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Discussion of diving methods and equipment available prior to the development of BCDs beyond the horse collar. This forum is dedicated to the pre-1970 diving.
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ScubaLawyer
Master Diver
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First Name: Mark
Location: Laguna Beach, CA

Mistral - to grind or not to grind?

Wed Aug 26, 2020 8:40 pm

I'm overhauling a Mistral and it turns out the volcano orifice is pretty severely nicked. The HP seat looks brand new so I'm sure it's the orifice causing the leak (The guy who sold it to me off the back of the truck told me it worked fine. :) )

I used steel wool, pencil eraser, etc... but no joy. Is it worth trying to grind / resurface the volcano orifice with a dremel or will it widen the orifice too much so the HP seat will not seal properly?

Your collective sage advice is welcome.

[URL=https://imgbox.com/7bczn7ru]Image
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

swimjim
Master Diver
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First Name: Jim
Location: Belgium WI

Re: Mistral - to grind or not to grind?

Wed Aug 26, 2020 9:31 pm

Doesn't look good... This will be an interesting thread. I hope it turns out well Mark. This is the place where it has a chance!

Vancetp
Master Diver
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First Name: Phillip
Location: Belmont CA

Re: Mistral - to grind or not to grind?

Wed Aug 26, 2020 10:03 pm

I think a little tool with a guide that fits down in the orifice could cut a flat on that volcano and make it seal. Would have to make one...

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captain
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Re: Mistral - to grind or not to grind?

Thu Aug 27, 2020 11:15 am

A wood dowel the size of the bore with a piece of 400 grit wet or dry sandpaper glued on it. Use a light oil and work the dowel back and forth between your palms. Lift it every few cycles and check progress often. You don't want to remove any more the necessary.

Similar to lapping engine valves.

https://www.youtube.com/watch?v=pVp04atZoc0
Captain

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ScubaLawyer
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First Name: Mark
Location: Laguna Beach, CA

Re: Mistral - to grind or not to grind?

Thu Aug 27, 2020 12:48 pm

captain wrote:
Thu Aug 27, 2020 11:15 am
A wood dowel the size of the bore with a piece of 400 grit wet or dry sandpaper glued on it. Use a light oil and work the dowel back and forth between your palms. Lift it every few cycles and check progress often. You don't want to remove any more the necessary.

Similar to lapping engine valves.

https://www.youtube.com/watch?v=pVp04atZoc0
Thanks Tom,

Herman suggested the same thing to me and I did that. I got it down to just a slight barely perceptible leak and then tried taking off just a little more. Big mistake. It leaks worse now than ever. Argh?
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

h2operations
Vintage Diver
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Joined: Fri Jun 29, 2018 9:40 pm
First Name: Paul
Location: Coronado CA

Re: Mistral - to grind or not to grind?

Thu Aug 27, 2020 7:41 pm

Can you see if it is out of round so to speak after sanding? Since the tolerance is pretty slim you may have to use a guide around the dowel to keep the sanding in a tight circular pattern. Please post a post sanding photo.

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ScubaLawyer
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Location: Laguna Beach, CA

Re: Mistral - to grind or not to grind?

Fri Aug 28, 2020 12:13 am

UPDATE: After further sanding, polishing and certain incantations involving goat-leggings and scotch I achieved a great seal of the HP seat and volcano orifice. Thanks for everyone's suggestions.
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

Vancetp
Master Diver
Posts: 306
Joined: Tue Oct 04, 2016 7:26 pm
First Name: Phillip
Location: Belmont CA

Re: Mistral - to grind or not to grind?

Fri Aug 28, 2020 7:26 pm

Toothpaste makes a great , very fine lapping compound. I've used it for years to open up bores, etc.

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