Converting your Oldsmobile 455/425/403/350/307 to a later model GM Serpentine Setup for less than $670.00

The ultimate way to convert your V Belt Style system to an Serpentine Accessory drive for your Oldsmobile 455/425/403/350/307 powered car

If you're reading this post, you already know that aftermarket support for your Oldsmobile motor is a bit more challenging then your buddy with a Small Block Chevy. You'll be shocked to see that full serpentine conversion kits cost upwards of $2,500.00 for these motors and on top of that, they're definitely going for an aftermarket look, which many are not looking for. When I upgraded my 307 to a 455, I didn't have all the stock brackets and finding brackets in Canada for a 1970 Toronado would have been really difficult. The search begin at that point for some way to convert the traditional V-Belt setup to a more modern Serpentine Setup and I stumbled across this thread and after 2 hours of trying to piece together what was required, I was off to the junk yard.

This guide does require you to convert to an electric fuel pump! This is an upgrade I'd do anyway, but it's a must with this conversion.

Difficulty Level - 3/5

This conversion requires a medium level of technical capability and tools. You can get away with a rough job if you're not as concerned about the look and only care about function. 

Parts List:

New Parts

  1. Pacific GBody Co Serpentine Adapter Kit - $400.00
  2. GMB 130240 Reverse Direction Cast Iron Water Pump - ~$50.00
  3. Alternator Lead Wire Repair Harness Connector $15.00
  4. ACDelco 6K938 (without AC) or ACDelco 6K956 (with AC)
  5. ACDelco 26003X Professional Upper Molded Coolant Hose
  6. Fuel Pump Block Off Plate - $60.00
  7. ~250ohm resistor

Junkyard Parts

When doing the junkyard crawl you're looking for early 90's GM vehicles with a 4.3l or 5.7l. These motors came with everything you need (including the above Alternator Lead Wire if you want to use a used one) except the crank pulley which comes from a 5.0l Ford. I was able to pickup everything I needed from the yard for around ~$170.00, including alternator, power steering and A/C pump.

  1. A/C & Tensioner Bracket from 1990-1994 S10 or Trailblazer - ~$30.00. The GM Part Numbers for these are 1018751 and 10233233.
  2. Alternator & Power Steering Bracket from 1988-1993 S10 or Trailblazer - ~$30.00
  3. Crank Pulley from Ford 5.0l (Early 90's Mustang, F150) - ~$10.00
  4. CS130 or equivalent alternator from the same vehicle (if it's good) - ~$50.00
  5. GM Power steering pump - $20.00
  6. Harrison A/C Pump (optional) - $30.00
  7. A/C Replacement Pulley (if you're opting out of using A/C)
  8. Water Pump Pulley (GM Part Number: 19418455)

Depending on the vehicles origin, these motors came with 2 different types of A/C brackets. You're looking for one with the long arm going downwards. I believe the other option is for vehicles equipped with a smog pump which we're clearly not using. If you're opting out of using your A/C, you need to grab the replacement pulley mount on a non A/C equipped Trailblazer or S10. Step 1 - Modifying Brackets The brackets you found at the junk yard are going to need some basic modifications to fit correctly.

The Pacific GBody Co. adapter brackets have threaded inserts attached to them to give more thread surface for the fasteners to grab on to. You'll need to drill out some of the material at the back of the bracket to allow the threaded inserts to sit inside flush. The GM accessory brackets should sit flush against the adapter brackets once this is done. I'd recommend using a drill press or a stopper bit to ensure you don't go all the way through the cast aluminum. Measure out the depth of the inserts, and go just as deep as you need. Next step is removing the long "leg" of the A/C bracket.

I thought about keeping it and having some sort of mount at the bottom of it to the bottom of the block, but it makes the water pump exit difficult to access. The simple solution is to hack the thing off. You can clean up the corner and give it a good sanding if you want to maintain a factory look. Once you've done this, give it a good clean, spray it with some black paint (I got mine powder coated) and put them aside.

