Modern Rack Rails in Classic Racks?

steven at steven at
Sat Apr 27 17:31:09 CDT 2019

Zan said
> How safe is it to put modern rack rails (HP) in a classic DEC Rack?  The DEC racks have small holes, while the new HP racks, IIRC, have big square holes.  It looks like the rails will work, they just won’t clip flush.
> Zane

I have been modifying quite a few hodgepodge pairs of modern server rack slides for
H960's. It's the same set of steps to go through on each one.

The very first consideration is to pick a pair of slides that are firstly a) strong
enough construction to hold heavy (Unibus) gear and b) thin enough so they will
accommodate the gear widthwise. Not all slides I've done could have worked for an
PDP-11/15 in a 19" rack as the box was really taxing the maximum width. Measure the
slide widths and gear with some degree of accuracy before committing time and effort
to modifying a pair of slides.

The next thing is to remove all the modern quick-release latches, levers, buttons, pins,
pivoting plates and other crap. Also the riveted rear bracket has to come off, so I use
a die grinder and cutoff wheel to do all these. It's very satisfying to end up with a bare
set of slides.

All ,modern slides are too long, so I set them up on the H960, scribe the length at the
back then disassemble and cut the outer rails to length then drill the outer ends for a
removeable backstop screw so the inner rails don't slide out the back.
Using the rear bracket removed in step 1, I cut to size and refit these at the back with
small countersunk screws so the inner rails can traverse over them. Often an extra pair
of rack unit-spaced holes are required to be drilled. Use an engineer's square to check
things are really square and accurate, or the slides will bind a bit when in use with heavy
equipment (you can almost bet on this).

The next inner rail is then fitted, 90 deg scribed and trimmed at the front. I use an air body
saw for this but sometimes also a plain old hacksaw. Each pair needs to be modified
accordingly to its locations of the front and rear roller guide wheels, the latches and
so on. Wipe everything clean of metal shavings.

The innermost rail is then fitted and cut to length at the front. Sometimes some extra
retract length can be obtained by repositioning something, or cutting away an indented
stopper or spring-lock latch. Again a stopper screw may be required to prevent the
gear falling out the front this time.

Then the mounting holes for the gear are drilled in the inner rails. For the PDP-11/15
I needed to have a pivoting arrangement so I machined a pair of thin stepped bushes
which worked well.

I use modern M6 server rack screws as they are easy and cheap to buy on eBay. Even some
of these need occasional fitting, for instance the Digitronics paper tape reader had
inset mounting screw holes in the front panel that were a tiny bit small for the pan-head
screws, so I machined the head diameter down a fraction.

For the square holes, I use soft aluminium washers which the rack srew draws into the
square and centres up when it is tightened.

In summary - you can adapt modern rack slides to fit old gear and racks, but it's a
surprising amount of metalwork even with a good arrangement of tools. I still have a lot
of them to do.


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