Transmission dipsticks and fill tubes are usually one of the last things on the list when planning out a project, but even a simple item like that can mean the difference between a long or short life for the transmission. I know you’re probably thinking what’s the big deal? Well, the problem isn’t that one dipstick is any better than another, but the issue can arise with how the dipstick is calibrated to your transmission. Today there are many different options for fill tubes, and they’ll likely be geared towards a particular type of transmission, but differences in the manufacturing process can cause one dipstick to read differently than another thus creating issues with fluid level that can eventually lead to a failure. We typically use a lot of the OE style fill tubes with our builds since they seem to have a better consistency with accuracy, but any brand or style can be made to work correctly for your transmission with the right steps.
If you’ve purchased a replacement fill tube for your transmission take the time to ensure that the full line is accurate for your transmission. The best way to make sure is by removing the transmission pan and installing the fill tube and dipstick to check where the full line rests when inserted. What you are looking for is a full line indication at the point where the pan and case meet. Anything outside of that range can result in potential problems due to an over or under fill condition.
Even looking at this particular installation, at first it may seem that it would be close enough, but this particular indicator was set too low into the pan which resulted in a low fluid level that eventually led to the transmission’s failure. That failure could have easily been prevented by taking the time to get it right before the car went on the road.
Here’s a comparison of what the correct length should be compared to one that is too short and one that is too long.
This is an example of a correctly calibrated indicator. Notice that the fill line is exactly where the bottom of the case is, where the pan meets.
This example is what an overfilled condition would look like. With the fill line up too high into the case you would be tricked into thinking you needed more fluid than what is required. An overfill condition usually results in excess fluid being pushed out the vent tube, seals or even the fill tube if enough pressure builds. No one wants to deal with cleaning up that mess.
This last example is probably the worst case scenario and what usually happens right before a transmission failure. The fill line is too far down into the pan giving you a false indication of full, whereas, you could actually be a couple of quarts low. We’ve witnessed quite a few transmission failures due to this exact problem.
Any and all fill tubes/ dipsticks can be modified to fit correctly so don’t forget to take the time to check this often overlooked item during the build process.