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Performa 5xx Series back panel screw size?

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I really wish we had replacement injection-moulded cases for the 520/550/575. 'Just went to investigate this and broke the only non-broken clip on the I/O cover. :(


Anyway, my 550 is missing those screws, so I haven't confirmed this yet, but let's see what we can figure out.


According to the Macintosh Computers Service Guide, Volume III (July 1994), page 108, there's two screw sizes used for the rear housing, but no indication of which one is used for what. (Not even how many of each one is used.) One is part number 426-1007, which a cursory Google search seems to indicate as the part number used for the I/O port, but I have some misgivings about that being the correct screw size.


The notation on that page is a little screwy (pun 100% intended), but comparing that part number to its occurrence on other computers in the service guide would indicate a zinc self-tapping pan-headed Torx screw with a 4.22mm diameter, 1.41mm pitch, and 16mm length. The self-tapping part really sticks out to me, as the 5 main rear housing screws just screw into plastic, so I could see them using self-tapping screws in the factory to tap out those holes. Also, measuring one of the main rear housing screws with my calliper, I get a 4.2mm diameter and 16mm length. The screw I pulled doesn't look self-tapping, and has a cheese head rather than pan head, but otherwise seems to fit the bill pretty nicely as 426-1007. (Also, 4.22mm? That's a weird diameter. I wonder if maybe 426-1007 isn't actually a metric screw.)


The other thing that makes me think that 426-1007 isn't the correct part number is that I can confirm that the I/O cover's screw holes are a smaller diameter than the 5 main screws on the rear housing, and the hole looks like it was meant for a machine screw. (Maybe I should have led with that?) The other screw listed on the page (part number 410-1308) is an M3x5.8mm screw, which is of course a smaller diameter than 4.22mm. (I can't tell by looking in the hole below the I/O cover if it's fine or coarse pitch, so could be either 0.5mm or 0.35mm.) So I think that's the one. Now, what kind of head does it have? No idea. I can't find any photos of the I/O cover's screws, but the installation instructions for the Sonnet Presto Plus seem to indicate that it's a Philips. (But the photos are really low-res.) If anybody has any of those screws and could take a photo, that would be helpful.


I'll see about taking a trip to the hardware store tomorrow to try and get to the bottom of this, but I'm also expecting an Xserve to be delivered tomorrow, so I may have to put it off until Tuesday.

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Okay! So I can confirm it is definitely the 410-1308 screws. My hardware store didn't have a 5.8mm long M3 screw (not terribly surprised) so I picked up some 5mm and 6mm screws. I tried the 6mm first, and it fits like a glove. So the screws you want are M3x6mm with a 0.50mm (coarse) pitch. I got ones that are pan head Philips, and they look just fine installed.

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