from that

    the increased displacement will respond that much better to other modifications as well.
     
  14. engineman312

    engineman312 New Member

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    i'm confused... the crank will give the engine a longer stroke, but you don't need new connecting rods? won't the stroke will be longer, and therefore go higher in the cylinder? won't it damage the heads and destroy itself?
     
  15. taylor0987

    taylor0987 New Member

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    all stock Ford 96-04 3.8's and all stock Ford 4.2 v6 97-08 have the exact same rod length.

    yes the rod will go higher in the cylinder.
    that is why you must use 4.2 pistons.

    in the 4.2 piston the pin is located much higher than it is located in the 3.8 piston. so the piston will not contact the valves or the heads.
     
  16. V6procharged

    V6procharged Moderator Staff Member

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    good question, good answer.
     
  17. engineman312

    engineman312 New Member

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    ah, so you need new pistons, but not rods. thanksfor the well formed annswer!! :thumbup: :D
     
  18. lgarza

    lgarza 00mustang

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    Another question on this subject?? I have an early 2000 5 speed which iam under the impression are externally balanced? And the 4.2 crank is internally balanced what if anything will i need or will a shop be able to balance the engine with just those parts?
     
  19. go-fast

    go-fast New Member

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    you need the flywheel and balancer off of a internal balanced motor
     
  20. myslow2002gt

    myslow2002gt New Member

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    In an ideal world, it should, however the off the shelf setups use the stock length rods.

    A stroker crank will increase dwell time at BDC (bottom dead center) and will pull the piston further down in the bore in doing so. The wrist pin is located higher up on the piston to keep the appropriate rod ratio.

    An ideal setup would involve a custom set of longer rods and a custom set of pistons, but is only needed on a race engine spinning high RPM's to reduce side load on the piston.