How to remove and refit a leaking output seal on the 6S850 gearbox

This information is also available as a video here.


To remove a leaking output seal on the 6S850 gearbox, you’ll first have to remove the 22mm bolt which connects the output flange to the rest of the unit.

The bolt is tightened to 150 newton metres so you’ll need an air gun to remove it. Once you have detached it from the unit, put it to one side for later.

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The next step is to take the output flange off the main shaft; this is an interference fit so it’s very very tight.

To remove it,  we use a three-legged puller as well as a special adaptor that we have in-house; if you don’t have a special adaptor, then an impact socket will suffice.

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Fit the three-legged puller to the output flange. Using the air gun, set it to 19mm and change the direction.

Make sure everything is central; you should be able to pull the output flange from the main shaft. If it goes tight at any point, make sure you haven’t hit the actual output flange when you’re pulling it out.

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video 3 Artlab.Still005video 3 Artlab.Still007Put the output flange to one side. Check the seal track is not worn on it.

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Removing the seal is straight forward – use a screwdriver to flip it out, being careful not to damage the housing. As they are tight sometimes, you might have to try to get the screwdriver in behind the seal to flick it out. Again, take care with the housing.

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Fitting a new seal

The standard seal for the 6S850 has a sealing spring washer so we recommend coating it all round with Vaseline to hold it in place while we’re knocking it into position, so that it doesn’t spring out.

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We use a special ZF tool for fitting seals to the right depth; these tools are specific to each gearbox and are available for purchase for this work.

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Put the seal on the tool and use it to place the seal into position; next, knock the seal evenly into place with a hammer, all the way down to the nylon stop. Put the tool to one side.

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Carry out a quick visual check to make sure that the spring hasn’t popped out; the Vaseline holding it in place should’ve done the trick though.

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Refitting the output flange

Before re-fitting the output flange, ZF recommends that you heat it to 120 degrees to go over the interference fit shaft; you won’t get it into position without heating the part first. An induction heater will warm it to the relevant temperature; please remember to use the appropriate equipment and protection for the task..

Once heated, the flange should slot easily into place.

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Because there is a spline between the flange and the shaft itself, you will need to put a little sealant around the washer to seal that area; put some sealant on the 22mm bolt also.

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Being careful not to touch the heated flange, screw the bolt (with the washer) into position; use the air gun to run it up.

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Next you need to torque the bolt to 150 newton metres. That tolerance changes depending on the width of the flange and it’s really easy to remember – 150mm diameter means 150 newton metres, 120mm to 120 newton metres for example.

Put a screwdriver through one of the bolt holes and against the housing to keep the flange in place; then tighten the bolt using a torque wrench.

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At Ecodrive Transmissions, we offer an extensive range of support services including exchange transmissions and axles, field service diagnostics, removal and refit service, gearbox repairs and overhauls.

Call 01204 701812 to speak to our expert team.