Parallelism is adjusted by means of a lever-operated cam that raises or lowers the right side end of the front carriage rail. The cam's lever is shown in the following photograph.
Swinging the lever to the right increases printhead/platen gap at the right side end of the platen. Swinging the lever to the left decreases the gap there.Note the ink daub. That was put there by the factory's technician to indicate the correct lever setting for this chassis.
Gap
Gap is adjusted by means of a screw at the front of the ribbon drive gearcase on the printer's carriage. Its head can be seen in the centre of the following photo.
Note the gear teeth surrounding the adjustment screw's head. Those teeth belong to a ring-gear that's kept engaged with the screw by spring tension from underneath. A short geartrain in the gearcase couples the gap adjustment lever to the visible gear teeth and to the adjustment screw. Here's a view of that.
The ring gear on the adjustment screw can be pushed down out of engagement with its geartrain, enabling gap adjustment to be done independent of the gap setting lever and its geartrain.The adjustment screw has a left-hand thread, and its action is a bit counter-intuitive. Turning it CW increases the printhead/platen gap; turning it CCW decreases the gap.
Tools
Very little is needed to perform the adjustment; only a small screwdriver, a 0.016" feeler gauge leaf and a bent paper clip for pushing the ring gear down out of engagement. (See this post for some information on feeler gauge use.)
Preparation
Set things up as follows:
1) Printer on a dead flat, hard work surface.
2) Main cover off printer.
3) Ribbon cartridge removed.
4) Control panel removed.
5) Printhead removed, card guide (ribbon protector) removed, printhead reinstalled.
6) Platen knob reinstalled.
7) Forms thickness lever set to position '1' (nearest platen).
8) Change lever in rear tractor feed position (lever all the way forward). (This is to get the friction feed pinch roller pressure off the platen.)
9) Give the platen a couple of turns to ensure that it's at ease in its bearings and all feels right.
10) Parallelism lever at its factory-marked position, or at design centre if there's no mark.
Measuring and Adjusting Gap
When taking a measurement, press down lightly on the front (gearcase) end of the carriage to bias the carriage against the front rail, and ensure that you're feeling the full, true gap. Check the gap at the centre and at both ends, and make gap and parallelism adjustments A/R. (Note that the factory parallelism setting may or may not be quite correct. Feel free to alter it if measurement reveals that it's off.) It's something of a back-and-forth, trial-and-error process. Platen age and condition are a complicating factor.
Platen Condition
Platens don't normally wear in the sense that they 'erode', but they're subjected to continual hammering in use that has the effect of 'compressing' their surface in places. A typical platen that's seen long service will exhibit variations in diameter along its length. The extreme ends of the platen will still be full diameter, but those areas where print was located will be slightly smaller in diameter. All you can do is average it out as best you can. A good practice is to set the gap to a 'snug' 0.016" at the extreme ends -- then the gap over the compressed areas will tend to be a little oversize. Although the gap adjustment is a fairly critical one, it's not as critical as Oki's service and training 'literature' make it out to be. Printers with manual forms thickness adjustment can tolerate quite a bit of platen compression effect with no perceptible effect on print quality or operability.
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