I found the article months ago but can't find it now. Olds 307 5A heads. What's the biggest valve you can stuff in there without hitting the water jacket?? My machinist says he remembers reading somewhere that 1.875 was the biggest intake and the exhaust was just fine at 1.624. Who actually knows, if anyone??? I got a guy stuffing a 403 in a G body but wants the 5A heads.


Scratch the above. It's going in a 307, real bad miscommunication. Anyway, I finally got a hold of a Mondello book....and it's true. 1.875/1.624

 

5A heads on a 403? Why? They were barely adequate on the 307 engine.

 

They're going to be ported....irrelevant. We're not looking for principle. Looking for facts here. Ha. its to look stock in a G body with a 403. So. What are the biggest valves you can stuff in a 5A head?

 

I had 1.875/1.56 in my 307 heads with the bowls blended to the 1.875 intakes... It's been 6 years, so I can't remember much more details than that.

Dale has a cross-section head picture in his EMC thread here, you can get an idea of where the thin spots on the exhaust ports are... it's behind the guide... I'm not sure the 1.62 is going to gain you much based on that picture, that's just my .02...

http://highperformanceolds.com/phpbb....php?f=14&t=90

 

All SBO heads use the same valve center-to-center spacing as the BBO heads. That means that 2.070/1.625 valves should fit just fine. With the large 403 bores, shrouding won't be a problem. The bigger question is, can you port them enough for the 403 without hitting water? The 5A heads are from Oldsmobile's "lightweight" motors, so there is much less cast iron than in older heads. Personally, I'd start with BBO heads on a 403, since these heads worked fine on a 400 BBO. Ports are much larger to start with. Use flattop pistons to get the CR where it needs to be with the 80cc heads.

 

I have a feeling he's trying to hide the 403 either from someone he's trying to race, or from the state EPA.

 

The big "403" on the sides of the block will likely be more of a problem than the head casting letter. Besides, Chris Witt can help with that.

 

The primary concern is making it look like a 307. And yes, if it were an option, big block heads would be on the table first.

 

As stated above, use the Chris Witt method and use BBO heads. Grind off the casting letter and make a 5A in its place using epoxy or the like to disguise them.

 

Looks like Dale used 2.02" intake and 1.56" exhaust. I have those heads stock on my 350 in my 94 4x4. They run out of steam fast, would need a lot of work to flow any decent number.

 

I have 2.07 1.71 in my #6 heads, I don't think you can go any bigger than that.

 

I'll be back with how it performs. They're just going on a low mileage 403. The bottom end isn't coming out yielding the big combustion chambers with a thicker head gasket pretty weak. Thanks for the replies, and yes I saw the 2.02 valves but it appears like it is shrouded and they only installed a seat. From the cutout, it looks like it would cut into a water jacket if pushed any farther than the seat.

 

I wonder if W31, 2"/1.625" valves would fit without issue? Hope it works out.

 

As is usual, Joe poses a reasonable question. In this application, the over-sized valves will grossly out flow the port, so what is the point?