Automatic transmission servo bore help!

[rustyknife]

Hey all, I'm overhauling a 2012 Ford Transit connect transmission, its the 4f27e thats used in the older focus. Problem is the servo bore for the 2-4 band wore out, the opposite side of the servo piston is used for the accumulator for the direct clutch, so when there is wear between the pin of the servo and the bore in the case, pressure is lost and the direct clutch burns up.

Northland transmission makes a nice jig that bolts in place and guides 2 reamers through the case and then you loctite and install a bronze bushing in the case to repair the case. Then you take just a metal sizing pin of .393" and drive it through the bushing several times and it supposedly makes the hole the correct size for the servo pin.

Well I put the bushing in and beat this sizing pin through the hole about 30 times and its clear it will never make the hole big enough. I have an old servo pin I put a long handle on to use as a gauge, it goes about half way and binds up. The hole is way too tight and it will never shift again if I don't correct this.

The servo itself measures .392" and the sizing pin the is labeled .393+ actually measures out and .3925". Id say the aluminum case just flexes out as the pin goes through and flexes back after. I have not measured the hole yet as its at work.

So my question is this, how do I enlarge this hole just SLIGHTLY so that my servo pin will slide freely in its bore. And remember an overly enlarged hole or out of round, will cause me a leak that will burn the direct clutch up.

I've considered some kind of polishing compound on my old servo tool and chucking it up in a drill for awhile. Not my favorite idea as it might be too easy to make out of round.

I've considered making a new sizing pin, but I really don't want to make more then one, and really have no idea what size I would need to make it.

My last and best option I have thought of, is perhaps an adjustable reamer, but I have never used one before. Is there a way to measure the hole and then set the reamer up with a micrometer on the blades or something? Can I place the reamer in the hole and adjust is outward? Or is it best to drill a hole in some scrap aluminum and ream it till my old servo feels correct? Can a reamer like this just be turned by hand?

Does any one have any better ideas? I could really use the help....

Thanks
Eric

 

[jixxerbill]

I have used adjustable reamers and they do a decent job for me but i always follow them with an AL lap and lapping compound..If you only need to remove less than .001 I would consider making an AL lap and using lapping compound.. The adjustable reamer will work but finish might not be what you want in the end.

 

[MachineTom]

I would try 1000 grit lapping compound use the pin that they set you as the lap, I'd say the onlly a little lapping will be needed, stay away from the DRILL.