4" bolt pattern and 4x100mm are one and the same. They're interchangeable. For a 23x8.5-12 tire, you want a 12x7 rim. So I need a 12x7 rim in 4x100/4x4" bolt pattern with a 70mm/2.75" centre bore.
There are a million garden tractor rims that almost fit this requirement, they're just 5x114.3/5x4.5" bolt pattern, so an adapter might be the way to go here. Adapters are like $300 for a pair though so it'll get expensive really quickly since I plan on upgrading the rear hubs too. Could be as high as $2k for adapters, hubs, wheels, and tires all said and done if done this way. Not a sum of money to just throw around without some thought behind it. The nice thing about this route is I can probably find a selection of 5x4.5" tractor wheels laying around in some junkyard.
4x4" and 4x100mm are NOT the exact same. 4x4" is closer to 102mm, which is enough to make the studs hit the edges of the holes on the rim and not want to slide through. However, it is usually pretty easy to fix. You can either tilt your press-in studs slightly by putting a nut on them and then tapping them back (ie un-seating them from hub slightly) and then tapping them sideways to put a slight tilt on them, or you can enlarge the wheel holes. These factory tractor front rims didn't even use a tapered seat, they just used a flat nut and washer, so even a not-properly-centered conical lug is good enough for this <8mph application. Ive done this on a lawn tractor i added 4x4" hubs too, works fine.
I have an early B7100 with 12" 4x100 rims. They do exist, but most B7100s have the newer 6bolt front.
4x100 to 5x114.3 adapters are fine, and they're actually around $30/ea at cheapest if you shop carefully. I've driven over 100mph, done drag launches on slicks, and towed trailers on the cheapest chinesium wheel adapters i could find and never had a problem. I probably own 20-30 total wheel adapters between all my vehicles that have them. Just buy the cheapest thing.
One downside of spacers is they increase your scrub radius, which can majorly increase your steering effort on a loader tractor with no power steering. If you don't have a loader, no problem!
One super easy option you may not like, is just to get some 12" 4lug trailer wheels. They are technically too narrow for anything but a 6-12 (that's a tire size) tire, but the downsides of squeezing a tire onto a too-narrow rim are also very minor on a tractor compared to a car. I have some 25x9-12 ATV tires mounted on some 12" trailer rims, for example. Drive tires on a golf cart in that case. But you definitely could mount a 23x8.5x12 on a narrow trailer rim. It will not look all that great, but you'd be hard pressed to
MEASURE a performance deficit resulting from it, on a tiny tractor application!
But, if you really do want to run a full 7" wide 12" rim on front, the easiest thing would be to buy some 12" golf cart 4x4" wheels and either modify the lug studs or the holes in the wheel slightly. Like these:
$49/ea
