I've cut and pasted one of Tim Engels post on the subject. His explanation was very clear and easy to follow.
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Shimming the yoke is just an assembly procedure, and requires no special tools. The yoke and the output shaft are both cross-drilled with matching holes for the roll-pin that secures the yoke to the shaft. If no shims are installed, then the yoke will slide onto the shaft too far. As a result, the roll pin holes will not align. The inner shaft's roll pin bore ends up to far outboard relative to the yoke's bore... which by comparison is too far inboard. If you use an awl/ ice pick/ small screw driver blade/ stiff wire, and slide it in along the inboard edge of the roll pin bore, it will run into the step between the two bores, and stop. If you slide the poker in along the outboard edge, it will just drop off the mis-match step, and keep going... and that's not the condition you want.
Start adding shims, moving the yoke outward, and reducing the mis-match step to zero. Then add one more of the smallest thickness shims, moving the mis-match to just beyond zero. Now when you slide the poker in along the inboard edge, it will fall off the step and keep going. Slide it along the outboard edge, and the poker will stub it's toe against the smallest of steps (thinnest shim available... 0.005") and stop. That's the condition you want.
After the yoke/ halfshaft are installed, insert the roll pin with the split vertical... either up or down. That will allow the roll pin to act like a spring, and compress to slip past the small bore mis-match step. With the step along the outboard side of the pin bore, the compressed roll pin will force the yoke inward, seating it firmly against it's stop inside spacer/ diff housing journal.
If insufficient shim thickness were used, and the step was on the inboard side of the pin bore, then the compressed roll pin would force the yoke outward, lifting it off it's seat on the spacer/ diff housing journal. Then the inward compression forces of the suspension geometry force the shaft inward when normally loaded, or allow it to move outward when the suspension is lightly loaded (ie, getting 'light' going over a rise/ hump). That little bit of loose motion can result in fretting wear in the spline joint, and result in a fatigue fracture in the roll pin. It's important that shims are added until the pin bore alignmennt shifts from too far inboard, to just a minimum too far outboard. None of which has anything to do with the transaxle's size nuts ("multi-tooth multi-slotted part").
In the past, Steve Veris has sold the shims. Or order them from rd Enterprises/ Richard Winter. Failing all of that, they're the same size as the shims used to set the end-float in a GMC 4-71 supercharger, so get them from a GMC truck service center.
The correct roll pins require not only the correct diameter x length, but also a slot of the correct, generous width. Many of the Lotus Specialists are selling some standard 'hardware' roll pin with a very narrow slot. That allows only minimal 'spring' compression during installation. Once the slot closes up during installation, the pin effectively goes "solid", and an abusive level of hammering is required to drive it in. That is not correct. A correct roll pin still has to be 'driven' in, but not "hammered mercilessly" to get it in.
The roll pin crosses two joint lines between the yoke's inside bore/spline, and the output shaft's OD/ spline. If insufficient shims are used, then it's common for the roll pin to fatigue fracture along one or both of the intersections. Once that happens, and the shaft & yoke move, creating a step in the roll pin bore, it can be an exercise in futility trying to drive the roll pin out (ie, the end you're pounding on hits the step, and stops). In that case, it's more productive to use a broken screw exctractor to remove the outer stubs, center the inner stub, slide the yoke off, and finally drive out the inner stub.
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