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Title says it all. I've gone to some of the various Wikis and not seeing it specifically.

I'm aware that Mark 1 and 2 aren't capable of being swapped around and used, and I know the collar on Mk 7 prevents the use of a 'Corvus' helmet. Mark X, probably doesn't go with any other set at all.

Does anyone have a reference for this or a general idea?

providing there was no obstruction or massive change as far as I know they were cross compatable

Read about mkv armour, which is the designation given to a suit of power armour compiled from mixed pieces.

All marks are cross compatible, but MK VII and MK VIII are the most modular, followed by MK 6. MK III, II, IV, and V bits can be slapped on other suits, but it's going to take some more work.

Marks IV and III are all but incompatible with each other. HH Book 6: Retribution notes (p26) that only Iron Hands Techmarines were capable of mixing pieces from Mark IV and III suits.

 Marshal Loss wrote:
Marks IV and III are all but incompatible with each other. HH Book 6: Retribution notes (p26) that only Iron Hands Techmarines were capable of mixing pieces from Mark IV and III suits.

Pretty sure we see in the 6th Edition Codex some Ultramarines wearing MK III and IV mixed together. Even some MK II thrown in.

There are a few places in the lore where we see marines mixing various types with no apparent difficulty. The impression here seems to be that they're very compatible.

Even if there was some compatibility problems the marine wouldn't dare complain. He's been allowed to use a piece of armor from Chapter history, maybe belonging to a great hero! It must magically be great, so if it doesn't work for you then obviously your faith is lacking...

Found it on my imgur album, here's all of the mixed sets of power armor from the Ultramarines Second Company.

Generally the only compatibility problem seems to be with the helmet. Mk VI helmets don't work with Mk VIII because of the raised gorget, for example. Mk I isn't really related to the other sets at all so it doesn't count. I think there was a change from fixed to moveable helmets at some point as well - Mk II to Mk III I think. Other than that there shouldn't be a problem mixing components.

 Marshal Loss wrote:
Marks IV and III are all but incompatible with each other. HH Book 6: Retribution notes (p26) that only Iron Hands Techmarines were capable of mixing pieces from Mark IV and III suits.

The particular example you are referring to is more than simply fitting separate Mks of components together. They have actually taken a damaged suit of Mk III, and spliced Mk IV parts in to repair half the torso and replace the left arm. In addition, this has been carried out by Iron Hands elements fleeing the Isstvan massacre- they were likely in a position where they were forced to use an ad hoc workshop to carry out the repairs. I think the reason the book states only Iron Hands Techmarines could do this is due to the uniquely complex situation surrounding that particular repair, not the general ability to use, say, a Mk IV helmet on a Mk III suit. It does state the two patterns have underlying incompatibilities, but that is likely less of an issue for exchanging entire components, like a helmet.

In addition, power armour had been a thing for around 200 years at the time of the Horus Heresy. It has existed for 10,000 by the events of 40k. That is a lot more time for Techmarines to tinker around with it and make components work together. Techmarines have a large amount of latitude in carrying out such modifications in comparison to other servants of the Machine God.

The Iron Hand in question:

According to "Deliverance Lost", Mk IV armour was not really readily with other Marks. It was intended as the new, best-of-the-best suit. It was designed in an Imperium that was looking increasingly secure. Supplies and logistics were not forseen as being a long term problem.

Then the Horus Heresy happened and everything went pear-shaped. Mk IV suits quickly become unusable due to minor damage or malfunctions because they could not be repaired when parts ran short because they were not backwards compatible with components in older Marks.

That was what led to the development of Mks 6 and 7. To try and keep most of the benefits of Mk4 but much easier to repair and keep in service.

I am sure that given time and parts, Techmarines can incorporate parts from Mk4 suits into other Marks (or vice-versa) but this modern techmarines spending years on venerable relics and does not reflect how easy it is to mix and match Mk4 with other designs.

In general, Mks 2 & 3 are pretty interchangeable. Mk4 is pretty stand-alone. Mk5 is rare in the Imperium. Mks 6, 7 and 8 are pretty interchangeable (apart from the helmets).

MkIV and MkV should be largely interchangeable, because MkV is essentially MkIV manufactured or repaired with inferior materials when the better parts are unavailable. The chest cabling for example is easier-sourced bulkier cabling because MkIV used much finer cabling. The studded shoulder pad is a compromise to make up for a shortage of adamantium.

It also seems like MkIV is reasonably interchangeable with MkVI and VII, because they are based upon it, but simplified. Those Mks are readily mixed in all recent plastic SM kits, whereas II and III are much less common to see mixed.

One for Kid Kyoto's Retro Review thread on the Warhammer 40,000 Compilation, this; originally the background was that mark 3 was a "highly modified Mark 2", so components should be cross-compatible. Mark 4 was a new design featuring several major upgrades (freely-rotating helmet, new materials allowing weight reduction, improved neural connection and so on). Since it's said that the Heresy interrupted supply lines and meant that lack of spares caued even lightly-damaged suits to be abandoned, one can infer that most components are not shared with earlier armours. Mark 7 armour is basically an evolution of mark 6 (with the major exception of the helmet; the mark 6 uses a design based on that of the mark 4 suit, while the mark 7 helmet is a new design). In particular, it states that the improved knee articulation of the mark 7 was actually a feature of later Mark 6 suits.

Mark 6 and 7 armours are heavily cross-compatible.

After all that, the miniatures designers didn't really listen. While they made a few miniatures representing each of the seven marks, they mostly decided to make miniatures which combined features of several into one. Hence the Long Fangs Sergeant in a Mark 3 suit with Mark 7 arms, the inclusion of components inspired by mark 5 and 6 in the redesigned Tactical Squad kit released for 3rd edition.

The very first Marine in Wyzilla's image has mk7 helmet, torso, arms and backpack, mk2 legs and mk6 shoulders.

Salient point from the original article:

"A notable feature of the resulting armour types (Marks 6 and 7) is the provision of dual technology circuits. These permit relatively rare or sophisticated functions to be temporarily replaced or repaired using common or very simple technology."

I've always assumed that would make it backwards compatible with the earlier marks as it stands to reason it would be easier to replace a faulty left hip actuator with a more primitive (Mk2) left hip actuator than it would to use parts from a waffle iron, in which case 'unfluffy' combinations of MkII/III & MkIV would be explained as the suits having been rewired with the 6/7 dual circuits.