I've got a '68 307 SBC motor. It's a 283 block with a 327 crank. My understanding is that the stock heads provide something like a 76cc combustion chamber, with a CR of around 8:1. Head casting is #3911032. Block casting is #3914636. I can score any of these junkyard heads: #3884520 60-67 283-327 #3795896 62-67 283-327 #462624 71-85 350-400 #3731539 1957 283 4bc-FI I am told that some or all of these would be a 61cc chamber with 9:1 CR. I plan to keep the bottom end stock (pistons and all)(there is not even a ridge). Questions: Is this information correct? This will be a motor for an antique daily driver. I want to run it on regular (87 octane) gas. Is 9:1 too much? Should I just keep the stock heads? (I hear they don't flow too well.) What cam should I be looking for? RV? Stock GM? 3/4? Or whatever. Looking for good performance up to, say 5000 or 5000 rpm, good low end torque, decent highway cruise mileage. This will be a budget motor build. Normal rpm range likely 1500 to 2200 rpm. Will use a stock cast-iron manifold with either a 2GC or 4GC carb. (I have both manifolds and carbs.)
The only number I remember of the ones you mentioned are the 624’s. This is definitely a 76cc smog head. It will certainly lower your comprehension ratio on a 283. Another bad thing about 624’s is that they are “light casting” heads. This means that GM redesigned them to use less iron and they are crack prone.
70 cc, perhaps? Does the appearance of the engine matter at all? If you are going for the look of a 283, then heads from a 283, with the correct rectangle, or rectangle with a triangle on top, might be what you want. If it doesn't matter, then the heads you have now, if they're in good condition, should be fine. If you want holes in the ends to mount stuff, the 1969 and later heads have those, so if you go for the 305 heads, that will happen. As for the cam...you probably want something slightly more performance oriented than the stock 307 cam, but not much. You might want to consult some cam catalogs, and get a feel for what RPM range cams with different durations at .050" duration, will produce. My guess is you'll end up with something around 210-220 degrees at .050" lift.
Appearance - somewhat. I want the look of an early SBC, so no center bolt heads and, almost certainly, no Vortec heads (I want to keep a stock cast iron intake). I have not pulled the valves out of the original heads yet. Do they have hardened seats? When did GM go to hardened seats on all SBCs? Concerned about the thin casting warnings on some of these (looking for a bulletproof, understressed motor).
There are a couple of threads on building a nice 307 motor, have you read them? Here's one of them: https://www.jalopyjournal.com/forum/threads/puttin-perfume-on-a-pig-307-tech.45615/
Those are 70cc heads, with 1.72/1.50 valves. Nothing very special about them, and pretty small valves for any performance. Not really known to be performance heads either. Could maybe have them opened up to use 1.94"/1.60" valves and hardened seats, and make them a little better.
Leave the 624's in the junkyard where they belong. If you want it too look like a 283 i think the 539 castings have staggered vc bolts.
If you're not working the engine hard, then hardened seats aren't really necessary. If it's a light car, you should be ok. If it's a heavy car or pickup or you're towing, you probably want hardened exhaust seats. The 1.72/1.50 valves should be fine for the performance requirements you've listed. The thin castings are more of a later 70s thing. So...305 heads would be thin castings, 307 would not.
I'd never rebuild heads to use on today's crappy unleaded gasoline and not have hardened seats installed. Considering how little it costs to add them, it's silly to only do the heads halfway. The machine shop I use charged me $550 to do hardened seats, all new stainless valves, screw in studs, high light spring set. and 3 angle valve job. The cost of adding hardened seats was $100 of that. Sure not where I'd cut corners with unleaded gas.
If you want to spend all the money, then definitely get hardened seats. But if you only want to spend the money you need to spend, don't bother. I've done it both ways, and only had trouble once.
EQ makes iron SBC heads with 3 different cc’s we ran a set on a 1/2 mile dirt car. They make one set that are legal on IMCA modified.’s where they take more of a beating than any street car will ever give them. We ran them on our 372”, but needed to go bigger on the runner cc’s with our 422”. Prices are fair in my mind.
Good dilemma right there. If I am going to spend $100 or more for exhaust inserts, I can just as easily put that money towards different heads that are already properly equipped. Maybe if I stick with these heads I will run them as is and wait for them to poo the bed. If I can score others, according to the 307 thread and advice already provided in this one, the 305 416 might be best. 56cc chambers, inserts, accessory holes, and usually inexpensive. Not sure if they are the thin castings that crack, though. I don't think it makes sense for me to purchase heads without hardened seats.
I agree with Squirrel's recommendations (like that really matters!!) about the non-hardened seats and the cam duration. Since you want it maxed-out by 5000-5500 RPM look at a a short LSA number 110* or 108*. (Isky grinds most all his cams on a 108* LSA.) Torque down low AND that hot rod idle! Advancing the cam timing a few degrees more than recommendations will move the torque curve down on the scale(where you're more apt to use it) Just be sure you have good PTV clearance. 6sally6
The "power pack heads with the triangle work well on a 307. We had what was probably a mid 80's Z 28 cam that I picked up at the Portland swap meet in my son's 70 C10 with power pack heads and a 300 hp 327 intake on the 307 block. It even had my old Holly spread bore 4 barrel on it as we put it together on a tight budget. With a compound 4 speed and 3.7 gears it went pretty good for a long bed half ton. Pulled 16 mpg on a road trip to Texas and back too.
How close are your to Buffalo NY? I have a set of 991 heads for a 307 Small valves but just machined with new seats and springs... 1.72 intake valve. Bought them by accident but they are fresh and ready. Come get em
If you ever decide to replace the heads, you might consider a later model head simply because they were factory built with hardened seats. That will maybe save you from spending money on having them installed, if the heads you find are in good shape, and valves are still sitting proud, which will tell you the seats aren't worn out. I've seen far too many old heads with soft iron seats that had valves sitting deep in the chamber, and seats were gone. So if they need new seats, I'm puzzled why anyone wouldn't spend a little extra to get all the seats replaced with hardened seats, vs. just replacing the bad ones with old style seats?
There's tons of stuff in TX, OK. Just watch the classifieds or FB Marketplace. There are guys that can't get their machine work costs back out of them. I have a set of 462's that have sentimental value and I have as much in them as I could buy a set of decent aftermarkets. Just throw a 350 crank in it and you'll have a LG 327.
14014416 and 14022601, mid 80's 305 H.O. heads. 58cc chambers, hardened exhaust seats, but they are 1.84 / 1.5 valves. 307's I've seen have dished pistons. The 58cc heads will help greatly, if you change the cam stay with shorter duration, something like this 218 duration @.050 .450 lift https://www.summitracing.com/parts/sum-1785/make/chevrolet#overview Thread listing popular factory cam specs https://www.jalopyjournal.com/forum/threads/gm-factory-cams-specs.1227162/
Why mess around with old heads, unless you have a purist reason to retain a particular casting number? New heads, steel or aluminum are a bargain over unknown used heads and are plug-and-play. Speedway has some nice steel WISSOTA/IMCA heads at a good price, 64CC IIRC.
Same reason to mess around with the rest of an old car....because they're old. Some folks get it, some don't.
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