40 point fluctuations between pre and post boil

Yesterday brewed an Irish red. It was supposed to end up at .51 and it ended up being .31 post boil. Pre boil it was .71. Mashed at 154 with 12.50 qts of water for an hour. Sparged at 170 with 20 qts. Ended up with 6 gallons. Brought another gallon up to 170 and slowly poured over grain bed to reach 7 gal. Would that extra gal. Throw off my #'s that much. It also could have been my hydrometer it is weighted by ball bearings but the seal for the bearings is cracked and some of the bearings are loose and able to roll around. When I get home today gonna put it in water to see what it reads

How did you get a lower gravity after boiling?

Did you measure just first runnings and not total wort collected.

We're the temps the same each time you measured?

I think that is Mercury (not positive) and not ball bearings in the hydrometer. Regardless, if it is leaking anything it needs to be replaced.

If water can get into your hydrometer then it won't be accurate you'll have to get a new one.
Your preboil Gravity should be lower than your expected starting gravity. As you boil, your beer will become more concentrated and the gravity will increase. By your numbers here your gravity decreased. Make sure you take your preboil gravity reading after you add all your water to the brewpot. As long as you did that adding the extra gallon of water wont give you strange results.

The hydrometer was borrowed because I broke mine the night before. The hydro itself was not cracked or broken at all just the wax or plastic seal at the bottom that let some of the bearing above the seal itself. Took a sample of total wort collected and put in fridge brought it down to 65 and the reading was .71 took the sample after boil at 70 and it read .31. so that's why I am thinking the hydro is messed up. This is my 6th all grain batch and never had a problem before

What temp is the hydrometer calibrated to? I would check it in distilled water, at that temp.
But, the reading don't make sense, as mentioned above, even if the hydrometer is off.
I am wondering if perhaps your first runnings and your later runnings weren't mixed well, prior to the pre-boil gravity reading?

Readings* Dang iPhone.

I asssume you are talking in units of specific gravity. So it would be 1.071, 1.051 and 1.031. In that order it would be pre-boil (actual), OG (estimated) and OG (actual).

The posters above are on the right track. Messed up hydrometer or poor mixing of the sugars in solution. The mixing problem isn't as big of a deal with all grain as it is harder to get sugar stratification on that big of a scale. Hydromters are super cheap. I'd buy a couple so that you have one as a spare next time you break one.

Already ordered