By now you should have the Serpentine Ford crank pulley available and ready to  drill.

Our kit includes an adapter that mates the larger inner diameter of the Ford pulley with the smaller diameter of the Oldsmobile crank. Place the plate on the outside of the Ford pulley and use a center punch to punch the holes. Make sure you've rotated the adapter plate away from the Ford holes before you punch! The adapter also helps balance the loss of weight going to the Ford Pulley. I'm not really sure this makes a big difference, but for me, maintaining stock specifications was paramount when doing this.

 

Since everything is off, it's a good opportunity to check for any leaks or issues from the Front Seal, Timing Chain Cover and Intake. Give it a good once over. Once you're confident that everything is sealed up, attach the Pacific GBody Co. Brackets to the motor with the fasteners provided. The lengths should be fine for any Olds motor.





A dab of thread locker is never a bad idea on the hardware, as it's a high tension area. Leave the lower L bracket a little loose so when you attach the longer drivers side adapter plate, you can get it adjusted. Tighten them down good and tight.

The water pump is a special order from ACDelco and seems to be available on Rock Auto. The biggest risk of the project is that this water pump won't be available forever, which is a concern. I picked up an extra one just incase as I don't want to be without one! I had some initial concerns that the cooling and flow wouldn't be enough to support my motor, but I drove all summer with absolutely no issues. Install the water pump using the included gaskets and the water pump pulley with 2 included washers behind the pulley itself on each water pump bolt. This is the only pulley that might require a bit of finicking to line up.

The crank pulley will mount on center with the flange adapter included in the kit. It's going to be a tight fit, and if it's not going on, take some 150 grit sand paper to the edge of the flange. Depending on the material used to paint/coat your pulleys, it could be a bit off.

Next up is the power steering pump on the drivers side. If you're like me, I try to purchase all the inexpensive items new. I ended up using a Dorman 300200 pulley with an outside diameter of 6.469". This pulley ends up being very close to the steering box bolts but it should clear. It's tough to predict if it'll clear perfectly as it depends on the position and wear on your body and motor mounts. You might need to grind down the head of one of the steering box bolts to be sure though.

Install the alternator and hook it up using the plug in the parts list. When doing anything with your wiring, unhook the battery and make sure you solder and shrink wrap any connection! This is a weather bearing connection that is susceptible to the elements. The last thing you want is a fire under the hood due to a bad connection! 

The S Terminal on the alternator pigtail goes to 12v power. I routed mine right to the charge post. The L wire goes to the stock exciter/ignition wire. I ran a new wire from one of the ignition outputs on the fuse box directly to the L wire. It's important to add resistance to this circuit in the form of a 35ohm > 500ohm resistor in line! If you don't, damage to the regulator will happen.

I found a really great PDF from American Autowire on the subject. Download it here

I opted out of Air Conditioning for now, but the replacement pulley should have been on many of the vehicles at the yard. I ended up grabbing a compressor for when I eventually hook AC back up. Install the AC delete pulley.

Do a once-over on the any bolts and hardware to make sure they're tight. You don't want one sheering off at highway speed and causing severe damage. Install the belt and unplug your main coil wire. Crank the car over a few times to make sure there are no hang ups or areas of concern. I'd recommend having someone do this so you can watch the motion of the belt. 

For the maiden voyage, take it easy! Don't be hot dogging it out of the hole. It's important to make sure things are buttoned up nicely before getting in to the gas.

 



Adapter brackets and plates are available for Small and Big Block Oldsmobile applications!


36 comments

  • Will this slide into a GMC Motorhome’s 455 Olds?

    Randy Hecht
  • Will this work on our 425? It’s externally balanced.

    Scott Christensen
  • So is it possible to find the waterpump now. I have a 69 olds 350 in my 87 cutlass that I need these v belts off of. They squeal early am

    dean jones
  • What kit did you use I’m trying to purchase asap

    Floyd Harris
  • Do I have to use a electric fuel pump? I was wondering if I can use the stock mechanical fuel pump

    Casey Gathers

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