I measured the vacuum input pipe of my TH-400 this morning. It's a 1/4" size on the transmission.

The intake manifold-to-modulator vacuum hard line running to it is 3/16"s. I don't think this is a factory hard line.

I've dug through various manuals and they all show the hardline, it's bends and retaining clips, but none comment on the size of the hard line.

The car drops into gear a bit "hard". Shift quality is about right, but a bit on the sloppy side.

I'm wondering if the too-small line is affecting the transmission. Or was a 3/16"s hardline the factory size?

If I replace the hardline with 1/4" pipe will that better replicate what the factory intended?

 

I don't think the diameter of the line (either hard line or rubber hose) will have any impact on the shift quality.

 

The size of the line whether 3/16 or 1/4 will not make any difference. The modulator has nothing to do with the car going into gear.

 

As long as the vacuum line is clear of carbon and not kinked, 3/16 should be plenty big enough. Make sure the u-joints are good, make sure the idle speed is correct, anything that can amplify the clunk going into gear.

If the trans has ever been rebuilt and the builder omitted the wave plate or center seal from the forward clutch it will bang going into gear. It won't hurt anything, but may be a bit annoying with a stock low stall torque converter.