rpjr42

Good Morning All - In my quest to replace my oldsmobile 307 engine with and olds 350, I may have also stumbled upon an olds 403. I'm still newbish when it comes to engines. Being a smallblock, I think everything will bolt right in, and all the parts that come with it (distributor, carb, manifold, etc) will be interchangeable with everything I had with the 307 (extra parts!)

So my questions are:

Is my assumption correct with the interchangeable parts?
Read that the cylinder walls are "siamesed" - what does this mean?
Also read this motor overheats more than others - is this true?
I bought a turbo 400 tranny for the 350 - will this be OK for the 403?

And lastly - is the 403 something I should be looking at? I have very minimal information on it right now, but will be hopefully taking a look this weekend so i can see the casting numbers.

Thank you for your assistance, hang in there will by newbie lingo!

joe_padavano

The 307, 350, and 403 (as well as the 260 and 330) are all externally identical and are bolt-in replacements for one another. The only differences on bolt-on parts are with the 1985-later 307s with the 7A heads. These motors have the smaller intake and exhaust ports, so the manifolds don't really interchange (OK, they physically bolt on, but the port mismatch makes that an undesirable thing to do). Also be aware that the 307 uses the mini starter. You probably want to use the correct higher-torque starter on the 350 or 403.

The siamesed cylinders refers to the cylinder walls inside the block. Normally there is a coolant passage between them, but on the 403 (as with the 400 SBC), to fit the larger bore in the confines of the existing SBO architecture, it was necessary to make the cylinder walls actually touch in the fore-aft direction. This prevents coolant from flowing between the cylinders. Some claim this leads to overheating, but for a normal street driven car this is not an issue.

Intragration

Lots of Trans Ams and full-size GM cars got the 403 and didn't have problems. I wouldn't worry about it, just make sure your cooling system is set up properly and you will be fine. Also, while certain parts might be physically interchangeable, I would personally ditch the CCC distributor and carburetor for a regular HEI and Qjet. I have heard some people have been able to get their 350s and 403s running with CCC, I wouldn't bother with it.

joe_padavano

I agree that I wouldn't bother either, but it has been done, even with 455s. The CCC system only controls the primary mixture ratio. The secondary side of the Qjet is a normal mechanical system and you can change rods and hangers just like on any other Qjet. The biggest problem is the timing curve, which requires you to burn a new PROM to get it right for the different engine.

The only other things the CCC system controls that matter are the A/C compressor clutch and the 200-4R converter lockup. The A/C just needs an accelerator pedal switch like the 84-down cars, and there are a lot of aftermarket solutions for converter lockup.

Octania

I have been pretty happy with the 403's I have had. The '85 442 does not overheat AFTER GETTING THE TIMING CURVE RIGHT. With the stock 307 radiator. Very well behaved.

I never got around to hooking the AC up, and the trans lockup I had on manual but kept forgetting to release it which gets ugly when you stop, so I quit using it.

rpjr42

Appreciate all the great feedback again - you guys rock