I'm delighted to see your progress - and happy that the delays in progress on my Chenery 9-cyl rotary engine are not so unusual!
Gypsy Engine
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cwelkie
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Excellent work on an interesting subject. I admire your tenacity; reminds me of a world champion aero-modeller I met many years ago. When asked how he managed to get all that detail into his model he replied, "I just keep working at each component until it's as good as I can make it.". Thanks for the updates.
Thanks. One subject that often arises with cylinder heads is their ability to hold pressure so I thought I’d leave a note based on my experience with the Gypsy. My last project (the Chenery V twin) was fraught with issues trying to get compression, and it turned out the main issue was the casting material. The valves in that engine are seated directly into the head, and the Gypsy is the same. The difference is that in this case the Gypsy heads are machined from 2024 billet rather than a casting, so I hoped that this would make life easier with respect to seating them. The valves are stainless.
Rather than attempt this on the engine where you have to worry about pistons and rings at the same time I decided to make up a test rig just for the cylinder and head.
This was the set up with just the cylinder to check for leaks with a blank in place of the head. There is an o ring in aluminum base to seal the bottom of the cylinder.
I experimented a little with a head gasket, trying a ‘paper’ gasket and a copper gasket. The copper being 0.01 thickness. Didn’t have much luck with the copper, even cutting it down until it was very narrow, annealing it etc. I don’t think I could get the head down tight enough to crush the copper into a seal. There are four 1/8 in studs holding it down. The paper gasket gave no problem and was re usable many times.
Having got the basic set up holding more than 60 lbs I assembled the first head, spark plugs etc. The first test was not promising and it found three basic issues.
1. The cylinder to head joint was not sealing with or without a gasket. This turned out to be related to the proximity of the spark plug holes to the cylinder sealing surface. When the plug threads were cut they raised the metal up enough to distort the mating surface to the cast iron cylinder. This was remedied by refinishing the surface with a plug that fitted the head recess with some 400 grit paper super glued to it and gently rotating till the raised portions were flat again.
You can see the surface of the head where l blued it and the area below the spark plug. Incidentally this also shows the technique to bed the valves i.e. cutting the head off a cross point button headed screw and super glueing it to the valve. I don’t recall who originally did this but it works very well. When the lapping is complete just ping the screw head off with a razor blade.
Rather than attempt this on the engine where you have to worry about pistons and rings at the same time I decided to make up a test rig just for the cylinder and head.
This was the set up with just the cylinder to check for leaks with a blank in place of the head. There is an o ring in aluminum base to seal the bottom of the cylinder.
I experimented a little with a head gasket, trying a ‘paper’ gasket and a copper gasket. The copper being 0.01 thickness. Didn’t have much luck with the copper, even cutting it down until it was very narrow, annealing it etc. I don’t think I could get the head down tight enough to crush the copper into a seal. There are four 1/8 in studs holding it down. The paper gasket gave no problem and was re usable many times.
Having got the basic set up holding more than 60 lbs I assembled the first head, spark plugs etc. The first test was not promising and it found three basic issues.
1. The cylinder to head joint was not sealing with or without a gasket. This turned out to be related to the proximity of the spark plug holes to the cylinder sealing surface. When the plug threads were cut they raised the metal up enough to distort the mating surface to the cast iron cylinder. This was remedied by refinishing the surface with a plug that fitted the head recess with some 400 grit paper super glued to it and gently rotating till the raised portions were flat again.
You can see the surface of the head where l blued it and the area below the spark plug. Incidentally this also shows the technique to bed the valves i.e. cutting the head off a cross point button headed screw and super glueing it to the valve. I don’t recall who originally did this but it works very well. When the lapping is complete just ping the screw head off with a razor blade.
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2. The copper washers that came with the spark plugs were a sealing nightmare. I just made new ones in the end. With these two issues taken care of the leakage rate dropped dramatically but still not holding pressure.
3. Air was whistling past the valve seats. Using the screw head technique (again sorry for not recalling where I saw this) I lapped the valves using 400 grit paste. This went quite quickly with the 2024 head material and no where near the pain of trying to lap into a porous casting!
I’ll note here for anyone new to this that I would not recommend trying to speed things along using an electric drill to rotate the valve, it’s likely to end in tears.
The valve seats are cut at 90 deg in the head and the valves were machined to 91 degrees so they touch at the outer edge of the seat. Mr. Rupnow recommended this in one of his posts. Trying to get the entire seat area to seal seems overly challenging
So in the end .. a head/cylinder holding over 60 psi. Calling that a win.
To any prospective Gypsy builders be careful with the clearance of the spark plug holes to the cylinder mating surface.
Regards, Alex.
3. Air was whistling past the valve seats. Using the screw head technique (again sorry for not recalling where I saw this) I lapped the valves using 400 grit paste. This went quite quickly with the 2024 head material and no where near the pain of trying to lap into a porous casting!
I’ll note here for anyone new to this that I would not recommend trying to speed things along using an electric drill to rotate the valve, it’s likely to end in tears.
The valve seats are cut at 90 deg in the head and the valves were machined to 91 degrees so they touch at the outer edge of the seat. Mr. Rupnow recommended this in one of his posts. Trying to get the entire seat area to seal seems overly challenging
So in the end .. a head/cylinder holding over 60 psi. Calling that a win.
To any prospective Gypsy builders be careful with the clearance of the spark plug holes to the cylinder mating surface.
Regards, Alex.
