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I'm having issues with the nipples the spoke threads are sticking out. I want to tension the wheel but when I tighten the nipples the spoke treads come out from top of the nipples before the wheel is tensioned completely.

Edit: Related to, and following on from Could I use a front rim to build a rear wheel?

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    You may need spokes of a different length. Check the diameter of the flanges of the rear hub and compare to those of the front hub.
    – Carel
    May 10 '19 at 16:32
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    Where did the spokes come from? What is the ERD of the old rim and the new rim? Are you using the same rear hub or a new one? Is the problem with all the spokes or just on one side? Is either rim asymmetrically drilled?
    – Swifty
    May 11 '19 at 10:19
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    Comments are not for extended discussion; this conversation has been moved to chat.
    – jimchristie ♦
    May 13 '19 at 13:33
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It seems the spokes are too long. Two possibilities I can think of:

a) If the rim had been built into a front wheel before, and you are using the same spokes, it's possible that the rear and front hubs have dimensions different enough that the recommended 2 mm margin of error on spoke length is insufficient. The main variable affecting spoke length (besides the ERD, but here I'm assuming the rim stays constant) will be the flange radius (1 mm less means about 1 mm less on radial builds with a smaller difference the more crossings you have), but a big difference in the flange offset from the center of the hub can have a significant impact as well. Use a spoke calculator.

b) you're doing a different number of crossings. This will affect the the required spoke length bigly.

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    Or the more glaring issue - most rear wheels are dished (hub not centered) to account for the sprockets.
    – Chris Stratton
    May 11 '19 at 5:32
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    Sources? For manufacturer it would be easier to order or cut correct length and avoid this extra step.
    – ojs
    May 11 '19 at 12:10
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    The asker's original title which was unhelpfully removed by a confused 3rd party made it clear that they were trying to re-use front wheel components to build a rear wheel. That leads to multiple causes for the spoke lengths to be wrong. Spokes are relatively cheap and using new ones has advantages even beyond proper fit.
    – Chris Stratton
    May 11 '19 at 16:25
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    @ChrisStratton we're not yet confirmed that the spokes were reused, but it does seem probable.
    – Criggie ♦
    May 11 '19 at 21:58
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    @ojs - My "source" is having seen the inside of many wheels. It's not at all unusual to see the evidence of a grinder being applied to the nipple and spoke end. I suspect that for some manufacturers it's easier to just grind the spokes to length rather than fussily manage their spoke inventory.
    – Daniel R Hicks
    May 11 '19 at 22:00
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The proper solution here is to get some accurate vernier calipers and measure the rim and hub dimensions, then feed them into a calculator.

Some useful spoke calcs I've used include

Basically feed the same numbers into each and you should get the same/similar answers.

Then measure your existing spokes, which will all be the same length because they're from a front wheel. The rear wheel should have a ~1-3mm difference between their lengths per side.

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    Those existing wrong length spokes can be a useful aid in measuring the critical ERD of the rim as the first step towards calculating and ordering correct ones. With the hub and all other spokes removed OP can measure between spokes positioned as desired in opposite nipples and then add the measured length of the two spokes. With a used rim take several measurements as it may no longer be entirely round on its own.
    – Chris Stratton
    May 12 '19 at 20:30

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