There's probably a chunk of the old tube still in the hole. When this happened to me I used an Easy-Out in the hole to grab the chunk of old tube and remove it.

 

I pulled the tube because it was snapped off. I used a punch from the oil pan side and drove out the entire piece. A real pain in the a$$ to get out!!! And even more difficult to get in. Any tricks would be great.

 

Is it a chrome-plated tube? If so, mask it above the flange and sand off the plating on the bottom. Option 2 would be to use a tapered rat-tail file and "massage" the hole until it fits.

 

Make sure part of the old in isnt still in there. Wiggle the new one out, slowly and carefully. Check the hole for the old one, use a grinding wheel to reduce the OD a few thousandths, it will be held in by a bolt on a flange anyway. I did this on mine, it went right in up to the flange, and never leaked.

 

These chrome aftermarket dipstick's don't fit. I had to grind mine thinner quite a bit to fit

 

Went through something similar recently. Also started to do the box wrench strategy but got concerned the thing would be mangled before it seated so stopped and ordered a Lokar braided dipstick because at least it had more meat to work with on the base to press in. The box end technique worked but initially wasn't cooperating so nicely.

What eventually got it was supporting a section of the wrench handle on a nearby eng. block tab as I tapped the wrench handle with hammer. W/o that added support, the energy from hammer wasn't quite making it smack dab where it was needed.

Talk about such a simple thing adding drama due to various potential consequences. Course I used some oil on the end. The Lokar uses 2 o-rings. I removed the first one when it was looking like was hanging me up but in fact wasn't. It just took some effort to knock it in. I like how it mounts and looks better than original/repro. Wasn't cheap but beats alternative outcome of breaking aftermarket dipstick off in a freshly rebuilt/installed eng.

 

The last chrome oil dip stick I got was too big for the stock hole. Fortunately, I also was re-gasketing the timing cover and the oil pan so I had everything off. After mic'ing the dip stick tube I got the right sized drill and drilled out the hole in the timing cover. I didn't think I would be able to keep the tube perfectly round if I ground on it. It still bleeds oil out down at the bottom though.