403 Exhaust Manifold Number/Shape

[MO 1979 TA]

Hi - Wondering if anyone can confirm if the passenger side exhaust manifold for the Olds 403 in a '79 TA is a #5 (411962?) that appears to have a 90 degree downward bend? I picked up one from an Olds 350 but the exhaust outlet bend is more of a 45 degree bend (#4) and don't believe is a compatible replacement. TIA!

[Wallington]

#411962 is an early 70's Olds 350 to suit A-body. It has a slight angle to the down pipe and only 2 mounting studs, from memory. Possibly different bore.

#417754 was common to many 77-79 Olds 350/403 cars, including Firebird models. The downpipe was almost square and was now a larger 3-stud mount.

It depends if you exhaust system is stock original or already modified to suit previous manifolds, and can be again if needed. Even the reproduction Thornton manifolds use yet another angle for the mounting plate outlet and requires further mods, for stock or custom exhausts.

Same manifold #417754 used on a '77 350 Olds in a Buick. Note the original locking tabs and the plated crossover cap for dual exhaust option. 3-stud downpipe.

350 Olds exhaust manifold locking plates by Aus78Formula, on Flickr

Exhaust from a '78 403 Firebird.

Firebird 403 Olds RH exhaust manilfold 417754 (1) by Aus78Formula, on Flickr

Firebird 403 Olds RH exhaust manilfold 417754 (2) by Aus78Formula, on Flickr

[MO 1979 TA]

This is beyond helpful. Thank you a bunch.

As you touched upon, the 411962 from the 350 may work as the exhaust system I have is not original and is currently set up for headers; so a custom pipe bend may make it work to connect the manifold to the exhaust pipes.  I would have much rather found the 417754 but haven't had much luck so far. Fingers cross when I install the block!

Thanks again for the info as well as all the pics.

[Wallington]

No dramas. Yes, if currently custom then good chance even the original manifolds wouldn't fit without work either.

The Thornton manifolds are back in stock, if you are considering an exhaust upgrade but like the 'stock appearance' and not using full headers. I have plenty of those photos too and comparisons to stock, just depends what you were trying to do.

Also, be aware that if trying to turn factory manifolds with crossover into true duals by capping the extra outlet, you will probably have different exhaust bore sizes from side to side, another feature the Thornton manifolds correct.

[MO 1979 TA]

Thank you. I have seen/read about the Thorton's; I spend more time on ClassicOldsmobile than here honestly. Lots of opinions on whether they are worth it and in my case, they probably would be since I did not have any manifolds to begin with and don't want to fight the headers. I hope to get these 350 manifolds to work and if not, probably go with Thorton.

https://classicoldsmobile.com/forums/