Background: I have a Dexter 2k# axle, 5 on 4.5 bolt pattern. I purchased automotive aluminum mod wheels 15x7 (most trailer wheels are 15x5 or 6); since they're a little "nicer" I opted for the auto wheels, however, on some auto wheels the bolt plate is quite a bit more thick than a standard trailer wheel and the hub hole on the auto wheel is considerably larger...so, when I bolted my wheels to the axle there wasn't enough thread left on the studs and the center hub opening wasn't riding on the axle hub...so
hmmm, what to do now. Most would say 1.) buy a regular trailer rim which is typically a 0 offset, the hub opening is 2.5" and the studs will be more than long enough or 2.) just press the studs out and replace them with longer studs...well... not so much the case.
Dexter axles: 4 lug is typically for 8-12" wheels; 5 lug is for 13-15" wheels. Trailer hubs are VERY different from automotive hubs in hub size and stud size...while automotive studs are made in 1/2x20 threads, the knerl (spline on stud) is different than automotive. When you replace studs there are 3 measurements you have to have to get the right one: 1.) knerl size (diameter of the stud at the splines) 2.) number of serations and 3.) bolt size and length. Well.. 1/2 auto knerl is about .68" and on a trailer, not just Dexter, the knerl size is .545". Hub diameter on a 5 bolt auto is about 3.2" and on a trailer it's about 2.5"... THUS, most auto stores do NOT carry studs for a trailer hub; but they can be found and I found my replacements at http://shop.easternmarine.com/index.cfm?fuseaction=catalog.catalog&categoryID=190 and they verified theirs were .545.
Now, the hub hole on my wheel is still quite a bit to big for the trailer hub...BUT, Dexter axles are NOT made for hub centered wheels (thus why they accomidate 13-15" wheels)... HOWEVER, the hubs are designed to be wheel lug centering via an "a-corn" (so MAG auto wheels simply cannot be used, so make sure you get wheels that have a conifer in them (recess for an acorn nut) and make sure you get a lug nut that has the same pitch as the wheel... 40-50-60 degrees. Another option is to not have studs but "tap axle" but I'd venture to say most of us have stud axles; If you are EVER in doubt, call Dexter... this is where I got all my info and they were so very happy to help me. You just have to go to their website, go to contact and call the corporate headquarters and ask for engineering... they can tell you everything about their axles and if you have your customer number or serial number or part number they can tell you EXACTLY what you want to know about "your" axle.
I purchased my axle direct from Dexter for only $40 more (shipping) than a "local dealer" and I had my axle in 3 days...when I called they had all my info ASAP, and told me exactly what I needed to do to correct my "use auto wheels on their axle" and it was awesome. This has turned into a VERY long post but I though some might like to know this lil tid-bit.
ALSO, before you order, you can call Dexter Engineering to size your axle.. you can tell them your frame width, how high you want it off the ground, etc and they will print out a report giving you all the specs of the trailer empty, 1/2 loaded and fully loaded.. min. fender height, space of arms, etc, etc. kinda takes the guess work out of ordering an axle.....it's free and they'll fax it to you WHILE you're talking to them...I can't say enough good things.
I saved this information it will come in handy when I order my Dexter Torsion axle, 
