Some questions on gaskets and packing glands

[kvom]

My Muncaster engine build is getting close to the debugging phase where I'll need to seal off the leaky parts.  Yesterday I bolted up all the parts for one half of the engine and applied air.  The good news was the the outer boundary  of the steam chest was quite air-tight without needing any gaskets since the mating surfaces are all ground.  However, I'll need to seal the glands of the valves with gaskets and packing.

I've read how people use teflon thread tape as packing.  Twist into a string and wrap around the value or piston rods.  On this engine the packing glands and their cavities are flat bottom.  So the question here is how much string is appropriate.  I also notices that tightening down the gland screws can clamp the rods as the clearances are fairly tight.  So I'd like to be able to not have to torque the screws down too tightly and let the string do the sealing.  On my locomotive build I used Viton O-rings as seals and could do the same here if that would work better.  On this engine the piston rods are .375" and the valve rods are .25".  I could also open up the glands and through holes as long as the string is a sufficiently good sealant.

The cylinder covers will need gaskets.  Here again I found that clamping down the cover cause the piston rod to be stuck, so the material chosen needs to have some "give" so that I can adjust the screw pressure to keep the cover aligned with the rod.  I've seen some people using teflon film for gaskets.  Wondering what thickness to use if this is the right choice.

My other issue is some taper in the cylinder bores, about .01" over 3".  Obviously there will be some blowby, but I don't know if it's enough to keep the engine from operating.  I made the first piston from bronze, buf haven't turned the second to fit.  At some point I'll need to do it.  Given that 2.25" bronze rod is expensive and that I don't have enough left to make two more pistons, I'm considering making a temporary piston from aluminum for the second cylinder in order to try to get the engine running.  Any thoughts on AL in a CI bore?

If the blowby is too much of a problem I did buy a big adjustable reamer that may straighten the bore, but I'd prefer not to resort to trying to use it until proven necessary.

[Jasonb]

I tend to use graphite yarn but the principal is the same with PTFT tape, you don't need to close the gland right down, it should bear on the packing that is why they are usually fitted with longer studs and retained buy nuts and locknuts so you have some adjustment.

If I do use any form of gasket on flat surfaces then its liquid gasket but that won't help if you don't want the surfaces to bolt up tight, may be better to find why things are tight and ease them.

0.010" is a lot to ask any form of piston packing or ring to take up, can you not machine the bore straighter, a between centres bar will flex less than a boring head.

I often use aluminium pistons in iron cylinders on steam engines with either iron or O rings, just allow for expansion of the piston if you are going to run with steam, about 0.003" per inch dia.