how to stop lower shock mount from rotating?

I like Spadaro's theory and will address that issue on a 32 with same malady.

 

It seems that there are TWO different situations.

1. Perch bolts WITH the key ways. And shock mounts WITHOUT the matching key ways.
2. Then the reverse; perch bolts WITHOUT the key ways; and a shock mounts WITH key ways.

Mine was perch bolt(s) WITH; and shock mount(s) WITHOUT.
(seems to depend on who's perches and mounts you have - Brand)

Cutting key ways on the OUTSIDE of the perch bolts is DEFIANTLY easier.
Broach cutting key ways, INSIDE the shock mounts ... well, that gets a bit more difficult!!

 

Here's a pic that may explain the mul***ude of stainless lines up front.
Four of them anyway.



Von Rig Fink is correct about the headlight wiring being in the one set of braided lines.
Works well, but aesthetic-wise it doesn't cut it.
Adds to the vertical styling component and in my opinion the Deuce has too much of a vertical style component front and rear.
Especially so in the rear when you run a top.

A different headlight bar, higher & curved - would do it in the front and what would help in the back is to lower it about 3" - 4".
Gotta "C" the frame rear for that . . . another back burner project.

The other two braided lines are trans cooler lines.
The front spreader bar is a skin effects radiator with 120 sq inches of radiation area . . . works well.

The braided lines to the rear of the pic are front disc brake lines.

 

Yep.

Fairly quick and accurate if you have a lathe handy.

Get your workpiece clamped in the chuck or set up on a faceplate so the bore of the lower shock mount is concentric and square with the centerline between headstock and tailstock.
Probably the hardest part.

Grind an HSS (High Speed Steel) cutting bit blank to the width and shape of the keyway.
Allow for depth of the keyway to be cut.

Put the lathe backgears in and also use a small machinists jack on a piece of steel across the ways and under the chuck/faceplate to help hold the chuck/faceplate in place and prevent moving due to slop in the backgears.
Don't crank up on the machinists jack very hard.
Minimal pressure will do the job.
The little devils will lift 2000# and as you can see, they can bend the headstock out of true.

Make doggoned sure the lathe power can't be turned on during this process.
It's all a manual deal.

Install custom cut bit in a boring bar.
Orient the bit to horizontal.
The bit and bar have to fit inside the bore of the shock mount.

Use the carriage travel wheel and run the boring bar and bit inside the bore to be cut.
Find the zero point by turning the cross-slide wheel till the bit contacts the bore.

Spin the carriage travel wheel till the bit is past the bore to the inside.

Turn the cross-slide wheel so as to move the bit into the bore .003.

Spin the carriage travel wheel so the boring bar/bit are pulled down the bore which cuts a very fine ribbon of steel.
It may take a few cuts to get a ribbon.
.003 every cut.
Any less will usually just drag the bit across the steel without making a cut.
Springback will be out of the equation by the 2nd or 3rd cut.

Repeat until the keyway in the bore is the depth you want.

After the initial setup and setting up the bit, it'll only take 5-10 minutes to cut the keyway.