I don't ride a 1300, so just interested in general, but isn't the crush washer at the top what ultimately stops the plug from threading any further into the hole? If so, then both plugs would extend the same distance into the cylinder.
Genuine question how would that work?
It wouldn't. As mentioned the crush washer is the sealing surface. The unthreaded portion isn't a shoulder for the 1300 but maybe it is for some bikes. It's wider than the threaded portion so you might look at that and assume it limits the protrusion into the cylinder. I've never looked into the cylinder but I'm guessing the plug bore is relieved to accommodate the unthreaded bit.
The EH part of the plug number is the reach - how long the plug is from the crush washer on. The non threaded part is the same OD as the ID of the cylinder head's threaded spark plug hole. The threads pull the plug, unthreaded part too, into the hole until the crush washer seats.
Not sure where that came from, since the "reach" of the plug is what determines how far into the cylinder the electrodes protrude. Since the specified reach is the same for both plugs, I'm not sure what you said is true. The EH plug is what NGK shows on their website for ST1300s.
Probably because
I tried to find this in the service manual because I want to know what they wrote about this. I can't find any reference to this. Do you know where this is written in the service manual?
I don't think the selling point of Iridium plugs has anything to do with performance, they are just a longer lasting plug because of the Iridium tips. They might cost 2x the price of regular plugs, but if they last 5x as many miles they're cheaper in the long run. Regarding the actual cost of either option, IDK the current prices because I put a set in my ST1100 so many years ago I can't even remember when it was and haven't priced spark plugs since.
Not really unless it mates tightly with the cyl head in the spark plug hole. If the unthreaded portion were tapered and was a tight fit in the hole, then perhaps. However, the cyl head is aluminum with cooling passages and conducts heat away better than steel. In order for the unthreaded part to fit in the hole it would have to be no larger than the threaded hole, i.e. not make solid contact thus heat transfer as a reason for its existence is suspect.
Except that many people who buy them do not use them for any where near 5 times as long so they are not benefiting from that benefit if it exists, which was one of my points in my earlier post. I have many times read of people who install iridium spark plugs in their ST1300 because they desire what they perceive as a superior spark plug and the increased change interval that they offer. They then replace them them at 24,000 mile intervals happy that they have extended the Honda recommended change interval by 50%. This makes no sense to me. I acknowledge that 24,000 mile change intervals is 50% greater than the Honda recommended interval of 16, 000 miles. I also submit that changing spark plugs of any kind at either 16,000 or 24,000 mile intervals in an ST1300 is nothing but a waste of money. The ST1300 has an electronically controlled digitally mapped fuel injected engine that runs every bit as efficiently and clean as a comparable electronically controlled fuel injected automobile engine. The spark plugs do not foul and deteriorate at any where near the same rate as they did in the old days of carbureted engines with point and condenser ignition systems. The recommended spark plug change interval in comparable automobiles is measured in the many tens of thousands of miles. I have always wondered how Honda came up with such a ridiculously low spark plug change interval recommendation, it makes no sense.
the benefit exists. Iridium has much higher resistance to spark erosion than standard plugs.
I'm with you,I only use stock oe spark plugs with that unthreaded portion like factory put in.There is a reason for it ,They got smart guys at honda
