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Old 03-03-2008, 12:56 AM
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425 olds motor

I was woundering if i can put a 425 olds motor in a 1979 trans am with a bad 403 olds ? If anyone could help that would be great! Thanks alot ! Also will a th350 bolt up to the 425 ?

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Last edited by ntrkooled; 03-03-2008 at 01:06 AM. Reason: forgot another question
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Old 03-03-2008, 04:39 AM
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YES!!

The 425 olds engine is an awesome 'plant, way better choice than that 403. I'm pretty sure it will use the same motor mounts as the 403, but there may be a few differences in mounting boss location, I'm just not sure... the engine will bolt to the trans as long as the trans is the BOP bolt pattern (different than the chevy pattern). What year is the 425? And what heads are on it?
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Old 03-03-2008, 11:47 AM
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The 425 will bolt in to your F-body with a few things to watch for. First, the F-body uses different motor mounts from those of any other Olds, so you must transfer the 403 mounts to the 425. The mounting holes are in the same location on the block, so this will not be a problem. All 64-67 Olds motors use a different crank flange bolt pattern than the 68-up motors, so you will need the correct flexplate or flywheel that goes with the 425. Big block Olds motors have a taller deck height than small block Olds motors, so the valve covers and carb will sit about an inch higher than those on the 403. The F-body Olds motors used a unique accessory bracket design, so you'll need to adapt these to the 425. Most will fit just fine, but any brackets that span from block to head might require modification due to the deck height differences. Use the water pump from the 403. Olds used three different pump lengths and the brackets and pulleys match the pump. Finally, you'll need either big block exhaust manifolds or correct headers. One last thing to look out for - the 425 could have come with one of two different lifter bank angles (45 deg or 39 deg). If you ever change the cam, you MUST get a cam that matches the bank angle of the block. The 425 had a forged crank and is a strong and underrated motor. Good luck.
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Old 03-03-2008, 06:28 PM
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Very useful information, Joe. That kind of stuff is hard to come by. Thanks for sharing your knowledge.

tom
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Old 03-03-2008, 07:31 PM
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403 to 425

thanks for the great info . If you had your choice would you go with a 425 or a 455 ? Also how hard is it to change to a 455 from a 403 ? Thanks again , this is a great website .
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Old 03-04-2008, 09:51 AM
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Quote:
Originally Posted by ntrkooled
thanks for the great info . If you had your choice would you go with a 425 or a 455 ? Also how hard is it to change to a 455 from a 403 ? Thanks again , this is a great website .
Frankly, the two motors are almost equivalent. Whichever one you can get is the one to use. The 425 and 455 are externally identical, so everything I wrote above still applies with the one exception that the 455 has the same crank flange bolt pattern as the 403, so you won't need a new flywheel. Also, all 455s have the 39 degree lifter bank angle (common to all 68-90 Olds motors) so you won't have that problem either. Due to the slightly shorter stroke on the 425, the rods on that motor are longer and the piston compression height is different (despite the fact that both have the same bore). The 425-unique pistons are a little harder to find and thus are more expensive than those for the 455.
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Old 03-05-2008, 11:55 PM
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Quote:
The 425 olds engine is an awesome 'plant
Quote:
The 425 had a forged crank and is a strong and underrated motor
I had a '66 Dynamic 88 with a 425 and it was the BOMB. I'd love another one of those motors and can't say enough about 'em. Funny thing to pull up to a stoplight in a 4 door and smoke the local 'hotrodder'. Too funny. Awesome motor.
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