Newbie question about piston seal installation

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Bones

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I am currently in the middle of my first transmission rebuild, but I have never had the pleasure of assembling anything with piston seals. Can anyone tell me which side of the seal is the sealing side? And, is there a general rule of thumb for knowing this? I attached a picture below for a reference. Thanks!
 

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The sealing side is the "wider" side of the seal. Fluid fills the cavity between the piston and the drum, which tries to force the piston out of the open end of the drum. Think of the seal as a one way valve; which end will allow fluid to bypass the seal, and which end will cause the seal to expand againts the sealing surface when pressure is applied. That being said, the general rule is that the "open" end of the seal should always face toward the enclosed space (the side that can/will build pressure). Good luck, I am currently in the process of rebuilding my first trans myself, a 904. Be careful installing those pistons back into the drums, its very easy to cut/roll those lip seals. I just had to order a full seal kit just to get the one drum seal that I screwed up.
 
The easy way to tell is that the sharp flared edge of the seal goes toward where the fluid supply comes from.
You have it correct on the piston in the picture.
The inner seal on that same piston faces the same direction.
 
Thank you both for the replies. I kind of figured it would act as one-way valve, but I was unsure. I appreciate confirmation on my install Trail.

Craig, I will definitely take care when re-installing, as I have already read your thread on that issue yesterday. Very good to know!
 
Bones said:
Thank you both for the replies. I kind of figured it would act as one-way valve, but I was unsure. I appreciate confirmation on my install Trail.

Craig, I will definitely take care when re-installing, as I have already read your thread on that issue yesterday. Very good to know!
Click to expand...

Sure, no problem Bones.
 
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