I was shipped NGK-TR5 (NGK#2238) spark plugs as a compatible replacement for my original NGK-BR6EF (NGK#3177). I believe that original plugs use a .025" gap, however, what gap should I use for the NGK-TR5? I think that the stock gap for these TR5 plugs are pre-gapped over .050". What should I GAP these plugs to? Please note: the original plugs (NGK-BR6EF) has a manufacturer heat range level 6, whereas, the newer NGK-TR5 have a manufacturer heat range level 5. Do you think that the heat level matters, or should I locate the stock plugs?

Submitted by stevejag@sbcgl… on Wed, 07/09/2014 - 21:30

You may get differing opinions on this.....but here's mine.

Send 'em back and tell 'em you want the right stuff, and here's why.

The TR5 is a hotter spark plug than the correct BR6EF. For NGK, the higher the heat range number, the COLDER the plug. For some plug makers, the higher the heat range, the hotter the plug. We all know the V12 is notoriously hard to keep cool, so you don't need a plug that will keep a hotter tip temperature.

The TR5 is intended for a .060" spark gap. You just can't narrow the gap to .025" without compromising it. You can't leave it at .060" because it can drive up the secondary ignition voltages significantly. That can cause undue stress on the ignition wires and coils and lead to early failures there.

The TR5 is a 'projected' tip spark plug, meaning the tip is extended into the chamber a good bit. I don't know if it's enough to pose a clearance threat, but it might. But it will place the spark in a different position in the combustion chamber than is intended. That and/or the hotter tip can lead to detonation issues. We also know we don't need that, either.

So stick with the short-nose, narrow gap, colder plug. I think you [and your V12] be happier in the long run.

My two cents......