I used this method to do a few vintage 70's chopper springer installs on aftermarket frames, typically where a new neckpost needed to be machined, and used a similar technique to mount a P&P girder (bit different for girders as a critical measurement is to keep upper and lower linkages parallel).

My bro is putting a +12 Santee springer on his old Santee 'Rigid Arm' swingarm frame, and kinda knows how, but his capable machinist has never seen one or built one, so I wrote this up for him.

There are variations on the neckpost attachment to the bottom tree, but the Santee springer has a threaded bore, so used that as a reference with a couple options.
Hope it is helpful:

Springer install; design neckpost and install springer January 31, 2022 TriNortchopz

For reference: Harley neck is 7.063” long, and CB750 neck is 7.46” long
1" tapered roller bearing is L44643 by Timken
7/8" tapered roller bearing is M12648 by NTN


1. Ensure bearing races are properly seated in the frame neck
2. Install bearings in the frame races and measure overall length with bearings installed… add thickness of top and bottom dust caps (about 1/16” each), record this measurement.
3. Install upper tree on springer and measure distance between the top of bottom tree and bottom of top tree (with upper surface of top tree flush with top of rear springer leg), record this measurement.
4. The difference between the measurements in #2 and #3 will be the thickness of the crown nut (if there is no step on the neck post below the bottom bearing… the crown nut fits below the top tree, and is threaded to fit the neck post – 7/8” UNEF or 1” UNS, at 24 TPI – the crown nut is used to adjust the bearing tension.
5. Measure thickness of top tree at center neck post hole and measure the diameter of center hole – confirm it is not out-of-round or bellmouthed or damaged. It will need to be rebored if it is…more on sizing later to fit spigots of crown nut and top nut.
6. Measure the thickness of the bearings and the ID – this will determine bearing fits on neck post, plus location of upper bearing on neck post – also research bearing part number for measurements.
7. Determine method of attachment for neckpost to lower tree – threaded hole or no? If threaded, determine thread my measuring size and TPI (may be slightly worn and oversize – easier to use a bolt or tap of known size than measuring internal thread). If worn, bore out threads to make hole round, or rethread it by inserting slug or sleeve, and re-boring and threading to original size. If bottom tree has just a round hole, measure for wear - both out-of-round and bellmouth. Rebore if required to make hole true.
8. Machine neck to fit into bottom tree; if threaded, add Ύ” length to thread for locknut to fit under the bottom tree. If not threaded, add Ύ” of threads below the .001” interference fit of neck in bottom tree to lock it in.
9. Machine bottom bearing surface on neck post at bottom of neck, to be flush with top of bottom tree and to be at least the height of the bearing thickness. Bottom bearing fit to be 0.001” interference fit (cool neck post and warm bearing to ease installation).
10. From previous measurements, calculate location of bearing fit on neck post for upper bearing, this is a size-on-size sliding fit of the bearing.
11. The space between upper and lower bearing fits is not precision and can be undercut from bearing size (about 0.010”) to allow easier install of lower bearing.
12. Upper thread (1” - 24 TPI is common for most Harleys, so easy to get chrome nuts) will start just above the upper bearing surface on neckpost. Length of thread determined by space between top of upper bearing and bottom of top tree, plus thickness of top tree, plus 1” of thread above the top of the top tree.
13. Crown nut and top nut (below and above top tree) have spigots that fit size-on-size into the hole of the top tree. The spigots must not touch each other when installed in top tree or they will interfere with tightening of the nuts (0.030” minimum gap between spigot faces when installed). Hole in top tree may need to be enlarged for spigots. Spigot wall can be thin over the internal threads of the nuts.
14. Thickness of crown nut (below top tree) is calculated from previous measurements.
15. INSTALL: install neck post into bottom tree, add bottom locknut. Install bottom dustcap and then the bottom bearing on neckpost – ensure it is fully seated against top of bottom tree. Install assembly into frame neck, install top bearing, then add top dust cap. Install crown nut (with spigot facing up) and tighten to adjust bearing tension. Install top tree. Install and tighten top nut (with spigot facing down). Done.
Note: top surface of top tree and top of rear springer leg surfaces should be flush – if not, add spacer or reduce thickness of crown nut.

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from CBH:
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from CBH:
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