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Old 10-16-2005, 08:13 PM   #1
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Default How To Swap A 350 Or 403 Olds Engine In Place Of Your 307

This question is very popular, so here is an overview:

To begin with, swapping a 350 or 403 Olds engine in place of your 307 is very easy, because ALL small-block Olds engines(260-307-330-403) share the same block dimensions, in other words they are all the same size. They all share the same bellhousing pattern,and in fact most everything in this engine family is the same as far as mounting locations, water outlets and things of that nature.As a rule all accessories bolt in the factory locations, and with a little attention to detail and some black paint, a 403 can easily masquarade as a weak 307.

This swap amounts to buying or building a 350/403, and simply drop it in. Things such as starters, flywheels, carburetors, exhaust manifolds and accessories swap directly over, or you can just use the 350 pieces.

You do not want to use any 307 heads on a 350/403. You can use 307 intakes and exhaust manifolds on a 350/403 provided that they are the 1980-84 versions with normal sized ports. Otherwise the later 307 pieces will restrict the engine unduly.If you are using the factory computer-controlled Qjet from the 307, at the very least you need to modify the secondary airdoors to open fully, and preferable rejet it to match the larger engine.


As with anything, there are exceptions. This is an overview of the swap, and not every detail has been explained. This does NOT apply to the installation of Chevrolet smallblock in place of an Olds engine. There are several differences including a different bellhousing pattern which dictates the use of a chevy transmission, and is generally discouraged.

This also does not address emission requirements, so it will be up to you to get through emissions testing.


The big-block Olds in basically a taller deck version of the smallblock Olds, so it is nearly as easy to swap in as a SBO. However, the engine is wider, so in most cases headers are a difficult proposition. Otherwise, with a little patience this engine can be dropped into your 307 car with little effort.


Since all Oldsmobile V8s share the same bellhousing pattern, any transmission that fit a 307 will fit the other engines. However, if the transmission in question is a metric 200, it's lifespan might be measured in hundreds of miles. In most cases, a 350 or larger engine used a transmission that was capable of handling the increased power, so try to buy them as a pair.

231 to a V8:

This is a popular swap also. There is more involved here, but it amounts to removing the V6 and acquiring some V8-specific items before installing the V8.

These include radiator, fan shroud, engine mounts and frame mounts(you CANNOT use the V6 frame mounts because they will sit the V8 too high). The V6 fuel line from the tank exits on the opposite side from the V8, you will have to run line over to the other side in a safe manner. The charcoal canister and battery swap sides when going to the V8.

There are some wiring issues that have to be addressed, but it mainly involves rerouting the wires for the sending units and the distributor.

The very best place to find the needed swap parts is from a junkyard G-body Olds.
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