The butterfly shaft rear support bearing seems to have disappeared from my lower plenum. Also, the cam that actuates the butterflies was fairly worn.

Here is the tale of my repair...

Here is the bronze drill bushing I used. It is a 19/64", I bought it from McMaster-Carr.




I used 24 hour JB-Weld to glue it in. I used the shaft itself to position the bushing, and I mounted 3 of the butterflies back on the shaft to help position things correctly.
Every now and then I'd turn the shaft back and forth to make sure I didn't accidentally glue it in.




The hole in the cam for the diaphragm actuator was in pretty bad shape.




I had a plan...that involved some experimentation...




I used a nylon flange bushing to position a washer on either side of the cam, in approximately the original hole location.
I clamped the washers with vise-grips, removed the nylon bushing, and then TIG welded them.
I didn't weld the whole circumference just in case I needed to undo it...but I think it's more than adequate as is.










It all seems to work fine!

I don't have a clip or keeper of any kind on the shaft of the actuator. The way the shaft fit through the nylon bushing, it actually feels like it has a bit of a detent when it comes out the other side.

Well written Nate. Good stuff. Sticky worthy...

How were the bushings between each plate. I have found them to be shot as well as the ends?

I've wondered if those weren't redundant. All you would see is some minor air leakage runner to runner, unless the unsupported shaft is prone to vibration.

If those bushings are loose the shaft will flex with each intake pulse. This is what fatigues the plates to failure.
I would machine up some new ones so the shaft will be snug all the way along its length. I started making them in Delrin for my SR but then got the parts truck and it had a new intake on it. Never finished them.

remove the shaft and install a small freeze plug in each end of the housing and be done with it you will not notice that the butterflys are gone

Willing to bet the engineers that designed the system would disagree with you. I know this got discussed ad nauseaum in another thread and I don't want to muddy Nate's beautiful write-up thread. But, I'd lay money on there being a dyno difference with and without them...

The bushings seemed to be in "good enough" condition. I suspect that they're probably just to reduce turbulence and the amount of crossover between the runners.

I can't really believe that they're for mechanical support, since that would have meant that Mikuni engineered a rubber-to-aluminum bushing interface.

I could be wrong...are there actually any documented instances of these shafts breaking due to vibration fatigue?

It seems to me that with the butterflies open, they would have a pretty small cross section to the airflow, reducing the possibility of intake pulse induced vibration...and when they are closed, the butterflies are basically braced against the walls of the intake. On top of that, just bolting the butterflies to the shaft would help stiffen up the whole assembly.

And yes, I am of the thinking that things are engineered the way they are for a reason. I'm sure there's either a performance or economy benefit, if not both, to having the butterfly setup functioning. I mean, that's a fairly complex setup that undoubtedly added significant initial cost to the engine, plus the additional parts count and potential points of failure...I don't think an auto manufacturer would put that on there without good reason.

Thank you for this timely post. this is exactly what I need on my "new" '95 montero SR with leaky manifold, although I may just do the freeze plug fix since my main goal is diesel swap.

The breaking part is the butterfly plate screws, and it's the flutter in the plates themselves that fatigues and breaks them. I don't know if the plates are whanging against the sides of the intake or just fluttering in free air, but it's the worn bushings that allow the flutter. I'm not sure if the internal bushing wear is enough to allow the shafts to flutter, or if the end bushings have to be worn out for it to happen. I guess Nate will find out for us.