I've always heard that the purpose of cylinder cocks is to allow condensed water to vacate the cylinders because water won't compress and will blow the heads. I've got a model live steamer (1:20.3 scale) that doesn't have cylinder cocks -- any water in the cylinders squirts out the exhaust pipe in the stack. I have to "rock" her back and forth when first starting to work the water out and warm up the cylinders.I assume that the forces are way higher in a full-sized engine, and the materials proportionately stronger on my model due to the smaller scale (4th law of model railroading -- physics don't scale). True? Where's the point that cylinder cocks become necessary?

Matt,Your engine probably has slide valves. If it does, they don't seal tight and let the water get past. probably eaiser than trying to build a small cock.Tom C.

Dear SirI am a live steamer also,,I have (2) 3/4" scale engine's,,and yes cylinder cock's are needed...I have seen,,cracked cylinder heads,,because the water could not get out...If you would like more information,,on this try going up too,,one of the Live Steam site's,,too get more answer's too your question...I hope this is of help,,,Pat

Matt:The only dumb question is the one not asked.Check the www.chaski.com site for a darn good site for such questions and other information. I also belong to the Gauge 1 Model Railway Association. Great group of people who can also answer many questions.To help with your current situation, go the hardware store and pick up an elbow that fits in the stack. Stick it in and aim it side ways. Water and steam will now shoot out the side rather than on the back of your hand or arm. If you remove the elbow during a run, use pliers. It gets very hot.Many engines as small as Gauge 1 do have cylinder drains. Without them damage would not be so much on the pistons/heads, rather it would be on driver/axle position. You can lose the quartering if you force the issue too much. I am quite gentle as I warm up the engine, letting it set its own pace. Also, the elbow helps create a good plume from the stack and a bit more talk as well. I just leave it in all the time. The effect off sets the odd appearance.Contact me at jrmay@monmouth.com if you need more information. J.R.

The engine is an Accucraft Ruby 0-4-0T that I modified into an 0-4-2T to hold radio the control gear. I'm pretty sure the valves are piston valves. It has "piston valve reverse" -- I haven't quite figured out how that works yet, it looks like Stephenson gear except it only has one eccentric per side.My question is, how come I can get away with not having cylinder cocks on that small engine, but bigger stuff needs them? Why won't water in the cylinders just squirt up the stack on a full-sized engine like it does on my little one?

yep, Ruby has piston valves.there are all kinds of tricks you can do to keep the water from squirting straight up the stack.crimp the pipe, block part of the top of the stack..etc..lots of options.and lots of work has been done on improving Ruby's performance.http://www.mylargescale.com/forum/topic ... .aspScotmy ruby!: http://gold.mylargescale.com/scottychao ... Angel.html

Matt,The answer has to do with physics, scale, and strength of materials. First of all, the model is made of brass, likely, and with heavier material sections than true scale would require. The prototype was made with cast iron, that did not have the same ductility. The force of water compression in the prototype would fracture the iron like peanut brittle (look at pictures, most of the cylinder heads look like potato chip fragments, and the bolts are intact.)Secondly, the model likely has more clearance between the piston and head at dead center. The prototype would have had very tight clearance, relative to scale. Less clearance means less allowance for water.Thirdly, if your valve gear runs in full cutoff, and has "line" or clearance valve design, then there is a place for water to go - out the exhaust - at forward/back dead center, even for piston valve designs. Fourthly, scale models are just not as tight. The pistons and the valves both leak a little, and allow some water to pass.Finally, damaging a cylinder head requires some force. Remember the force in the cylinder scales at the square of the dimension. A 3/8 inch scale model has 1/1024 the force at the piston of its prototype, but the metal bulk of the cylinder is greater than 1/1024 of the prototype (else the model would be too delicate to handle). Also, the weight of the model is not enough to force the piston, with dynamic momentum, to break the metal.On a riding model, in 3/4 inch scale, there is some risk, because the weight of the driver, and train and passengers, are pushing the model. The risk would be priming. The train, at speed, sucks water into the cylinders. The engine shudders, and the weight of engineer and 4 passengers overcomes the resistance and pops the cylinder head. The prototype had a spring loaded relief valve on cylinders for this purpose, especially on high speed engines.

The cylinder cocks I put on my 4.5" gauge 2-6-2 and my 7.5" gauge Clishay were "automatic" ones. They were a basically a ball bearing with a spring behind it sealing the ball onto a seal that was on the part of the valve that was attached to the cylinder. The screw cap on the bottom has a small hole and was where the spring seated. When the water in the cylinder would not compress, the spring would compress and let the excess water drip out. The cap was adjusted to keep the ball seated against steam pressure only.Doug

Thanks, that's what I wanted to know.I don't mind my Ruby squirting hot water. I just wondered why it would do it when a real engine wouldn't. Now I know.