Are the correct peanut plugs for 291 heads RXXTS or RXXT? I'm thinking the first but want to be sure. I had one set aside for reference but I have no clue where I put it yesterday and I haven't used these heads in over 25 years! :clonk:

I used R44XLS

Dana,
Big Block 291 heads used in 70 were originally equipped with R43T spark plugs. The XLS extended reach plugs were not the right plugs for these heads. They were used on Aluminum BB heads, not the cast iron.
Chris

Chris knows his stuff. I use R43XLS in my aluminum heads.

Well they do work. Advance lists R43T for 360 and 390hp which would be 290 heads. I was under the impression that closed chamber heads too the XLS style plug.

Dave,
Both the 290 castings and 291's for 70 are tapered seat spark plug castings which was designed for the short reach tapered seat peanut style plugs. The R43XLS or R44XLS plugs are gasket style plugs that won't properly seal to the closed chamber cast iron 3964290 or 3964291 cylinder head. I would not recommend using that style plug in these heads.
Chris

THE ONLY DIF BETWEEN FOR EX THE 5/8" TAPERED SEAT R43T & R43TS AC PLUGS IS THE TS IS A LATER DESIGN PLUG WITH A SLIGHTLY EXTENDED TIP OF SAME HEAT RANGE PLUG.

SAME GOES FOR THE FOR EXAMPLE THE 13/16" LONG REACH STD TIP DESIGN GASKETED R44N PLUGS VS THE NEWER DESIGN 13/16" GASKETED EXTENDED TIP R44XL OR R44XLS PLUGS ALL BEING SAME HEAT RANGE LONG REACH PLUGS . THEY WERE USED IN BBC'S PRIOR TO 70 ACCEPT FOR A FEW VERY EARLY 70 BUILDS WHERE EITHER A FEW 063 LONG REACH PLUG HEADS FROM 69 SLIPPED THRU INTO THOSE VERY EARLY 70 MOTORS OR WHERE SOME VERY EARLY BUILD 290 HEADS WERE TAPPED/THREADED FOR THE OLDER DESIGN 13/16" GASKETED LONG REACH PLUGS.

Anyway,the TS /extended tip 5/8" tapered seat plugs were designed to run cleaner at idle and lower engine speeds due to the tip being placed slightly further into the comb chamber where theres more heat to help keep it clean.

But then when your on the motor hard it's the additional fuel & air charge comming into the cyl passing by the extended tip that can actually cool the tip to help avoid detonation and glazing of plugs due to overheating too which was a great design upgrade. The tips on the extended tip plugs ran hotter when at idle or when at lower load engine rpms and ran cooler to help avoid detonation/plug glazing leading to misfires at higher rpms or when motor was under heavy load at WOT,best of both worlds IMHO.

Thats why in older catalogs they rec the std T tips and then sometimes in later catalogs they rec the TS extended tip designs.

I have had no clearance issues at all when using the newer design TS slightly extended tip design plugs in stock apps that called for the older design std or T tip plugs .

I always use the extended tips design in the stock perf apps (not rebuilt motors with questionable piston dome heights etc) because they run cleaner at idle and lower engine speeds. Street perf motors can make better use of of the extended tip design because they arent always dialed in perfectly timing/fuel calibration wise so the extended tip plugs that stay cleaner are a help.

Scott