Technical Confused sbc head question

It may depend on where you look but Chevy hardcore says
They are listed one above the other on the page

3998993: 1972 and 1973 307ci and 350ci, 75cc chambers, 1.72/1.50-inch valves.

3998997: 1972 and 1973 400ci, 76cc chambers, 1.94/1.50-inch valves.

Swap Meet Guide To Small-Block Chevy Cylinder Head ID (chevyhardcore.com)
I go along with someone sticking a mismatched pair of heads on it back in another of it's lives. Possibly when it went though a valve job with a cylinder head exchange. Speaking of that take a look at the valves and measure the stems. A cylinder head shop that I did head and engine changes in about 30 years ago years ago machined the valve stems down to a smaller diameter so they could save the valve and just use a guide with a smaller diameter hole. They have been out of business for a few years but those heads are still floating around.

 

have a nice fresh set of 991 heads larger chambers but smaller valves if you need them

 

Both head have the 1.94 and 1.50 valve in them I measured both sets with a micrometer.


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Ok so my kit showed up and instead of dish pistons it came with flat tops would these work with the heads I have? It would give my compression back.


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Depends on the pin height, rebuilder pistons assume the block has been decked some, so do some calculations. But you were probably running a low CR anyways with a 305 and the current heads

 

GM used the big open chamber heads on some lower compression motors. These heads seem to be changed as 305s did not come stock with the bigger valves (they could have but not probable). The smaller bore should need the intake to be unshrouded.

Your rebuild kit does not care what heads you got. Where you would need to be concerned is if you had aluminum heads. I run aluminum heads on my 355 and felrpo makes a specific head gasket for them.

I would do this before I screwed the heads on I would put a little lump of playdough on the top of the piston and roll it through. Then you can see where the valve strikes and tell of you want to unshroud it. Might be that a little nick in the top of the cylinder makes all the difference in the world how your 305 pulls.

 

You could always use a 14014416 head. This is a 305 head with a 58cc chamber, 1.85intake and 1.50 exhaust. You can punch them out to 1.94/1.50 or 1.85/1.60. You can get a little more compression out of these, and the rumor is that they flow air good and respond well to do a bit of porting. I have a set to go on my 283 that I had machined and built for about $300.

 

Later heads for hi output 305 are very light castings and prone to crack . Having large valves on a 305 usually shrouds the valve and reduces the flow . So far as even having a hi lift cam COULD possibly hit the small cyl walls found in the 305 . Make sure your head gasket doesnt rest in the combustion chamber , match the gaskets that came off the should be 305 other wise the 350 gaskets would be exposed to the combustion and soo fail .
Good luck . If you dont needthe front accessory bolt hole on your current heads , you could source a set of 283 heads and raise your compression, performance and have a head less prone to cracking . Your head was cast 4 month of 73 ( D 73 ) forgot the other alpha numeric code you pictured .
Gary

 

Bad news is even at best, a true 1.560" comppresion height replacement flat top, at stock block deck height and a composition gasket you are at a miserable 7.7:1 compression. If they are "rebuilder" height then it could be a 1/2 point worse yet.
That 305 originally would have had a 58cc chamber head on it from GM, the much bigger 76cc chamber just kills compression.
You really need a domed piston to get a decent compression ratio in a 305 with 76cc heads.

Or a blower LOL

BTW, I've had 2 sets of 993's in the past, both were 1.94/1.5" valve combo, right from GM.

 

GOOD CALL!!!!!! I never would have thought of that . Whoever put these heads on used 350 head gaskets as well. Fortunately I have a set of 350 head gaskets I can use. Thank you that saved me a ton of aggravation


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Hmnn, being part of a different site that the cars either came with a 350 or a 305, this is the first I’ve heard of the issue (prone to cracking)

 

Around 1974 chevy started to lighten the castings . Early changes include dual exhaust heat to intake manifold passages ( both center exhaust ports go all the way thru to the intakes center heat passage to heat the underside center of the intake manifold ) before this only one exhaust port was used . This increased the amount of heat and reduced the amount of material. The next move was a hollow passage under the rocker arm studs . Prior to this change it was solid and easily converted to screw in studs . In 1974-75 the exhaust seats were induction harderned (cars ) trucks usually had hardened exhaust seats with rotators and very heavy duty had sodium filled exhaust valves . As emissions and mpg figures became more important, weight was reduce across the board on most all castings . The only problem with using early 283 heads or for that matter any head befor 1974 is having non hardened seats so lead substitute is recomended if compression is high and pinging becomes an issue.
Gary