Grooves on reverse/low servo piston

[jamie1974]

Hi everyone. I've got a 94 Explorer with an A4LD transmission. I had problems getting reverse and low to engage (slow to engage), and pulled the reverse/low servo piston.

I noticed some grooves scored onto the piston. There are 3 of them, close to the same size, and close, but not quite, equidistant from each other on the diameter of the piston.

Here are some photos:

The servo (See red arrow. It is pointing to one of the grooves):

And a closer view of a couple of these grooves:

Are these grooves ever normal, or ok? They are deep enough that they catch my fingernail. Maybe .002 inch thick

When I saw this, I suspected a problem, and showed it to some family members when they were over for Father's Day today. One of them pointed out that the three grooves are the same distance apart so it was probably done at the factory.

Later on, I measured the distance and found that they are not quite the same distance apart. Close when eyeballing, but not quite.

So I went online looking for other pistons like it, and I don't see any that have these kinds of scores.

So, I'm back to suspecting this is a problem.

I did not find any scoring in the cylinder bore it fits in, and it feels as smooth as glass in the bore.

Has anyone seen this kind of scoring before? It is bad, right? I'm guessing this was done by a chunk of metal and is NOT ok.

Thanks...

- Jamie

 

[BrooklynBay]

Maybe some previous rebuilder poked around the sides of it while trying to remove it from the bore. Use a fine grade of sandpaper or a sanding block to make it as smooth as possible.

 

[jamie1974]

Thanks for the idea. I bet you're right! I hadn't considered that but it makes perfect sense..

For the cost of a replacement servo, ($25), I decided to just go ahead and order a replacement. I took another closer look at the scratches and one runs a bit deeper than the others...I think it would be very hard to get out.

 

[BrooklynBay]

Make sure that you get the correct servo since there are 3 different types. Where did you order it?

 

[JK080]

Grooves on the piston in that area is not uncommon and shouldn't cause any issues. You will want to make sure that its not raised in that area, sand paper should be able to take care of that.

You will want to look in the case were the piston rides for scratches as well, as long as the scratches are not in the area were the seals ride it shouldn't cause issues either.

These scratches are caused from metal debris entering the servo area.

 

[BrooklynBay]

I would recommend using TransTec Viton D rings instead of ordinary O rings.

 

[jamie1974]

Thanks much for the tips. I did go with the D ring kit and I got a replacement servo before I read the rest of the posts about the scratches not being a big worry. Unfortunately I put everything back together and still have delayed engagement into low. So...now I'm looking into next steps.