If you're using a billet rod (and flywheel), then the 18# springs will be a great upgrade. However they allow the engine to rev 7k rpm, I believe, depending on the cam, so I wouldn't use them with a stock rod or stock flywheel, since the stock cast rods have trouble staying in one piece over 6k rpm, and the cast iron flywheels like to grenade at those speeds. And that brings me to my question for you, is this engine staying governed or is the governor being removed. If the governor is staying, the springs are a waist of money, since you won't gain any rpm. And If the governor is bound for the junk bin, then I'd recommend a billet rod and flywheel for safety reasons, both the engines and yours.
Now, a quick disclaimer, I do not recommend that you do this, since the retainer can slip off and drop a valve in the cylinder, but I have been running my Baja warrior with a stock rod and flywheel, ungoverned, but with stock springs for about a year now. I have taken it to valve float, but not very often. The stock springs will float at around 5200-5500 rpm iirc, which will keep you from snapping a rod (as long as you don't stay there for more than, I'd say 20-30 seconds max). But on my bike valve float occurs somewhere just north of 55mph, and it takes it about 1/8th of a mile to get there. But I don't go there very often, because I don't want to be that guy that, because of his own stupidity, and inability to back off the throttle, ends up pushing his bike home with a valve in the piston and other various cylinder damage. Also I have a job, and I'm going to get myself a billet rod and flywheel (and maybe a cam) for Christmas.
And, yes this is the correct section for engine questions. And I'm sure that itsid will be here shortly to correct any mistakes I may have made.
