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Topic: Hobie ST fin mast thread size  (Read 1948 times)

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Larry_MayII_HR

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  • Location: Corvallis, OR
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Finally, a nuts and bolts machining post related to kayaks!

A while back I found a pack of older ST turbo fins in a bargain bin at a local paddle shop and decided that I wanted to upgrade my boilerplate ST fins which came on my 2016 Hobie Outback. When I went to install them I found that the turbo fin masts were the newer design which appear to be held in by cotter pins and not by threads at is the design with my fins. See photos below.

Instead of pay $28 each for $10 total worth of metal, I figured I would make my own masts by threading the ends of an SS rod. When I took the fins apart today I found that the design isn't as simple as I thought...

Does anyone know the thread size and/or standard rod size used for the threaded fin mast design? The rod measures 0.285" in diameter and the threads appear to be 5/16-18 with the thread crowns squared off (I got this size nut to thread all the way in without binding). Is this an M7 rod with 5/16-18 UNC threads? Can anyone confirm?  See photos attached and thanks in advance!  --LM_II / Steve





kredden

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If you mean that the masts are held in by a grub screw then those are actually the old v1 style masts, not newer ones.

Kevin


Larry_MayII_HR

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I found out that the mast design for the part I need is V2 - here is the product from Next adventure. Just looking to see if anyone can confirm the thread size...

I found metric 7mm SS rod on McMaster for $20 total for both masts.

https://www.nextadventure.net/hobie-kayak-mast-v2-st-turbo-threaded.html




craig

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My guess is they went with the 5/16 inserts, but 8mm rod was too large so they downsized to 7mm.  I was going to do this once and tested with a 5/16 nut. I just never got around to doing it.
« Last Edit: September 14, 2021, 10:40:07 PM by craig »


Larry_MayII_HR

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Thanks Craig - its weird to mix metric and imperial, so I really just wanted to make sure I wasn't going to thread on male imperial threads into a female metric blind fitting.  I've done this before where the thread pitch and diameters are very similar and you end up stripping the bottom end threads out after a few revolutions, but the top threads feel fine when twisting the fastener on by hand.

Since the metric SS rod I'd need is about $30 shipped I decided that I'm just going to cut the bottom 1/2" off the turbo fin masts, thread that end, and just live with the fact that my fins will overhang the mast by 1/2" or so.  Not going to make a hill of beans of a difference unless I'm not careful with the drive and keep ramming the fins into the rocks.

Will post photos when done and let folks know how it turns out.

Cheers,

Steve


craig

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I wonder how well a 5/16" aluminum rod (7.9375mm) would work. It is cheap and available at Home Depot. I have one in my garage that I bought a while back for a project, but it was the wrong size. Maybe I will see if it works.

How long do they need to be? Here is 5/16" stainless at amazon for $9.69 .  https://www.amazon.com/dp/B08Z8K4XN4/ref=redir_mobile_desktop?_encoding=UTF8&aaxitk=30772a14c7ab2aa2d983165c0cc6ecd8&hsa_cr_id=1504377270501&pd_rd_plhdr=t&pd_rd_r=4264b324-6ffb-4811-aa6e-8c8267854a07&pd_rd_w=PB2Mk&pd_rd_wg=hTZR9&ref_=sbx_be_s_sparkle_mcd_asin_1_title


craig

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« Last Edit: September 16, 2021, 10:44:17 PM by craig »


INSAYN

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I wouldn't use aluminum for a fin mast. Bends way too easy and will eventually break if bent back.
 

"If I was ever stranded on a beach with only hand lotion...You're the guy I'd want with me!"   Polyangler, 2/27/15


INSAYN

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Thanks Craig - its weird to mix metric and imperial, so I really just wanted to make sure I wasn't going to thread on male imperial threads into a female metric blind fitting.  I've done this before where the thread pitch and diameters are very similar and you end up stripping the bottom end threads out after a few revolutions, but the top threads feel fine when twisting the fastener on by hand.

Since the metric SS rod I'd need is about $30 shipped I decided that I'm just going to cut the bottom 1/2" off the turbo fin masts, thread that end, and just live with the fact that my fins will overhang the mast by 1/2" or so.  Not going to make a hill of beans of a difference unless I'm not careful with the drive and keep ramming the fins into the rocks.

Will post photos when done and let folks know how it turns out.

Cheers,

Steve

Couldn't you just tap the other end of each mast, rather than cut off the V1 mount side?  This way you don't lose that 1/2" of the mast?
 

"If I was ever stranded on a beach with only hand lotion...You're the guy I'd want with me!"   Polyangler, 2/27/15


craig

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Quote

Couldn't you just tap the other end of each mast, rather than cut off the V1 mount side?  This way you don't lose that 1/2" of the mast?

This is why we keep you around here.  ;)

But there may be a flat ground on the other end. I  can't remember.


INSAYN

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  • Location: Forest Grove, OR
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There is a flat grind one end and a small hole at the other end. I would just tap the end with the small hole in it.
 

"If I was ever stranded on a beach with only hand lotion...You're the guy I'd want with me!"   Polyangler, 2/27/15


Larry_MayII_HR

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You guys beat me to the post!  I independently decided to do what you all suggested - cut off one end of the mast and thread it instead of buying new material.  I decided to put a small spacer in the bottom of the mast hole to make up for the portion I cut off.  That way if the mast started to back out of the threads and loosened all the way out my fin wouldn't fall off (see 1/4" dia. ~1/2" long piece in the last photo).

I forgot how much of a PITA it is to thread 304 SS.  Not a quick process.  It took about 1 hour for the first one, and 30 minutes for the second one.  I also had to 'turn down' the excess material extruded outward by the dye (threads extruded larger than the rod diameter since the rod is smaller than the standard 5/16" blank) by turning the mast in my drill and filing/sanding down.  But on paper I saved $50-$60 for my efforts (minus the cost of the 5/16-18 die, but that doesn't count since I always wanted a set of dies for my tool collection ;)) so feels worth it to me.

Thanks all for the input.  Problem solved.