Unless the engine TCDS specifies the accy, or is altered by the accy, it goes in the airframe log.
In the case of the CHT gauge, ... the only aircraft required to have CHT are those with cowl flaps. The CHT in Frank's installation is required neither by the engine TC nor the airframe, but it is an alteration of the airframe, so the unit is logged there. If the installation of a CHT probe required a cylinder change to accomodate a head-mounted probe, then both logbooks would be so noted, but future replacements of the probe would be airframe mx. (I doubt anyone would complain if it were entered in both logs but consider this: If you later replace that engine with another in an exchange program...would you remove that probe and install it on the new engine??? or would you send the old engine off to it's new owner with that probe still installed??
If the former...then it's clearly an airframe matter. If the latter is the case, then the probe will have to be logged off the airframe log and a new probe installation noted there. Considering those kinds of scenarios may help you understand where the log entry should go.
Generally, cockpit instrumentation, fuel transfer/boost pumps, generators, vacuum pumps, etc (those items which serve the airframe systems) are airframe log items. (They are not req'd per the engine TCDS.) Fuel injection pumps, oil pumps, etc are engine items, as are engine oil changes.
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