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GMC 6 buildups

Discussion in 'The Hokey Ass Message Board' started by mtkawboy, Aug 20, 2009.

  1. mtkawboy
    Joined: Feb 12, 2007
    Posts: 1,213


    Anyone in the process of building up a GMC 6 either for street or pure race ? Always wanted to do one, my cousin had a street 302 in a 53 Chevy back in the late 50s ,early 60s
  2. Mr48chev
    Joined: Dec 28, 2007
    Posts: 27,544


    The guys over at Inliners International have a section for those that is pretty active. There are several of us from here on the HAMB there also.<input id="gwProxy" type="hidden"><!--Session data--><input onclick="jsCall();" id="jsProxy" type="hidden">
  3. revkev6
    Joined: Jun 13, 2006
    Posts: 3,351

    from ma

    how much power do they actually make anyway?? I might have a possible lead on a wayne head but don't know anything about it. guy that has it was an old hot rodder turned sports car racer. they any good on the street??
  4. panic
    Joined: Jan 3, 2004
    Posts: 1,450


    Even the biggest engine can't keep up with a 292 or 300 Ford, but it's fun to try.
    After Wayne etc. the best head is the -417 big-port (1950-52?). The SSR is bad on all of them. Rods and oil system are pretty good, pistons terrible (heavy and weak). Needs pushrods, some fabricated rockers, serious porting work, and much more cam.

    I have a gut feeling that the late Toyota Land Cruiser L6 (12 port, reverse flow, wedge chamber, iron) may be adaptable - very cheap, big improvement over the original.

    Attached Files:

  5. Truckedup
    Joined: Jul 25, 2006
    Posts: 3,864


    Power varies with the amount of bullshit being thrown. I've built a few,a moderate 302 build,stock head with bigger valves,forged 9-1 or so pistons,cam around 215 degrees duration at ,050 lift,400-500 cfm worth of carburation and some kind of reasonable headers will make 225-240 HP at 4200 rpm,lots of torque,maybe 325 ft lbs at 3000 rpm.An engine like this will idle with a mild lope at 700 rpm,very flexible around town and pretty much lay waste to other vintage non overhead valve engines of the era.
    You can get a lot more power,spend a lot more money.....................
  6. Gigantor
    Joined: Jul 12, 2006
    Posts: 3,819


    I got a 302 that has been relegated to the "for sale" pile since I got my Hemi. I might just keep it on the back burner indefinitely since I can't part with my 308 Hudson before that Jimmy 6. Too many engines. But man, they are cool.
  7. sdrodder
    Joined: Feb 8, 2008
    Posts: 507

    from Houston TX

    Well first off is how much is the head? The wayne heads were mainly used for raceing. I will look around in my picture collection to see if i have any pics of them. Also what u need to consider is what condition is the head in? is it complete with all the exhaust manifolds and such. Also as i heard i think that if u put on a wayne head i think u need to build up the bottom end also but not quiet sure on that.
  8. RichFox
    Joined: Dec 3, 2006
    Posts: 9,950


    If you go to a Wayne head you will need new pistons. the Wayne head has no combustion chamber in it. It's flat like a 348 Chevy or a 430 Merc. Maybe you could find a 4.060 BBC piston that would work. The early 302 had a .990 wrist pin same as BBC. As has been pointed out intake and exhaust will be made for a Wayne. I ran Howard and Hourning heads on stock bottom ends without and trouble.
  9. MichaelDorman
    Joined: Apr 27, 2001
    Posts: 849


    The Toyota Land Cruiser heads will not fit. While they are similar in dimension, the water pasages and valve ports are all off.
  10. Truckedup
    Joined: Jul 25, 2006
    Posts: 3,864


    The stock 302 head is a wedge chamber very similar to early GM ohv V-8's.The stock valves are kinda small,it's easy to fit Chevy V-8 valves .They are shorter than the stock valves so the pad on the stock stamped sheetmetal rockers that contacts the valve can be driven down and welded to maintain reasonable rockerarm geometry.After market valve spring seats for SBC slip over the guide perfectly so low cost Chevy springs and retainers can be used.The stock 302 combustion chamber is 118 cc's.I milled the head .020.decked the block.040,had Venolia make forged pistons with a very slight dome for an actual compression of 9.3 using common SBC 1/16 inch compression rings.
    Some guy say you fit a small valve and combustion chamber 270 head on a 302 for a higher compression without expensive pistons,you be the judge on this .
    The rods on my engine were the latter ones with the 1.090 or whatever wrist pin.I had custom bushings made and the holes were offset to make allowance for the rods being different in length as much as .007.
    The block needed milling because the factory machining was shitty ,the deck was tilted front to rear and side to side.The bores were offcenter and so on.The machinist did say the crankbore machining was very good.
    Stock GMC 302 crankshaft is hardened,maybe Nitrided for durability.My machinist didn't want to turn the crank and remove the hard surface treatment.But the never turned crank was right on the limit so it got turned.
    All this machining gets a bit expensive........I don't know if it's really necessary for a street engine...but that's what I did.
  11. mtkawboy
    Joined: Feb 12, 2007
    Posts: 1,213


    Ive already been down the Ford 300 six drag racing route in 1970. Went pretty good with a Sissel head & cam, Holman Moody 4 barrel manifold, 600 Holley and I forget whose headers in a 51 Anglia in H/G. When NHRA allowed the hybred V8 heads welded together everything was instanly obsolete. Now Im interested in vintage motors

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