Early One-Piece OPRV Rebuild Tool

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928Kyle
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The one-piece oil pressure relief valve (OPRV) for early 944s (1982-1986) uses a large black o-ring between the valve body and screw plug, which is nearly impossible to reassemble by hand without damaging the o-ring. I designed this simple tool to facilitate rebuilding the early OPRV without tearing the o-ring. The ramped collets (with the help of a little grease) effectively compress the replacement o-ring and guide it into the bore of the screw plug.

Print details:
  • No special filaments were used for this print; basic PLA was more than sufficient.
  • The press piece is optional; it just makes the valve a little easier to hold/work with
  • Print 2x collets and 1x collet retainer
Early one-piece OPRV rebuild tips:
  • To disassemble the valve, mount the hex portion of the screw plug in a vise
  • Carefully heat the screw plug with a heat gun or small torch until the residual oil begins to smoke
  • If using a torch to heat the screw plug, be careful not to overheat the steel! Work the flame all around the threaded area to heat it evenly.
  • Twist/wiggle/pull HARD on the valve body until the o-ring lets go. Don't lose the small parts!
  • Thoroughly clean and inspect all components. Make sure the inner piston moves smoothly in the valve bore
  • Note the orientation of the internal components before reassembly
  • Apply grease liberally to the ramps of the collets, replacement o-ring, and inside the bore of the screw plug
  • Assemble the two collets around the bore of the screw plug. Secure them in place with the collet retainer
  • (OPTIONAL) install the press piece on the end of the valve body
  • Press the valve body with new o-ring into the collets. This will compress the o-ring and allow it to slide smoothly into the screw plug
  • Remove the collet retainer/collets and press piece and you are done!
For reference, here are the replacement o-ring part numbers for both early and late style OPRVs:

Early one-piece OPRV (up to 1986):
  • qty 1 of 999.707.145.40 (green, fits on exposed end of OPRV)
  • qty 1 of 999.707.146.40 (black, fits between the valve body and screw plug)
Late one-piece OPRV (1987):
  • qty 2 of 999.707.144.40 (black, fits both ends of OPRV)
NOTE: this tool does not fit the late one-piece OPRV, as the late version is much easier to reassemble without special tools.
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Last edited by 928Kyle on Tue Feb 24, 2026 9:16 am, edited 2 times in total.

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Tom
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Love this. 3D printing really shines for single purpose tools like this. Any chance of a video showing it in action? I'm trying to envision how it works...

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928Kyle
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I should have taken more photos while this unit was still apart, but I didn't know if the tool was going to work yet :lol:

I have one more early valve. I will eventually add more photos and possibly a video of the start-to-finish process.

Basic description is that the collets serve as smooth, gradual ramps which compress the o-ring gently so that it will slide directly into the screw plug with minimal effort. The screw plug has nearly zero bevel on the mouth of the bore, and the o-ring is oversized as it is meant to expand once inside, so it's extremely tight. Without special tools, I really don't know how anyone has successfully reassembled these early valves, as the o-ring likes to squish out otherwise.

The collets are split into two pieces so they can be removed after the valve is reassembled.

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