1961 wheels would be of the riveted variety I think.......usually replaced due to failure under VERY hard use.

Sigh, I'm not surprised about the variability of fitment and different wheel sizings through the decades.  I think I have original 1961 3.5x10 steelies but then again, they could have been swapped out to another generation 3.5x10 wheel, who knows.

I guess I will find out if they fit when time comes to put on the wheels over the 7.5.  I'll take some pics either way so we have closure on this

I did a Cooper s 7.5 front end rebuild last week. It was on non S 3.5 steel wheels. I believe the guy said it was an 80. 3.5 S wheels are hard to find and not cheap. I have a fair collection of 8.4 stuff but still like my 7.5 s and 10 inch option although I'm riding on 12 s now. I have bought many 10 inch wheel sets and still haven't found the ones I love. Steve (CTR)

I have to shamefully admit that I attacked the inside of pair of scabby 3½" x 10" wheels with a grinder and relieved a bit of the caliper with a gert file a few years ago so that the rolling shell would actually roll...

It *can* be done, but I'm not sure I'd want to drive on the wheels.

Many moons ago (probably mid '70s) I remember "discussing" fitting 3.5" non-S wheels over S discs. I insisted that it couldn't be done. I'd never found a set that would work. The guy from Canada with whom I was "discussing" insisted that they would all fit.
We were both wrong.
I, being U.S. and only having worked on Mk I and early Mk II cars was right...for early wheels.
He, being Canadian and only having worked on Mk III and later Minis was right...for the later cars.
I don't know when the changes were made. I do know that a few years ago when one day I went through all the non-S 3.5" wheels I had only about 1/2 of them would work with 7.5" discs.
Skyr., I'm betting that if the 3.5s on your '61 are original, they will not fit 7.5" discs.
Now, whether you could do some filing on the calipers that would allow the 3.5s to work, that's another story.

Interesting point regarding the fitment of 7.5 disk brakes and 3.5x10 wheels.  I talked to someone on the Minimania live chat help and they confirmed that I will have no issues and can use the 3.5x10 wheels.

I agree ..there seem to be at least two kinds of standard 3.5” steel wheels - NOT including the 3.5 S wheel.

I have never been able to find a set of standard Mini wheels that will work with S discs.... you’ll be able to bolt them on but the discs interferes with the inside of the wheel preventing it from rotating. 

Cheers, Ian

Have you tried fitting your 3.5" over the calipers?

I had thought that regular 3.5 steelies may have issues going over 7.5" calipers.  ( not S 3.5" of course )

Most  145/12 inch tyres here in the USA are 21 inches high NOT 19 inches hight and will NOT work.. they are actually 145/80/12s.. later bc

We appreciate you updating us with your decision!

Thanks for all the responses.  I ultimately ended up deciding to get a new drum to 7.5" brake conversion kit so I am able to retain my 3.5x10 steelies for now.  I will take my time to look for a 4.5x10 wheel in the future.

Cooper S discs provide more than adequate stopping power in my experience.  We had them on Minis from the early 1990s and drove those Minis all over North American...the last version towing a custom trailer.

The OZ window van had 8.4s & 12s when I bought it.  I think the 12s are OK on the LWB and appropriate for my use of this Mini.  It has NOT required any modification to the arches.

However, I will be using the Mini Spares Special Tuning 4.5"x10" wheels with Cooper S offset on the '61 Countryman...eliminating any need for flairs.

I'm a big fan of 10-inchers, but 10" or 12", let me offer some personal experience with offset. I started with 1 1/2" offset wheels and the torque steer was outrageous. I have a Quaife LSD, but still... I switched to 1/2" offset wheels (same tires, same alignment) and the torque steer was all but eliminated—Rosebud tracks as straight as my wife's Audi. I was astounded that an offset could make such a difference.   

Not attempting to hijack your thread but i have a set of original 7.5's i would sell or possibly trade. They have not been used in years so will need the usual going through.

Stay with the 10's you will thank yourself later.

Have you driven a mini with the 10” front and 7.5 disc brakes before? Why a 12? I hope you don’t end up cutting anything with those flares you’re thinking and then end up regretting it. 

If you've already got the hubs, CVs etc, then the conversion kit to 7½" is less than $200 plus shipping.

Thanks for the responses.  I am located in California so no legal issues I am aware of.  My preferred choice is to go with 7.5" disks so I can have the option of staying with 10 inch rims, but getting the 7.5 drums to disk conversion is a mouth watering $1000.  Used full 7.5 sets are far and in between.  I was fortunate to source a full set of 8.4 (with drive shafts, drive flanges etc) at a fraction of the cost.  I need new tires as well so decided on this route for the time being.

It seems there is minimal difference between the 5.5x12 and the 5x12...

Here are the measurements between the most common 10" and 12" tires.

165/70/10 = 19.1 tall x 6.5 wide

165/60/12 = 19.8 tall x 6.5 wide

or a 12" is .70 taller less than 3/4"total or less than 3/8" taller at the top/front/back.

You can be the judge in regards to performance, looks, fit. 

You'd probably need spacered drums for the rear, too.

Actually, buying a set of 7½" discs isn't the bank breaking operation it once was.  Indeed, the MS/MM own brand calipers work very well and are exceptionally competitively priced.

12" wheels on a Mk1 imo just look wrong, as they do on an Elf or a Hornet.  At 5½" wide you're running the risk of rubbing on full lock, and both will require arches.