4L60E Input Drum Differences--B-29 & B-85 - Hot Rod Forum : Hotrodders Bulletin Board
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Old 05-25-2011, 11:45 AM
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4L60E Input Drum Differences--B-29 & B-85

Okay, as some of you know I've been building quite a few Chevy transmissions lately.

A while back I posted about how an input drum can be bad on the 4L60E and yet there was no way to test them before you put them in... Since then I have been air-checking each and every drum with a puddle of trans fluid around the spot where the shaft is pressed into the aluminum drum checking for air leaks around this press fit.

What I have witnessed is that both drum styles have an orifice hole right there close to where the bearing sits. This orifice hole bleeds fluid off from the 3/4 piston when it is pressurized. This makes the 3/4 arrangement a feed/bleed set up.

Now the differences between the two styles of drums is that the B-85 style drum bleeds off a LOT more fluid/pressure (larger diameter orifice hole) than the B-29 style.

I'm wondering if anyone can account for why they did this? Are they trying to move more fluid through the 3/4 pack to aid in cooling? Additional lubrication to the input drum bearing? Did they alter the programming in the computer to raise line pressure in 3rd/4th to account for the greater pressure drop due to the larger bleed hole?

As an aside, I've found out that B-85 style converters seem to be far more likely to come apart than the older B-29 styles did. Are these 2 things correlated?

I know these are technical questions but I was hoping someone with more experience than I could shed a little light on the subject.

I've been toying with the idea of reducing the orifice hole on the B-85 drums down to the same size as that found in the B-29 drums. Anybody ever tried this? I know GM opened that hole up for a reason, but given that the B-85 drums and converters are far more likely to fail than B-29's I was wondering if there was any merit to altering them. Since the 3/4 clutches are the weakest link in these transmissions, I'd think reducing the orifice hole may raise line pressure to the clutches and help them live longer. Another benefit.

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