Originally Posted by dwoodward
OEM-quality spark plugs are cheap, and you may not be able to visibly detect functional issues. I once found one that would spark fine in open air, but not under compression at TDC.
Sort of like doing a timing belt on a car- most likely you have to muck with the water pump anyway, might as well replace it while you're there, and hey, crank main seals are cheap and you're right there- why would you not replace it?
When a customer was paying me 5 hours of labor to replace a timing belt I always recommended a set of front crank and cam seals. This was because it was a labor intensive job, and I couldn't "see" the hardness of the seal material or the condition of the seal lip without removing the seal, which of course destroys it.
Things are a little different with a spark plug. I can see the condition of the electrodes. I can measure the gap and determine the amount of wear already incurred. I can (and did) re-gap them to spec. NGK plugs have a legendary reputation anyway. And lastly, if I'm wrong and start having issues I can replace all of the spark plugs in less than a half hour in a parking lot with a 5mm Allen, a 14mm socket, and a 16mm socket & ratchet. Practically any auto parts store in any city will sell me the plugs at half of the cost from Triumph and loan me the tools to do it.
I did my 12k for less than $90 including a gallon of amsoil synthetic 10 w 40, a triumph factory air filter, and a Triumph factory oil filter. Or I could have spent $200 to get the complete service kit from triumph, dealer oil, and dealer plugs, and had a cam cover that doesn't leak just like the gasket I re-used, and spark plugs that spark just like the ones I re-used, and had that oh so good feeling of throwing parts in a landfill that were still within service limits.
Do I recommend you do the same? Not necessarily, unless you know how to truly determine when something should be replaced. I know a thing or two about engines and felt comfortable making the decision. As always, ymmv.