Issue with jam boiling out of jar when canning.

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May 15, 2020
Prior Lake
I made cherry jam a month ago with great success, and now I made peach jam. I used a recipe I always seems to work, 2 cup diced peaches, 1 Tbsp lemon juice, 1.5 Tbsp. no sugar pectin, sugar to taste, it jelled properly. I did 1/4 in. headspace, pint jars, brand new lids, new rings. Boiled in my big canner 20 minutes. Came out with peach jam boiling so hard it boiled out of the jar, leaking out underneath the lids. So I cut back on the amount of peach jam in the jar to leave 3/4 in headspace to account for the jam boiling a bit, wiped the rims of the jars clean, got new lids, another 20 minutes in the water bath (as I've had luck with that in the past) but it's still boiling out of the jars just a little and I don't want to store it as I'm not sure boiling out will make a good seal. Any suggestions or help for my third round on this same jam?
Not knowing where Prior Lake is - at what altitude do you live?

Perhaps 20 minutes is just too long? What size jars are you using?
Pints? = 2 cups
1/2 pints = 1 cup
1/4 pint = 1/2 cup
ooopsy just reread: you are using pint jars (approx 2 cups)
I will assume your large boiling water canner has a rack to hold the jars?
Standard head space is 1/2 inch.
If your jam is boiling and reaching it's gel and then you ladle into hot jars - almost all of my recipes just call for 5 minutes after the canner has come back to a boil.
Thank you, I will decrease my processing time the next time I can! On a different question, what is the standard jelly jar size people use for canning? I bought my mason jars about a year ago on sale, they were brand new pint size, rings and all, a 12 pack was 3 dollars (25 cents a piece.) Should I buy half pint jars to make jam in the future, or does it not matter?
Recently, the nchfp or an extension service came out with recommendations that the 1/2 pint recipes for jam could be made in the 1 pint size. and 5 minutes for half pints, 10 for pints.
The reason this was so helpful, was that the rule is you cannot put food in larger jars than the recipe calls for, but you can put it in smaller jars than the recipe calls for, due to processing time. But, this frees up people to use the size they find most convenient. Notice the amount of time that is needed.

There are 12 oz jars, 1/2 pint= 8 oz jars, and 4 ounce jars. These are all fine for jam.

I usually do jam or concentrated puree in 8 oz jars, my favorite are the flat 8 oz jars. I use bigger jars for apple and pear sauce.

processing times jam:
5 minutes for 1/2 pint
10 minutes for pint
Fruit purees, for pints or quarts, 15 minutes
For all of the above, make sure the contents are HOT and the water in the canner is HOT when you put them in the canner.
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