Hi guys! Random question. I have dart iron eagle heads on my SBC, part number is 10510020P. I noticed in the pamphlet for these heads that they recommend a tapered seat spark plug with .460” of reach. Previous owner ran the AC delco R45TS’s with no problem, those are tapered but only have .430” reach. Is that okay? I noticed the NGK UR4’s have a tapered seat and are .460” of reach and I tried those for a bit but I fouled them and swapped them back out to the R45TS’s and plan on adjusting my carb, I just don’t want to mess anything up now. Thank you guys!

 

R45TS has a heat range of 5. The UR4 plugs have a heat range of 4. If it's fouling up, the plug is too cold but that's already a very hot plug. I bet if you put in UR3 or 2 plugs, they won't foul up as much or you have bad intake valve guide or seals problems letting oil into the cylinders. AC plugs get hotter as the number goes up. NGK plugs get colder as the number goes up.

For best performance, you run the coldest plug possible without it fouling.

As for the reach, it's going to depend on the pistons. With a flat top piston, it won't make a big difference. A longer reach will all the tip to go more into the cylinder but may also make contact with the top of the piston depending on a lot of factors. The R45TS plug has been a standard SBC plug for decades and works with just about any head.