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2-Bolt vs. 4-Bolt Engines - What's the Difference?

Discussion in 'The Hokey Ass Message Board' started by Heckler, Apr 2, 2007.

  1. Heckler
    Joined: Mar 20, 2005
    Posts: 200

    from Austin, TX

    Help me out here,

    I know that 4-Bolt engines are more desirable, but why? What's the difference?

    Are there instances when you shouldn't use a 2-Bolt? Like if your motor is going to exceed X horsepower, or if you're going to run a blower?

    How can you tell a 4-Bolt main from a 2-Bolt main by looking at the motor?

    Any other info would be greatly appreciated!

    Thanks! Ben
  2. 53sled
    Joined: Jul 5, 2005
    Posts: 5,818

    from KCMO

    get a (cheaper) 2 bolt, then have it machined for splayed caps and you have MORE strength than a factory 4 bolt.

    you can't always tell by the outside, the casting #s were shared in the 70s. mortec can help you find one
  3. buzzard
    Joined: Apr 20, 2001
    Posts: 4,335

    Alliance Member

    Pull the oil pan and look at the caps that hold the crank. It it has 2 bolts holding the main caps on, it's a "2 bolt main". If it has 4, then it's a 4 bolt.

    What 53sled is talking about is the 4 bolts will all be drilled straight into the block. All 4 bolts being parallel. If you get a 2 bolt, a machine shop can drill it for 2 more on the outside of those. The will drill them at an angle so the 4 bolts are not parallel. It'll be stronger than a stock 4 bolt.

    For a blower, I'd for sure get a 4 bolt main, or have a machine shop drill it for 4 bolt splayed.

    Go by Crushproof this weekend and I'll show you on my 406, on the stand. It's a 4 bolt splayed main. Seeing it will make perfect sense.
  4. squirrel
    Joined: Sep 23, 2004
    Posts: 52,400


    For a mild street blower motor, a 2 bolt may or may not be ok on a small block, and would most likely be ok on a big block (chevy). For a more serious engine, go for a 4 bolt....factory 4 bolt small blocks are generally pretty easy to find, and the $premium is not that bad. As mentioned, a set of aftermarket 4 bolt caps properly installed on a 2 bolt block is better yet, and also there are a bunch of aftermarket blocks available now if you're really worried about it.

    Most guys who have a 4 bolt 350 have it only for bragging rights, they'd be fine with a 2 bolt.

  5. SinisterCustom
    Joined: Feb 18, 2004
    Posts: 8,274


    4-bolt blocks are more prone to cracking in the main cap area, at the inner bolts of the caps on the block. .... FWIW........
  6. scottybaccus
    Joined: Mar 13, 2006
    Posts: 4,109


    I've seen more than one durability test over the years showing 2 bolt blocks (properly built and driven) reliably holding 500 to 600 HP, especially the ford 5.0. SBCs are sometimes dependent on the particular casting. Unless you plan to turn one to 7000+ rpm routinely, or use crazy compression ratios/blower pressures, a 2 bolt is fine.
  7. whatever
    Joined: Oct 27, 2006
    Posts: 116


    ive heard two sides to the story people say 2 bolts are stronger because there is more material in the block and also ive heard 4 bolts are stronger because there are 4 bolts holding the caps bla bla bla :) i say run what ya got
  8. RacerRick
    Joined: May 16, 2005
    Posts: 2,757


    2 bolt vs 4 bolt is a chevy thing. Also remember that there are lightweight blocks when dealing with small blocks. These blocks started in 1980 and will not take as much power as the earlier heavier castings.

    2 bolt mains in a small block are fine for under 6500rpm or 450hp if you stud them on a typical 350 chevy. Smaller motors can get more RPM out of them because the shorter stroke cranks have less deflection.

    If you run a blower with a gilmer style belt, I would only run a 4 bolt and a steel crank. The blower puts a ton of strain on the crank and block as it yanks up on the crank snout.

    4 bolt main factory blocks are good to about 650hp in a SBC.

    In big blocks, you can get away with a lot more because of the beefier bottom end. The factory rods are the weak point. a big block 4 bolt main will take 800hp reliably and a 2 bolt main will take 550hp without a strain.

    Remember you can go higher in rpm and HP but these are all pretty safe numbers where you will never have to worry about block strength.
  9. Gotgas
    Joined: Jul 22, 2004
    Posts: 7,077

    from DFW USA

    Two bolt means each main cap uses two bolts to hold it down. Four bolt means there are four, which means more clamping force, and less likelyhood that the crank is going to push itself out of the bottom of the engine at high RPM / power.

    Here is the "splayed" 4-bolt mains. These are always aftermarket.


    The strength comes from the "extra" bolts being out of alignment with the force the crankshaft is putting on the main caps. Top fuel (and the Ford 4.6!) use six main bolts per cap, four like these above, and another pair that come in from the outside of the engine block.
  10. temengines
    Joined: May 9, 2006
    Posts: 10


    People are right. If you have a four bolt use it, unless your building a 400 small block. If you have a two bolt find a GOOD machine shop and have your block magnafluxed and stress relief. Then have it align honed using ARP studs. Splayed caps are killer but a bit expensive and overkill for street.
  11. One other thing to consider is that there are certain factors that can greatly affect the main caps (& bearings for that matter).

    Detonation under load is the #1 factor in cap walk & main bearing death (assuming that the oil is kept changed & the owner actually gives a crap :) ). In a street engine, the caps/bearings may not show as many signs as the rod bearings & pistons; but in a racing application it can be a tough call as to which will go first. When you see cap walk, it is almost always caused by detonation- & four bolts, even splayed, won't hold the cap in place under that. Doweling might, but it doesn't save the bearings forever. Just another reason to tune carefully. :)

    Given all other things equal, nitrous tends to be harder on mains, than, say, the same power made by a turbo- the shock load is greater. As mentioned, a blower puts a lot of stress on the front main & crank snout. All good reasons to use four-bolt mains....though I have two-bolt mains with studs in a 402 BBC with over 550 hp on the dyno, naturally aspirated, not counting the nitrous, & it lives just fine. :) The two-bolt Ford BBF blocks hold up under even more.
  12. john56h
    Joined: Jan 28, 2007
    Posts: 1,767


    Four bolt blocks were developed by Chevrolet mainly for trucks. The two bolt main bearing retainers were fialing due to the excesive "lugging" that was typical with a truck that was loaded. This was strictly a low RPM problem.

    Racers of course liked the idea of the extra bolts retaining the main caps, so they became commonly used in racing and were installed in many factory Muscle Car engines.

    As the others have said...a fine bottom end can be built from either. The 2 bolt is plenty strong for most street driven and mildly modified racing engines. If you are getting into serious big power modifications, then the aftermarket caps may be the way to go...which will require a 2 bolt core block.
  13. I ran a 383 sbc with ARP studs on a 2 bolt block, dyno'd at 475hp, was just fine, I think the best tip is have the rotating assembly balanced by a good shop, and it will have alot less stress....

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