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TOPIC: 200k/combat case oil mod

200k/combat case oil mod 13 May 2023 08:51 #1

  • dynodave
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Due to the potential of the oil return line sucking up broken off thrust washer 06-2601 commonly broken off locating tabs or other bits of schrapnel and causing harm to the oil pump. This relatively easy and quick modification might be an improvement to block larger pieces and reduce the allowed size of pieces allowed to pass.

The basic case scavenge return pickup hole is .250"(1/4) . The flow passage going toward the rear to the pump draw passage is .2188" (7/32). The final case oil pump feed hole is .1875 (3/16).
The opening surface areas for these holes then follow in sq inches as .049089, .0376, .027612.
If the original large access hole can be substituted by several individual small holes but still of adequate opening size to allow adequate scavenge flow.
The original 1/4" hole will be plugged and be replaced with several new and relocated holes.

On the drive side case 2 holes of .118"/3mm will be drilled.
On the timing side one hole of .111"/#34 will be drilled and on the case joint line an additional matching set of 1/2 round grooves making 1/8" total, provide the final scavenge passage surface area of .010588 +.010588 + .009677 = .043125. This is 1/3 larger area than the basic return passage of 7/32" but changed to several smaller holes.

Drive side:
1. Establish and transfer the timing side hole across to the drive side, then center punch.
2. Drill 1/4" hole ONLY about .100" deep measuring the side wall not the drill center. This will provide a secure starting point, otherwise it will be very difficult to properly start the hole if you go too deep.
3. Drill .118/3mm at a shallow horizontal angle and aiming downward to break through toward the bottom of the rear trough. Be very gentle when approaching the break through point since it is very easy to break the drill bit.
4, Drill the center hole another .100" deeper to create the second passage drill starting position. Again aim the drill another 20 degrees horizontally inward and down to again break through with the drilled hole bottom at the bottom of the rear trough.




Timing side:
Make a center punch on the rear face of the pump draw passage at about .060". From this point you will drill inward at about 20-30 degrees horizontal/inward and 5-10 degrees downward.




Center passage:VERY shiny rearward line line on each pix above
This is a manual or machined 1/2 round in each face of the case from the mutual hole toward the rear trough. I have used a dremel with a 1/8 or smaller ball bit. make it generally horizontal with the bottom edge of the case. Do not make each side too big/deep. You do NOT want a full 1/8 bore to result.

Final notes:
Which ever resulting hole is the HIGHEST, is the LOWEST level the oil will get evacuated to. This is when air will get sucked in (scavenged) instead of oil. Once oil rises the evacuation will continue.
As reported to me by more NHT expert than me, the oil windage gets spun off the crank and the weir helps to strip this off the crank and retain it where it will get scavenged from this location.
This was evidenced by the rear combat breather hose flowing excess oil directly back to the tank as the front oil pick was ineffective. Removing the weir to me is fool hardy. The 300000 series benefits from this weir configuration. The weir did not exist before the 200000 combat cases.
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Dave Comeau
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