MG Sports Cars

My 2nd hand compressor started popping the blow off safety valve in the tube between the low & high pressure cylinders... Popped open the head this evening & the gasket was blown. It's blown in a narrow section between the intake & exhaust sides of the head. What concerned me more than the gasket blowing was there was a layer of oil on both cylinders & on the top of the valves. Does it make sense that the high pressure side leaking back to the low pressure cylinder could build pressure somewhere causing the oil to be forced past the rings? Or is it more likely that the rings are bad -- letting oil by & the oil getting into the gasket caused it to blow?

Rob, that's a tough question you ask because most of us don't really know that much about compressors. But I've been into a few of them and I'll give it a shot. First off, it's fairly clear that the blown gasket section would cause the blow-off valve to pop since it's sending second stage high pressure air back down the transfer tube. It isn't all that common to see a blow off on the transfer line but as you describe it that's how it would function. Oil on the gasket should have no effect, it probably blew either from age or from loss of torque on the attaching bolts, and maybe to a lesser extent from warpage of the mating surfaces. If you use a good strong sealant when you put it back together it should hold fine. I like "Right Stuff" for things like this but make sure your parts are clean and use sparingly. The oil in the chamber is probably not cause for concern unless your oil level in the sight glass drops requiring frequent refilling. Some oil will naturally find it's way up there and unlike a piston engine there is no flame to burn it off. Worn rings would be indicated by longer running times to build pressure and loss of crankcase oil. If neither condition existed, why worry? HTH

Jim

Thanks Jim. There was probably about 1/32" of oil on the cylinders, maybe a little more than that which is what got me concerned... I was able to work on the car this weekend without the compressor so am going to contact the manufacturer(Northern Tool) tomorrow to get the diameter specs on cylinders so I can check them out before putting it back together. With luck it'll be ok, but if they do need attention, I'd rather take care of it once & for all rather than have to open it up again.

If there's any cylinder wear you'll see it as a ridge at the top just like in any car engine.

Some compressors do pressurize the crankcase I think and that would be the only reason I can think of for that much oil in the cylinder. But again, blowing oil out into the tank is really the only accurate indicator and if it wasn't doing that I wouldn't worry.

Jim

Rob, I blew a head gasket in my compressor and found parts at this place, good service and worked great. Hopefully they can help you with your compressor. [www.sanborncompressors.com]