"Slim" as in not an SK roundhead or the cheapest HF plastic coated units.
As I said, next time you remove a 15ft/lb torqued spark plug, or even on the light side at 12ft/lb, choke up to a 4" ratchet equivalent length.
That 4" ratchet that we normally use for 1/4 drive is usually driving 10mm headed hardware, around 5 ft/lb max. Your typical M6 ingnition coil bolt with a 10mm head will be around 7ft/lb. Yes 1/4 drive can handle more as a drive size, but not in the common length accompanied with the drive. And people still ask why I love my 10"+ 1/4 ratchets... The typical gearset of a 4" 1/4 ratchet can do the job, just not the handle length paired with my strength.
That's the deal with 3/8 for spark plugs. One needs the ratchet length, and given that a "standard" 3/8 ratchet is the smallest drive capable of dealing with removal, that's the defacto. The other concern is extensions when required. I had a 3' extension on a crowfoot last week, you can feel it wind up and twist. Imagine needing a 10" extension, 1/4 drive, to remove a spark plug. It would be wound up into a twizzler. Even on DOHC subarus, 3/8 "chunkiness" isn't an issue. Given the space requirements, 3/8 typically has plenty of room to work. The issue isn't the diameter of the tool, it's length and manipulating the tool in tight quarters. An extension and ratchet 1/2 the diameter would not drastically improve a DOHC spark plug change on a subaru. Your hands still don't fit.
Yup, 99% of the time. I am certainly capable of installing by hand with no issue. That said, I have several high quality torque wrenches available, and the additional time required to find a spec, and use it, is very low compared to the entire job. I remove them typically with an electric ratchet. That more than saves the time required to set the wrench and swap the extension/socket over. You're talking 10 seconds a plug tops? Remove ratchet, torque, swap back, next plug?
