The Seca II uses spark plug caps with resistors built in (resistor covers) to help minimize electomagnetic interference caused by the firing of the plugs. The service manual indicates that these are 10k ohm resistor caps. Measuring a set of 8 stock caps yielded 10k resistence on stock caps on a '92 Seca II.
The stock caps feature a 90-degree bend and fit a CR8E spark plug with a 10mm base and stud-type terminal. This lends to the NGK resistor covers with the model number LD(resistance)F
The 10K ohm spark plug cap would be NGK LD10FH (LD10F). <- Though these are difficult to find.
Yamaha recommends using replacement caps (PN 4BP-82370-00-00) which cross references to an NGK 8060 (LD05F) which is a 5k ohm resistance plug cap. The NGK 8060 is black in color. A red cap is also available (NGK 8231).
The dimensions and specs for the LD05F are as follows:
You can find more information on the resistor caps from NGK:
http://www.ngksparkplugs.com/docs/Resistor_Covers.pdf
The OE Yamaha part number for the plug caps is 4BP-82370-00-00. They list for $29.33 each.
You can purchase the NGK 8060 and 8231 at several online stores for $3.50 each.
Replacement Spark Plug Caps (Resistor Covers)
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