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Stirling hot air engine.

Heat proof joint sealing

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Mike Brett09/08/2019 12:08:46
118 forum posts
18 photos

I m building a Stirling engine and need to fix a brass end cap on a stainless steel pipe. It needs to be airtight and to withstand a certain amount of heat. All I can think of using at present is fire putty that sets hard when heated. Anybody have any other ideas.

Thanks Mike

Jeff Dayman09/08/2019 12:29:45
1818 forum posts
45 photos

Many Stirling engine hot cap designs I have seen use a stainless cap on a stainless tube. Usually they are specified to be welded on with TIG process. The cap could be threaded on and the threads sealed with high temp silicone sealant, but welding would be easier. You could also turn the hot cap from solid bar, or try and find a one piece drawn stainless cup of the right size online. Food containers and salt shakers for camping are often made from one piece drawn stainless cups.

Brass cap may not take the heat, and will have different thermal expansion coefficient than the stainless tube. This means that the parts will move apart very slightly every time they are thermally cycled. Fire putty will crack if there is any movement, so it may develop leaks quickly. In my opinion brass would not be a suitable material for a hot cap.

Edited By Jeff Dayman on 09/08/2019 12:30:12

Barrie Lever09/08/2019 13:37:31
574 forum posts
56 photos


My father and I made a Sterling engine in 1974 with a cap of brass attached into a stainless steel cylinder exactly as you propose, initially it was silver soldered in place but that failed on the test run, then it was brazed in place and it has remained in place ever since. We use a meths burner as the heat source.

The CTE of brass and stainless steel are fairly similar, certainly in the same ball park, however the expansion of a cylinder is a little more complex than just comparing CTE figures. I don't think expansion is a problem though.

What heat source are you using?





Mike Brett09/08/2019 14:47:29
118 forum posts
18 photos

I have not decided the heat method yet, anything that works I guess will be ok. Unfortunately I do not have the equipment for either tig welding or brazing, hence the idea to use some sort of compound. I intended to use a tube of fire cement that I use when I seal the Pyrex glass in the door of my log burner. It sticks to cast iron and the heat makes it as hard as concrete. There must be a lot of expansion in the glass also so I am hoping it will make a good seal.


BOB BLACKSHAW09/08/2019 14:56:56
299 forum posts
64 photos

I tried the compound method when doing my end cap but failed after a few goes.

Best to silver solder the brass to stainless, mine has held with no problems.

A small torch with Map gas, or propane is a must in any workshop.


Journeyman09/08/2019 15:34:48
802 forum posts
141 photos

James Rizzo in his book "Modelling Stirling and Hot Air Engines" describes an interesting method of making a stainless steel displacer cylinder. The cylinder is relieved at the outer end so that the diameter is slightly larger than the cylinder bore using a V shaped boring bar. A plate the diameter of the relieved section has its edge finished to a V section rather than just parallel. The plate is a tight fit in the recess and is pushed in and the edge of the cylinder (tube) is gently hammered inwards to make the plate captive. The cylinder is filled overnight with a salt solution, just ordinary table salt, and apparently the tight fit and the slight chemical action seals the displacer end-cap in place.

Have not tried this method so cannot report on it's effectiveness.


Mike Brett09/08/2019 16:12:34
118 forum posts
18 photos

I do have a 35 % propane 65 % butane torch , as its a small item perhaps this would get it hot enough to silver solder it.

Is there any particular silver solder I need to get to weld brass to stainless steel.


BOB BLACKSHAW09/08/2019 16:36:44
299 forum posts
64 photos

Others on this forum will put you right, but I used Easy - flow flux and silver solder wire from Mac Models,works a treat.


Barrie Lever09/08/2019 17:30:34
574 forum posts
56 photos

This one did not need an end cap seal !!

I will see if I can get a video of it running this weekend.



JasonB09/08/2019 17:43:40
18127 forum posts
1996 photos
1 articles

You need HT5 or Tenacity No5 flux for stainless as it contains chemicals to remove the chrome oxide that forms on stainless steel, 55% solder will do

Edited By JasonB on 09/08/2019 17:43:58

Mike Brett09/08/2019 20:11:08
118 forum posts
18 photos

Many thanks, much appreciated.


Ian S C10/08/2019 05:59:51
7468 forum posts
230 photos

I make my hot caps usually from stainless steel, on the odd occasion from mild steel. At first I made the hot cap by boring out a bit of stainless bar and the open end got a thread cut in it to mate with the rest of the cylinder as in a BETA type motor, ans sealed with Copper Cote. Now I make the hot cap from thin walled stainless tube with the end TIG welded on the end, when I got down to .007" thick, my friendly TIGer growled a bit. The displacer should be made the same way. The joint between the cap and the rest of the motor is via 4 to 8 studs depending on diametre, and just a metal to metal joint. DON'T use aluminium for the displacer, do use cast iron for the power piston, and either cast iron or steel for the cylinder.

Ian S C

Ian S C17/09/2019 13:23:13
7468 forum posts
230 photos

The steel case of a used NiCad battery. Make sure it is flat, zero volts before you attack it there is every thing from AAA to D size. Alkaline batteries also have steel cases, but the + tit is on the closed end of the case.

Ian S C


Ian S C30/01/2020 12:05:20
7468 forum posts
230 photos

It took me 3 goes to get it right, don't use aluminium displacers unless you want to see a melt down, also less efficient than stainless steel, or mild steel (it corrodes). the end of the stainless tube is atached by TIG welding, try and keep to about 0,010" or a little more, 0.007 is possible, but getting difficult.

I have used copper gaskets, these should be annealed, for sealing, but tend to use metal to metal joints see the hot cap photo among the groupe of batter cases, it is bolted to the motor with 4 1/8 x 40 Whitworth bolts. 

Ian S C

dsc01133 (800x600).jpg

Edited By Ian S C on 30/01/2020 12:15:04

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