Can someone tell me the difference in these two threads

On the left is my stock oil pressure gauge. On the right is a brass adapter i ordered. Both are M10-1.0 The adapter is a hair larger and begins to screw in the thread but then starts to cross thread. I don't understand why if they are the same size?

 

the one on the left looks like it tapers, starts small and gets wider. the one on the right looks to be the same diameter for the length of the threads

 

Correct.
The one on the left is pipe thread, a.k.a. NPT.
It has a taper so it starts in easy then tightens up really well as the tapers come together.
A decent sized hardware store should have lots of brass fittings with different thread types so you can find what you need.

 

x2, could just be the camera but the one of the left appears to be a tapered bolt as Zombie Runner said. If it don't fit, don't force it!

 

NPT, you should be able to find that at Home Depot

 

Here is a better picture of the stock sender. It does look like it tapers but only towards the last part of the threads and not at the beginning hmmm...

 

yup its pipe thread, tapered. the new one is not

 

The brass fitting is for a feral or compression fitting

 

So what i need is a NPT M10-1.0 to SAE adapter? I will be putting in a nylon tube on the SAE side of it for my oil pressure. Will this work?