I'm re using some old lag screws to fasten a new treated 2x8 and I can't get any of the lag screws to fully tighten. They just keep spinning. Any ideas what the problem might be? Are the old ones no good anymore?

Stripped out threads or stripped out hole or hole too big?

I'm thinking the threads are stripped out if that's possible.

Probably best to just replace them one at a time with a wider lag screw. Perhaps add some wood glue and/0r stuff some wooden matches or toothpicks in hole prior for the threads to regrip the wood fiber.

Yeah, sounds like you've stripped out the hole. What size? Did you use a pilot hole?

3/8"x1.5" is the size of the lags. At forst I wasn't using a pilot hole but then did with same results.

I went and picked up some new lag screws early this morning and it seems to be better.

Would y'all recommend using a pilot hole or not?

100% use a pilot. For 3/8" lag you should be at 1/4" or just under depending on what your mounting to (hardwood vs pine). And drill the pilot beyond the length of bolt penetration.

You're mag screws may not have the threads on them that they once did. New lags are always a smart move. If they still don't tighten, drill out your hole with an appropriate size bit that matches a readily available
Dowel of the same size, glue it in, flush cut it, pilot hole it and reinstall the new lag.

I guess I should clarify what I'm using the lags for. I'm re doing the wood bunks on my pontoon boat trailer so it's important to make sure they're good and tight. I got most of them fully tightened yesterday but there's still a couple that just spin and spin and won't fully tighten even after using new lags and pre drilling. I've run plenty of lag screws in my life and have never had this problem before.

Use Carriage bolts and lock nuts; head on top, nut under wood. They won't scratch the toons as bad as a lag bolts if worn wood and are way more consistent and last longer.

Great idea thanks!

Another solution is to try using a larger, and perhaps longer lag bolt. Trying to reuse the same hole without a larger screw (especially in a soft wood) is problematic.

Your pilot hole should be slightly smaller than the core of the lag screw (IOWs, the solid centerpiece- not the threads).

How are you attaching a 2x8 (1.5" thick), with 1.5" lag screws? Unless I'm not understanding something. Depending on what you're attaching to, I'd usually use a 3.5-4" long lag screw at minimum.

Edit: never mind, I missed the post a couple above mine.

Drill the pilot hole smaller than the screw size? Seems obvious, but some people match the bit to the thread width. It should be the size of the center shaft on the threads, so a 3/8" screw needs a 3/16, maybe a 1/4" bit. I usually lean to the smaller side of things for the pilot bit, assuming you're attaching to wood.