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  1. #1
    rob2337's Avatar
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    Counter Balance Shaft Rebuilding 787

    I am rebuilding a 787 engine. I need to change the bearings and seals on the counter balance shaft.

    I bought a gasket kit, but it seems that it only came with one smaller oil seal for the counter balance shaft. The one I have has two seals back to back to seal the oil cavity. When looking at the cases, it seems that there is only one seat that is machined for the oil seal. Any ideas if I need two seals or one?

    Also, the new / used cases that I purchased doesn't have the oil filler for the oil cavity. Should I drill and tap for a npt plug?


  2. #2
    96XPSS's Avatar
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    There is enough room to install two seals, one will walk back and forth. I think that is why they installed two.

    I'd drill and tap a drain plug. You use an oil bottle with a small tube and install through the small drain hole on the top case, to fill it. An 800 engine never needs changing, as the oil constantly exchanges.

    But, the reason why I recommend a drain plug is "if" the engine ever gets hydrolocked. This is the lowest point of the engine, and you will never get all the water out of this area, otherwise. If you really want to get snazzy I'd put one on top too, to ease filling if you have to drain the water out.

  3. #3
    Quote Originally Posted by 96XPSS View Post
    There is enough room to install two seals, one will walk back and forth. I think that is why they installed two.

    I'd drill and tap a drain plug. You use an oil bottle with a small tube and install through the small drain hole on the top case, to fill it. An 800 engine never needs changing, as the oil constantly exchanges.

    But, the reason why I recommend a drain plug is "if" the engine ever gets hydrolocked. This is the lowest point of the engine, and you will never get all the water out of this area, otherwise. If you really want to get snazzy I'd put one on top too, to ease filling if you have to drain the water out.
    I like your way of thinking . Very good advice in my opinion.

  4. #4
    rob2337's Avatar
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    Quote Originally Posted by 96XPSS View Post

    I'd drill and tap a drain plug. You use an oil bottle with a small tube and install through the small drain hole on the top case, to fill it. An 800 engine never needs changing, as the oil constantly exchanges.
    So you're saying rig a small bottle of SAE 30 oil that has a tube that goes to the drain plug of the oil cavity, or the filler plug on top?

  5. #5
    96XPSS's Avatar
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    SAE 30 wt oil is only used in 951s! Refill with XPSII oil.

    When building the engine, fill through the case. Lift the upper half of the case and you'll see the drain hole goes straight to this cavity.

  6. #6
    rob2337's Avatar
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    According to the 1997 BRP shop manual it says SAE 30.

    Also, I am getting a bit confused about the counter balance oil. It looks like the oil in the counter balance area can make its way into the crank case. Does this mean it slowly burns off, or does one of the crank bearings prevent it from making its way into the crankcase?

    Also about the drain, I have no fittings in the cavities as of yet. I was just looking at the cases and there is a boss on top where I could drill and tap a plug. I am just worried about the plug protruding into the cavity and hitting the gear.

  7. #7
    96XPSS's Avatar
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    Rob, that's actually a typo. SD was supposed to have the 951 in there in 97, but they didn't get the bugs out yet. They left that part in the manual thinking it was a 951, remember, these are people and they make mistakes too. Read the 96 manual for the correct instructions.

    Yes, the fuel/oil mixture constantly transfers between the crank bearings. So the oil left from evaporated fuel is constantly mixing. Now you know the reason why you wouldn't use 30wt oil with XPSII oil. A 951 CB shaft cavity stays sealed to the intake/case cavity.

    If you tap the hole correctly (not tap too deep) it will tighten up snugly with some 518, and be perfectly flush.

  8. #8
    rob2337's Avatar
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    Ok, so what is the point of the seals on the counter balance shaft if it is not isolated from the crankcase? I know it looks like it is to keep the oil in the cavity, but if oil goes everywhere in the crank case, what is the point? My seals don't look to great so I was going to press it apart, but if they are really not important I can just change the bearings on each end.

  9. #9
    Hydrotoys's Avatar
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    It's been a while, but if memory serves me correctly,

    it minimizes cross-cylinder "talk" on the vacuum stroke of the crank which improves low end response. All cranks/ balance shafts bleed a little back and forth, but keeping the amount down, keeps the cylinders getting the same amount of air/fuel mix.

  10. #10
    rob2337's Avatar
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    Quote Originally Posted by Hydrotoys View Post
    It's been a while, but if memory serves me correctly,

    it minimizes cross-cylinder "talk" on the vacuum stroke of the crank which improves low end response. All cranks/ balance shafts bleed a little back and forth, but keeping the amount down, keeps the cylinders getting the same amount of air/fuel mix.
    Interesting, but after looking at the open cases, I don't think that is the reason.
    Is seems that the other end of the counter balance shaft is not connected to the other cylinder. There are two bolts that go through the counter balance shaft area on the other side of the seals. I think the two seals are just to prevent an air leak in the crankcase.

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