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Old 03-18-2018, 08:56 PM
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Oldsmobile 425

Hey guys. I'm new to this group. I recently bought a 1971 C10 with an olds 425 in it. Can anyone tell me about this engine? It's a strong running engine but I'm just not familiar with them. Any info would be great!

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Old 03-19-2018, 03:04 AM
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Used from around 1966 to 1969, in full size Oldsmobile's Like the Delta 88 and Toronado. It is the precursor to the 455 that came out in 1970 and replaced the 425. Part of the Big Block Olds family.

A decent engine, often overlooked because it as never used as a high performance engine other than a couple of the Toronado packages.

IIRC it should have a forged steel crank and have a desirable head casting on it.
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Old 03-19-2018, 05:41 AM
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Actually, the 425 was made from 1965-67. Every one of them has a forged crank. Nearly all were high compression. 4.25 bore, 3.98 stroke. Externally they are dimensionally identical to the Olds 455. Beware that most 425s used a 45 degree lifter bank angle instead of the 39 deg version more common on Olds motors. The cam is different as a result, so be sure you know what you have if you ever plan to get a cam. Head castings were different each year. The 1965 "A" castings used 3/8" rocker studs. The 66-67 heads used 5/16" bolts. All 455 eternal parts (intake, headers, distributor) bolt right on. The 1964-67 Olds motors use a different crank flange bolt pattern than do the 1968-up motors, so flywheel needs to match. Factory ratings were 365-385 HP for 4bbl motors.
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Old 03-19-2018, 05:55 AM
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GMC pickups in the 1950's used Olds and Pontiac engines stock because Chevy just had small V8's... or even only 6's earlier...
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Old 03-19-2018, 01:29 PM
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I have a customer that has one in a 70 Bronco of all things. They did a hack job installing a GM power steering box on it and screwed up the front end geometry. I wondered why he always brought it over on a tow bar til I drove it. Lots of torque, pulls the front end up on acc and goes hard right, went left on decel. Beautiful looking rig, just not driveable. He even rolled it in town once showing off. I worked on the motor and trans, he never mentioned the steering. Guess it has always been that way.

Anyway, I talked him into letting me fix it for him as a winter project. Now you can let go of the steering wheel on acc or decel. Great engine, just a bit much for a Bronco. It has the stock cast iron manifold which is for a Quadrajet, someone put a square bore adapter on it and a Holley, I don't like the mismatch. Plus it raised the carb up high enough to smash the air cleaner down enough to stop the choke from opening. With a drop base air cleaner, it runs pretty strong again. He wants me to change the intake next.

I was not aware of the 425 lifter bore angle difference, thats good information to know.

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Old 03-19-2018, 03:56 PM
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I do need to correct a typo in my earlier post. The bore is 4.125. Sorry for the bad info.
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Old 12-14-2018, 12:35 PM
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Had one in a g body cutlass once, scary man. Thing had balls like a brahma bull.
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Old 12-15-2018, 12:32 PM
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Joe Padavano is right about the HP rating of 375 HP @ 3200 RPM with 470 ft-lb of torque @ 3200 RPM with 10.25 or 10.50:1 comp ratio. With those torque numbers, it will put your eye balls in the back of your head. They could pull real hard. My father had one when I was a kid and I always thought it would be awesome in a light car.
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Old 12-17-2018, 06:33 AM
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I had a buddy with one in a Delta 88... can you say tire-fryer?
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Old 01-01-2019, 08:46 PM
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The Olds 425 produces tons of low RPM torque. Have installed many of them (as well as 455) in flat-bottom inboard boats with either jet-drive or v-drive. They are awesome boat engines, as well as hot rod engines.
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