727/904 Vent Relocation
Submitted By: Hybrid Date: February 17, 2010, 11:09:31 PM Views: 4536
Summary: Steps for relocating the front pump vent to a higher area of the case.

Spend a lot of time four wheeling in 8' of water? Get in mud so deep your engine starts sucking the stuff in? Have future plans of fording the Mississippi?

Probably not. But have you ever gotten yourself in a situation you didn't intend on getting into and really wish you had taken a couple of small steps to help yourself out? I know I have.

This How-To is geared toward preventative maintenance. It is not a performance upgrade. It won't make you go faster, farther, and you won't even notice you've done it until the time comes that you need it.

The 727 has a built in style vent in the 12 o'clock position of the front pump. It's not a bad location, but there is certainly a better choice.

Moving the vent to the exterior of the case accomplishes a couple of things. Firstly, it allows you to move the vent point to a higher location (cab, engine bay, etc.). This will prevent your transmission from sucking in water and/or mud in the event you ever get your transmission that deep. It's unlikely, but if could very well happen. It doesn't usually happen if you blow through some deep water, but that vent can suck up a lot of liquid should you get stuck.

Also, the stock vent has been known to "burb" fluid occasionally and could lead to an improper diagnosis of a worn or bad front seal in the transmission.

I had the luxury of doing this directly before a trans rebuild, so my entire case is gutted and cleaned. You may not. I would not recommend pulling your trans down if the only goal you're looking to accomplish is relocating the vent (unless water in your stock vent is a common issue for you).

If you do want to pull your transmission and do this without completely tearing down yours, all that needs to be removed is the front pump assembly. Before doing that, be sure to tighten down on the front band adjuster to hold the front drum in place. The front pump can be removed with a piece of wood across the bellhousing face and two threaded 3/8 rods with some washers and nuts. Take extra care to make sure no aluminum filings get into the main body of the transmission during this process. A piece of cardboard with grease smeared on it and placed between the inside of the case and the front drum will catch the filings as they fall through the hole.

Parts required:

1/8" MIP plug
1/8" MIP X 1/8" FIP Street Elbow
1/4" Hose bard X 1/8" MIP Adapter
1/8" NPT Tap
11/32" Drill bit
Drill
Pipe dope
Sockets/wrenches for fittings



This is the rear side of the pump assembly to show you what the vent looks like. In earlier versions (mine is a 76), there is no round piece of plastic. It is a baffle-style vent.



From the front, we see the opening for the vent on the reverse side. This is the culprit that needs to be plugged up.



Fortunately, the hole in the front of the pump is roughly 5/16" and the 1/8" NPT tap will work in it without drilling to a larger size. Beware that their may be a ring of casting flash on the inside of this hole. If there is, simply drill it out and then begin tapping. The plastic vent can stay right where it is- no need to remove it.



The 1/8" MIP plug gets installed. I used a small amount of high temp PTFE pipe dope around the plug (it is the white aspect you see at the base of the plug). Snug it down good-and-tight but no need to over do it.

Next we'll move onto the important part: Drilling and tapping the case.



Here, I've already drilled the hole. It is roughly 1" off centered to the driver's side of the casting line (never drill or tap into this area). It is also roughly at the point where the taper of the bellhousing comes into the straight part of the case, slightly favoring the taper. Be sure to drill the hole flush with the surface. It will be at a slight angle.



Here is a view from the inside for reference. The picture is taken from the front of the transmission. Notice the face of the pump mounting area. You can barely see the casting line from the inside and where the hole is in relation to it.

Same as before. Run the 1/8" NPT tap down the hole you drilled. Do not run the tap all the way. I went about 3/4 of the length of the tap and it put the underside of my fitting flush with the inside of the case.

After that, test fit your fitting.



Here is the 1/8" MIP to 1/8" FIP fitting in the case.

After that, I removed the fitting to paint my case.



The case is painted (optional, of course) and both the 90* fitting and the hose barb fitting are installed.



I attempted to have the fitting hug the profile of the case to avoid it hitting or getting snagged on the underside of the body at all.

Once my transmission is finished and installed, I'll run a 1/4" piece of hose from the fitting up into the engine bay with a breather loop on it. It's important to note that you can also put a fluid catch can on the end of the line if you know you're definitely going to put your truck on it's lid so the trans doesn't puke all over the engine/exhaust.



Hope you enjoy.

-Pat

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