Stock plugs, splitfire and S/E (splitfire) platinum. The S/E platinum by far I like the best. Easy starting and the things seem not to ware any. Im know there are many choices, but the platinum are hard to beat in any brand for a street engine.
Unless you've got heavy mods, stay with the stock plugs or cross reference to same heat range in a different brand. I run the Screamin' Eagle EX12P, the old Sportster plugs. They've got a split ground electrode... Maybe mfg'd. by Split-Fire.?.? These were recomended in the build list for my particular engine mods in the P&A catalog.
Stay away from Accel plugs. I bought a used sporty that had them in it. Harder than hell to get started. I was ready to kill the thing when I grabed a old set of HD plugs from the bagger and it fired right up. the Accel looked like new and the engine had less than 100 miles on it so I'd say they were new.
Well, I've put probably a hundred thousand miles on 2 Evo's using Accel plugs and never had a problem. I've never used any other of their stuff though. It's always possible to get a bad plug now and then with any manufacturer or someone messing with them, may have dropped the plug which I have seen happen. I used them for a while on my TC88 with no problems but have since switched to Autolite just because they are easier to get for me now.
Louz, Bosch doesn't have HD's listed in their parts finder on their webpage. So no dsirect way to find out if they make a Platinum 4. Only thing to do would be to do a double cross reference from the HD plug to a champion or NGK, then to a Bosch. Too much research for my small mind.
I tried all that with no luck. The reason I'm looking for them is from an article at Donny Petersen's web site (AIM writer) that states they are very good for the TC.
Of course he doesn't mention the stock number and doesn't take e-mail without charging.
Here's a site with a lot of cross references. Might help. It's big so it loads slow if you're on a dial-up connection. I'd just run a decent plug like a NGK or Autolite and change them every 5K. They are cheap.
If you want Iridium plugs go with the Denso plugs. They are of the correct heat range for stock up to stage II. The correct Denso plug for a TC88 is the IXU22.
Denso was the first with Iridum plugs. Denso also makes plugs with other companies names on them. Just look in your GM, Toyota, Honda and so on. They do not have Denso on them but they were made at the plant in TN or Japan.