Pop Off Valve Tech.......
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Pop Off Valve Tech.......

  1. #1
    Senior Member Boat 405's Avatar
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    Default Pop Off Valve Tech.......

    Here are a couple of issues that I have mentioned before.

    First off, the internal piston oring groove is not machined to the correct size, it is too deep and it is not tall enough thus not allowing the oring to seal to the walls of the valve correctly, so this is how Roy Miersch decided to "fix" the problem. Install a thin shim behind the oring........ Note, I paid him $90 to install this shim which still doesn't work right and doesn't fix the problem, which it has never worked in the first place.



    Now if any of you know how orings work they must allow movement of the oring to seal correctly, They must be able to move up and down within the groove and air must be allowed to get behind the oring too. Today I'm going to make a new internal piston with the correct oring groove size.

    The second problem is the lower guide oring on the piston is cutting itself on the valve housing. This what it looks like after 8 passes......



    I will post new pictures up today after I make a new internal piston with the correct dimension on how orings work and seal in a dynamic situation.

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    Senior Member Boat 405's Avatar
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    Here is how I started, MEASURE EVERYTHING TWICE!!!!

    Here is what found. The bore of the valve in which the internal piston is 4.746. This measurement will stay consistant throughout the project.

    The piston that came with my valve is 4.726. That means a .020 difference in diameter. Giving me a piston to wall clearance of .010, which is a lot.

    A piston to wall clearance needs to be between .003 and .006 for a dynamic oring situation. (Or a oring that seals when moving up and down in a cylinder) This clearance will be appropriate given temperature fluctuations.

    So not looking too good to start the pistion diameter is at best .010 too small.

    Next time to start cutting a new piston. I first cut out a round piece of aluminum 6061 grade to as round as I could cut, next I drilled a hole in the center to be able to bolt to a mandrel and chuck in the lathe.
    Last edited by Boat 405; 02-19-2009 at 09:19 PM.

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    Senior Member Boat 405's Avatar
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    Next item to address now that I have cut the piston to the correct diameter, I decided to use 4.735 which will leave me with a piston to wall clearance of .005. Which is within spec of necessary clearance.


    Now on the piston that Meirsch made the oring groove height is .210. The necessary height for an oring groove for this application is .283. That is a .070 difference.

    Here is picture of the two side by side.


    Now as you can see the new piston is made of a much thicker material. The old piston is made from .375 material and the new piston is made from .500 material. When putting a .283 tall groove in material that thin the upper and lower skirts of the piston become less than .050 in thickness which would be prone to failure.

    Here is the piston with the oring and the proper height groove


    As you can see the groove is much larger allowing the oring to move laterally as the piston goes up or down. The oring movement is essential in proper sealing.

    Finally the oring groove depth. The old piston oring depth was .188.The proper depth for this oring needs to be .178. But wait remember that the piston diameter was .010 too small to start so so really the oring goove was .015 too deep because the piston diameter was too small where we were measureing from. .015 because half the diameter .005 and .010 too deep groove.

    The groove on the new piston is .178 deep with the correct piston diameter. Orings seal because they are sandwiched between material. This is called the "squeeze" When looking at the squeeze of an oring it needs to be between 8 and 14 percent to work in this size application. We are now at 11 percent after final machining. The squeeze on the oring with the Meirsch piston was below 5 percent, which would explain why it didn't work.
    Last edited by Boat 405; 02-19-2009 at 09:25 PM.

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    Rollin With Fink fasterthanu's Avatar
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    Raw materials. Pretty silly to fix it with a shim when they made it to begin with. I would expect that with something from overseas, not here!! So much for american quality. Get your money back, unaceptable. It's a safety device WTF.

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    Senior Member Boat 405's Avatar
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    Quote Originally Posted by fasterthanu View Post
    Raw materials. Pretty silly to fix it with a shim when they made it to begin with. I would expect that with something from overseas, not here!! So much for american quality. Get your money back, unaceptable. It's a safety device WTF.
    Honestly, I think most of the machining was a wild ass guess, to be that far off, I don't know how it worked at all. It took me about 5 hours of prep work, calculations and machining. All with about $25 bucks in material. It helps to have an engineering background when figuring how this stuff works too.

