INSIDE THE 307 CHEVY.... It was produced by Chevrolet from 1968 until 1973, and was used in passenger cars and some trucks. They were all two bolt mains blocks and all came from the factory with two barrel carburetors. The 307 was slated from the start as a low performance, economical replacement for the 283, and was never considered a performance engine by Chevrolet. Looking at the bore and stroke specifications, you will see that the 307 comes with a 3.875" bore and a 3.25" stroke. What it essentially was, is a 327 crank dropped into a 283 block. No mystery combo there...just an easy to produce base model V8 for Chevy cars using existing parts. (So, you can think of your 307 as either a stroked 283, or as a small bore 327!) To put into easily read chart form: 283 = 3.875" bore X 3.00" stroke 327 = 4.001" bore X 3.25" stroke 307 = 3.875" bore X 3.25" stroke The journal sizes on a 307 are 2.45" for the mains, and 2.10" for the rods. Again, no hidden surprises here, just a common, cast 327 crank in disguise, folks! All 307 blocks are two bolt mains, but there are several different casting numbers. One isn't really much better than any other, but I'll list them here for the purpose of aiding in identification, so you can be sure that you do indeed have a 307 on your hands. 3914636 (1968 passenger car) 3931174 (1968 truck block) 3956632 (1969) 3932371 (1969-1973) 3932373 (1969-1973) 3970024 (1969-1973) 3970020 (1970-1973) The blocks are really nothing special. Externally, they look just like any other late 60s to mid 70s small block (400s excluded!), and they accept all such oil pans, accessories, and other bolt-on parts such as manifolds, water pumps, fuel pumps, etc. Cylinder heads are a point of concern on 307s. Although they used common castings often shared with the tamer versions of their 327 and 350 cousins, they were saddled with small (1.72") intake valves due to their bore size. The many 307 head castings available are not your best performers, but we can work around that with proper machining and parts selection. More on that later. For now, let's just see what casting numbers were available on factory built 307 engines: 3911032 (1968, 70cc) 3917290 (1968, also used on some 327s) 3917293 (1968, 75cc) 3931633 (1968-1973) 3986388 (Used from 1968-1976 on 307 and 350 engines) 3927185 (1969-1976 307/327/350, 70cc) 3932454 (1969-1973) 3927188 (1970, 74cc) 3986339 (1971 307/350) 3998991 (1972-1973 307/350, 75cc) 3998993 (1972-1973 307/350, 75cc) As can be determined from the list above, some low performance 327 and 350 engines were also equipped with these small valve heads, so they can be found easily for next to nothing or free, as any serious 327 or 350 build is going to involve discarding those castings in favor of better heads with 1.94 or 2.02 valves. In addition, although I didn't list them here, any 283 head castings can be used on 307 blocks. This is good news for those wanting to keep a nostalgic look and flavor about their project, as it allows for the use of the old 283 "Power Pack" heads, and any number of choices from the 283 line-up. This expands your range of selection, and means that you can use 60s style heads with no accessory bolt holes for a cleaner, more traditional look if your 307 build is slated for use in a trad rod or custom. Just more fuel for thought. Aside from their bore and stroke numbers, small valve heads, and specific casting numbers, the 307s are just another small block Chevy on the outside, so building one is still a fairly affordable deal. You just have to know what you're dealing with going in. BUILDING THE 307... 307s got a real bad rap early on as poor peformers, and it stuck with them. The primary reason for this awfull reputation, however, comes from one single mistake on General Motors' part...they equipped the 307 with a wimpy, unhardened hydraulic camshaft. These cams wore out very quickly, leaving the engines way down on power and way up on fuel consumption. Many owners failed to diagnose this condition, and just sold their cars in disgust, or swapped in 327 or 350 replacement engines. The fact is, a simple cam swap would have put their wheezing 307 right back in the fight, had they taken the time to check it out! So, obviously the first thing you should know about building a 307 is that you're GONNA have to buy a cam. Luckily, there are litterally THOUSANDS of factory and aftermarket grinds to choose from, so fear not, brave warrior! The small valve heads, along with the bore/stroke relationship will play a critical role in camsfaht selection. It is easy to "over cam" a 307 and make it perform WORSE if you ignore these factors, so avoid the temptation to use the same lumpity cam your buddy is running in his built-to-the-gills 350! You want a hydraulic cam with sensible duration and relatively small valve lift. This leaves plenty of options open, and it should be said right now that building a 307 for anything other than mild street/strip duty with limited RPM (under 6200) expectations is a foolish affair given it's specific shortcomings. For this reason, we will be looking at cams in this relatively mild range.