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It can be changed without taking the pulley off.
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Banjo is right... in most cases. Originally there was one bolt that sits partly behind the pulley and can require the pulley to be removed... sometimes. Often, if the pump has been changed in the past (and most have been in the 40-50 year life of the car), that bolt was very commonly changed out for a nut on a stud (the other two mountings are already nuts on studs). It's easy to tell just by looking at all the fasteners behind the pulley. If all are studs with nuts, it will be obvious. There is only just enough room behind the pulley for a nut and lock washer. So often these are installed with a bit of Locktite that might make the entire stud come out of the block. If so, once the pump is off the car clamp the stud with Vice Grips and try removing the nut again. A little heat is sometimes needed to get certain types of Locktite to release.
Note, if you do find a bolt and happen to need to remove a stubborn pulley, don't use one of those two, three or four-legged pullers that hook over just the edge. The pulley is cast metal and it almost certainly will break at the edge, and you'll be shopping for a replacement. Instead, take the pump and pulley assembly to a machine shop that might have a collar-type puller, which will be able to remove the stuck pulley without damaging it. The pump housing itself doesn't need to be completely removed when changing the pump. It's held on by a fourth bolt.
In fact, not all pumps even have removeable pulleys. Some are simply pressed on. The removeable ones will have a nut (usually a Nylock) at the very tip of the pulley. The non-removeable just have the flush end of the shaft, i.e. no nut. The non-removeable-pulley pumps were (and are still) common aftermarket replacements. If this type pump is used, that one partially hidden bolt absolutely had to have already been replaced with a stud and nut, or there would be no way to fully install the pump.
The original pumps with removeable pulleys were a direct carryover from the tractor engine. In fact, they were user rebuildable and parts to do it yourself, or factory rebuilt units, used to be widely available at any Massey Ferguson tractor dealer, often a whole lot cheaper than the same parts at auto parts stores or Triumph dealers. I haven't checked lately, but you might see if there is a local Massey Ferguson dealer, if you need a pump or parts (including gaskets).
A final clue to the pump's age, the original pumps (at least up through TR4) had a grease zerk to supply an occasional squirt of lubrication to the bearings. Nearly all replacement pumps over the years lack the zerk and some don't even have the flat in the casting where it would have been fitted (others do have the flat, and I've even heard of people seeking these out and then epoxying fake zerks onto them, for sake of "authenticity".) Modern pumps don't use or need any bearing lubrication .
As Banjo said, it's a pretty straightforward and easy job, plus replacement parts are widely available. One last thing to watch out for if you do need a replacement is that many of the current crof pumps seem to have a problem with their casting, that causes the inside of the pulley to rub in certain areas on the housing. Watch for this and if necessary grind the pump housing a little to create some more clearance. (The last pump I bought was an uprated one from British Frame and Engine and the housing had been carefully machined so that there were no clearance problems at all, but these high performance pumps are also quite a bit more expensive.)
Hope this helps!
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