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    Quote Originally Posted by widowmaker View Post
    Honestly, I think most of the machining was a wild ass guess, to be that far off, I don't know how it worked at all. It took me about 5 hours of prep work, calculations and machining. All with about $25 bucks in material. It helps to have an engineering background when figuring how this stuff works too.
    Did you adjust for the thicker material used to make the new piston?
    The stroke may need to be shortened so the piston doesn't slam into the cap at full stroke. You could have used a 1/8 o-ring instead of going to a thicker material.

    You can cure the cutting of the o-ring by opening the exhaust port in the valve so that the o-ring never has to reenter the bore.

    Send me a PM, Glad to help with any dimension you might need.
    The original o-ring grove was .250 tall,.180 deep on a 4.490 dia. with a 4.500 bore.
    Some still in use after more than 15 years.
    Last edited by John Mills; 02-17-2009 at 06:24 PM.

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    Senior Member Boat 405's Avatar
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    Quote Originally Posted by John Mills View Post
    Did you adjust for the thicker material used to make the new piston?
    The stroke may need to be shortened so the piston doesn't slam into the cap at full stroke. You could have used a 1/8 o-ring instead of going to a thicker material.

    You can cure the cutting of the o-ring by opening the exhaust port in the valve so that the o-ring never has to reenter the bore.

    Send me a PM, Glad to help with any dimension you might need.
    The original o-ring grove was .250 tall,.180 deep on a 4.490 dia. with a 4.500 bore.
    Some still in use after more than 15 years.
    Yes I did check before using larger material. Funny thing, there is about quarter inch or so gap when the piston is all the way at the top under pressure before the piston hits the cap. That could have been removed in my opinion for weight consideration. I have already pressure tested it and it works great! Another thing I did find that the 1/2 - 13 machine head screws used to hold the piston onto the rods were no where near long enough for correct thread engagement. (at least a bolt diameter) it was 3/8. This was with the old piston. I countersunk the holes .125 deeper into the new piston to counter the thicker piston. I'm going to order longer bolts to be on the safe side. Remember with 100psi in that valve. There is 1500+ pounds of force pushing up on that piston.

    If I had used an 1/8 oring which was possible, I would have to re calculate the oring depth, and height, and this is an option if one wanted use the same size 3/8 material for the piston.
    Last edited by Boat 405; 02-17-2009 at 10:23 PM.

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    Senior Member Brendellajet's Avatar
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    Wow, that shim is pretty chicken shiat. Nice work, thanks for posting.

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    Senior Member bp298's Avatar
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    Quote Originally Posted by widowmaker View Post
    Here is picture of the two side by side.

    kjell, is your oring groove off center? looks like the groove is higher on the left than on the right? is this just trick photography??? if it is off-center, will the oring care???

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    Quote Originally Posted by bp298 View Post
    kjell, is your oring groove off center? looks like the groove is higher on the left than on the right? is this just trick photography??? if it is off-center, will the oring care???
    nah that is just the photo. LOL looks weird though huh... If you look at the other piston it does the same thing in the picture, almost like I took the photo with a bad lens or something.

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    Quote Originally Posted by widowmaker View Post
    nah that is just the photo. LOL looks weird though huh... If you look at the other piston it does the same thing in the picture, almost like I took the photo with a bad lens or something.
    well, if it's any consolation, it looks faster...

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    Another update after further work on the valve today. All of the orings on this valve are wrong. I had to remeasure and go to my o-ring guide book and order all the correct size orings.

    Each of the orings that are in use with this valve are too small thus stretching them beyond the acceptable values, which lessens their sealing abilities.

    Just doing my homework. If anyone would like the correct orings for their valve, let me know and I'll bring them with me to the first race. The orings i ordered only come in packs of 10 or more so I will have a few extra. I'd be glad to help anyone out.

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    Quote Originally Posted by widowmaker View Post
    I'd be glad to help anyone out.
    cool, could you swing by here and help get my chip pan to look as good as yours??

    That's some good all around O ring tech. thanks for posting.

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    Quote Originally Posted by superdave013 View Post
    cool, could you swing by here and help get my chip pan to look as good as yours??

    That's some good all around O ring tech. thanks for posting.
    LOL, sure....

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