: 403 Swap?

01-02-07, 12:43 PM
This will make you want a 403 Olds in place of the 307...


I can attest, these cars aren't lightweights....

01-02-07, 06:56 PM
But at that point, it's not THAT much more effort to drop in a 455 Olds...

01-02-07, 10:05 PM
But if you are good, you can make the 403 look like a 307....

I am regretting letting my 68 455 Olds go with "C" heads....

01-03-07, 09:25 AM
But if you are good, you can make the 403 look like a 307...

In fact, you don't even need to be "good" - just a little paint and reinstallation of all the accessories. I submit, however, that to the casual observer (ie, most smog test technicians) a 455 can be made to look like a 307 also...:yup:

01-03-07, 09:53 AM
I agree, not to many techs are out there who probably have SEEN a 455 Olds in a long while! But it IS a big sucker....

I did my 350 like a 307, only "issue" was the "3A" on the heads (which I could have ground off and faked something) and to drill out the AIR manifold ports so the AIR pump could have gone on. But all else was there, intact and functioning. No one ever was wise to it.

I did ponder getting some 5A 307 heads to drop on, really wanted some 5a heads. I even had a 82 307 intake on there and a 84 307 carb (the 85 got damaged by a gorilla mechanic who over tightened the front mounting bolts and squashed the TPS)

My 307 valve covers had the proper 85 inspection stamps still on it. It was very original looking. I wish I had dug up a 403 instead though....

01-03-07, 06:14 PM
455 is only 1" taller and about 1.5" wider than a small block. It's all in the taller deck, as you probably know. Also, FYI, the 66-67 California cars (like my 66 442) came with A.I.R., so you can get good flowing heads from either a 330 or a 400/425 that already have the A.I.R. ports in them.

01-27-07, 02:06 PM
I'm considering this swap in my '83. Will the TH-200 tranny take the added torque? It's an Olds 350 now, used to be a diesel...

I already found the complete 403, supposedly from a '7? Formula Firebird.

01-27-07, 06:25 PM
Officially in stock form the THM200 4R was rated at 275 lb/ft torque.

Modified, LOTS of hp.....

01-27-07, 09:07 PM
If that's the original trans from the diesel, then it's a TH 200 three speed, not a 200-4R. Not one of GM's better transmissions. I'd at least swap a TH350 in with the 403.

01-27-07, 11:20 PM
I would agree, the THM350 and 403 do well together. The THM200C is ok, it did last 275K untouched in my dad's 85 Cutlass, and the last 75K behind a 350 with 2.56's. Only "mod" is Amsoil around 50-100K. Same fluid in till 275K when the car went to the boneyard. Honestly, I can't knock a THM200C. It did hold up well. It wasn't abused, but WOT was seen quite a few times in the last 100K miles....