The problem of bore wear on torque-converter-clutch
pistons has been around since lockup torque converters were born. The
rotational restriction on the pistons is handled by splines, pins or
fingers. But centering the pistons is generally the sole responsibility
of the bore. Any out-of-balance movement of the piston will cause the
piston bore to scuff against the turbine hub when it moves to apply or
release the clutch. Indexing the piston to the turbine and balancing
this unit helps minimize bore wear. (See the article on indexing
TCC-piston damper assemblies to turbines, Transmission Digest, March
2006.)
But having a perfectly balanced assembly and
expecting it to remain that way throughout its lifespan is not
realistic. Torque-converter rebuilders will always have to deal with
piston-bore wear.
The area of the piston bore that is the sealing
surface seldom shows any wear because it is not used for centering. Of
course, the rest of the bore will wear faster because of the small
surface area remaining.
The E4OD piston in Figure 1 is a good example of
bore wear. Machining the piston and installing a replacement sleeve is
the current industry fix. This method works well, and most replacement
sleeves are harder than the original bore, for longer life.
Dick Lewis of Midwest Converter Supply decided to
address his bore problems at a different level. Dick’s shop
renews the bore surface by spray-welding the original bore (see Figure
2) and then remachining the bore to its original dimension.
The piston is placed onto a standard welding
turntable. A sleeve attached to the turntable centers the piston and
also keeps any welding debris from getting into the damper assembly.
The turntable is set at a 45° angle, and the piston’s own
weight keeps it in position.
The torch operator shown in Figure 2 is using a
“Stoody” jet-spray torch (model No. JS-100, made by
Thermadyne Corp.). The media used for welding is held in the canister
mounted on the torch. The type of welding media used controls the
hardness of the finished weld. For this application media No. HSW25 was
used, giving a finished hardness of 36 to 38 Rockwell – hard
enough to extend the life expectancy of the part and still soft enough
for machining to a good surface finish. The amount of the weld being
applied is controlled by the lever on the torch below the
welder’s right thumb.
There are many different gas combinations that will
work with this torch, but the oxygen/acetylene combination being used
in this instance seems to work best. The welding process takes about
four to five minutes from start to finish.
Normally, bad bores are found during the cleanup
process and repaired before bonding. But if you don’t discover a
bad bore until after the piston is bonded, you can still make your bore
repairs on the bonded piston. 
Ed Lee is a Sonnax technical specialist who focuses
on issues of interest to torque-converter builders.
