Repairing an SGS 50L compressor

I’ve had this compressor for a couple of years now. I’ve used it to spray paint my workshop, drive die grinders, sanders and nail guns. More recently I got a plasma cutter (which is awesome). however somoewhere along the way the compressor has developed a fault, it started out as an intermittent issue where it would reach the cut off pressure, but immediately hiss a leak until it dropped beflow the low point and kick in again. what started as once every so often became every time. and now it’s just a constant problem. these things aren’t really expecting to run continously like that so it’s time to see if I can fix it.

first suspicion was the check valve. So I ordered what looked like a replacement online.

as is quite clear in this picture, they are not the same… apparently the SGS one has 1 of the same threaded ports as what seems to be the standard(?) but not all three. However having taken it off, and taken it apart, I’m less convinced that this is the problem.

There is really not much to these things, they have a spring with a piece of rubber that presses against a hole that separates the tank port from the fill port and a smaller overpressure port that connects to the main switch. the idea I believe is that whilst the compressor is running it easily pushes the spring back and air flows. once it stops the same pressure in the tank closes off the connection and an inbalance through the switch does something to bleed excess? slightly hazy on that part

nonetheless, it’s hard to see anything wrong with this part, no dirt etc holding the port open. so I guess next culprit is the switch itself. This is where the hissing air sound is coming from afterall.

I search reveals replacements for this run to about 20 quid. and all the ones I saw on amazon had at least one person complaining that they’re very cheaply made out of some fairly cheap casting. So it seems likely these are ostensibly built to fail (at least on the cheaper consumer end of the market)

So, order placed, and off came the old one. Whiilst i’m at it, something that has been annoying about this compressor since pretty much day 1 has been that it leaks oil. Havng just removed the whole pressure switch and guage assembly, I now have clear access to the ‘head’ so I could remove the cover and check out the situation. I’m pretty sure the gasket is not really working, and at this point I realise that the random peice of rubber I’ve kept with this thing for years in-fact… a spare gasket. Now of all the things to provide a spare for, this seems suggestive that these are known to just fail and need replacing also.

in for a penny… now we’re really getting into maintenance mode. the oil looks contaminated with partculate, so I reckon the plan is now to:

drain the oil,

clean out the head.

clean up the mating surfaces and check for any burrs or bumps.

Install the fresh gasket.

refill with good oil.

install the new pressure switch assemly

hope for the best.

just need to wait for stuff to arrive. then we get to see if I can make this thing work properly or if i’ve just made everything worse 😉