Autohelm / Tillerpilot extension thread specification - the definitive answer

winsbury

winsbury

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So, this has been asked a gazillion times but I have never seen a definitive answer.

This didn't worry me until this weekend when in a moment of anal tidying up I tried to unscrew the old bunch of short extensions to replace them with a proper length one on our ST2000 only to find the last tiny one was jammed in tight. A lot of faffing and a pair of stilsons later revealed the reason for the resistance was that someone ( not me, honestly guv) had used high strength loctite to 'weld' the extension in place because the thread in the *steel* pushrod actuator of the actual pilot somehow got knackered at some point !

So, after a cuppa, a rich tea and a moment of calm to figure out what to do next it is clearly a case of having to rethread the actuator rod or throw the whole thing away and spend £600 on a new pilot. Opting for the lesser of two weevils (I love that film) I reckon someone in this hallowed bastion of boaty knowledge must know the exact thread....but no guesses now, because I have to buy and expensive tap to rethread that damned hole in 316 steel and your name will forever be Bilge Water if it's the wrong advice.

Answers below please, and if it works I PROMISE to let you all know ( so many say it but don't do it ) so many similar threads about this have been left hanging like tassels on a flapper dress, and that's just not cool folks. This is your chance to be a cool engineer ....
 
I will do better than that. I have a Simrad ap, and a 100mm extension for an Autohelm (which I really must put on eBay) and a complete set of thread gauges so I can definitely give you a definitive answer this evening. And define the difference between the two brands.

You do know that a bit of heat can make Loctite Loclooser?
 
Oops accidentally deleted my post, here it is again:

I await your answer with bated breath and credit card at the ready to buy the appropriate tooling.

Yes, I tried some heat, but only gently; there is a lot of plastic in a tiller pilot to drip on the floor if melted. An engineers vice and 12" stilsons proved the only way, brute force and the time tested engineers prayer "please don't break" did the rest.
 
OK, this is where I squirm a bit. You chose a bad day to ask, I've just come from an eye clinic having a mote removed (two motes actually, but the good book doesn't say whether that's good or bad), so I can't see properly this evening, hopefully better tomorrow. And it turns out that I haven't got an absolutely complete set of gauges. The tpi go from 18 to 20...

What I can tell you is that I've measured and measured, and both Autohelm and Simrad are 19tpi.

The Autohelm male has an OD of 0.67"/17.2mm, and a female ID of 0.61"/15.6mm.

The Simrad male has an OD of 0.65"/16.5mm and and a female ID of 0.57"/15.1mm.

(The male Simrad will easily screw into the female Autohelm, and with a bit of epoxy would probably get you home perfectly. The Male Autohelm will not fit the female Simrad).

Using the online tables that I've briefly looked at, the obvious candidate is a 3/8 BSPP = but that's the suggestion for both?! Looking at screens isn't very comfortable either, so I've stopped looking.

I need to come back to it with better eyesight tomorrow and some bits of feeler gauge to help measure the minor on the male thread (my house are calipers too thick) and a magnifying glass. The A/helm threadform appears to have a wide flat valley, the Simrad more rounded. I should also be able to measure the thread angles.

Anyone else knownof any other 19tpi threads? They're categorically not metric.
 
anoccasionalyachtsman said:
OK, this is where I squirm a bit. You chose a bad day to ask, I've just come from an eye clinic having a mote removed (two motes actually, but the good book doesn't say whether that's good or bad), so I can't see properly this evening, hopefully better tomorrow. And it turns out that I haven't got an absolutely complete set of gauges. The tpi go from 18 to 20...

What I can tell you is that I've measured and measured, and both Autohelm and Simrad are 19tpi.

The Autohelm male has an OD of 0.67"/17.2mm, and a female ID of 0.61"/15.6mm.

The Simrad male has an OD of 0.65"/16.5mm and and a female ID of 0.57"/15.1mm.

(The male Simrad will easily screw into the female Autohelm, and with a bit of epoxy would probably get you home perfectly. The Male Autohelm will not fit the female Simrad).

Using the online tables that I've briefly looked at, the obvious candidate is a 3/8 BSPP = but that's the suggestion for both?! Looking at screens isn't very comfortable either, so I've stopped looking.

I need to come back to it with better eyesight tomorrow and some bits of feeler gauge to help measure the minor on the male thread (my house are calipers too thick) and a magnifying glass. The A/helm threadform appears to have a wide flat valley, the Simrad more rounded. I should also be able to measure the thread angles.

Anyone else knownof any other 19tpi threads? They're categorically not metric.
Click to expand...

I have an Autohelm here. I can confirm 19tpi as i have a couple of thread gauges that include 19tpi

the OD is 43/64", ie 1/64" under the 11/16" in the table for 3/8" BSPP, or 17.1mm ( measured with a vernier caliper0
 
It is interesting that you found a difference between Autohelm and Simrad and possibly Raymarine too, I had expected them all to be the same.

In hindsight what I probably should have done is measure the brand new thread on the new extension, but what I did was to measure the knackered thread so will have another go this weekend. I don't have thread gauges and my micrometer is far too thick to get down into the grooves but certainly all the extensions I have all mate with one another in any sequence so at least they all appear to be the same or compatible threads. I have an Autohelm 1000 as well that I can measure and all the extensions definitely fit that so I am beginning to wonder if the Raymarine ST2000 unit is slightly different again.

More anon
 
Raymarine and Autohelm should be the same the first having taken over the latter. I'm fairly sure that Simrad is the young pretender, and why they didn't just copy the Autohelm thread I can't comprehend. Either way, until I get home I can only offer that I have some 3/8 BSP stuff in the workshop and will take it home to compare this evening - as well as measuring more of the thread specs.
 
Adam ? ... really you should know me better than that !

I do like the blocky-functional look of the Pelagic stuff , certainly better made than the Raymarine ST4000 ram, but would require a course computer, controller and a heck of a lot more money and I think still has to use the same extensions so the question remains about the threads.

I found a broken Autohelm 800 in my box of bits at home, and having had a measure am pretty close to convinced it is indeed 3/8 BSP, so much so that I just ordered a tap and die so will know for sure in a day or two.
 
anoccasionalyachtsman said:
The Autohelm male has an OD of 0.67"/17.2mm, and a female ID of 0.61"/15.6mm.

The Simrad male has an OD of 0.65"/16.5mm and and a female ID of 0.57"/15.1mm.

(The male Simrad will easily screw into the female Autohelm, and with a bit of epoxy would probably get you home perfectly. The Male Autohelm will not fit the female Simrad).
Click to expand...

So, I was excited this morning when the postie delivered 3/8 bsp tap and die from Tracy Tools. The tap screws easily into the autohelm 800 pushrod, but it is loose on the threads with some lateral play too. Next, trying the die on the thread of the extension, it simply does not fit and is attempting to cut a new thread. This result implies the 19 TPI is indeed correct ( there no binding up at all ) but the profile of the threads is slightly different.

Between the measurements and this experiment it seems that the Simrad gear probably is indeed 3/8 BSP but the Autohelm/Raymarine is something slightly different.

I have hunted through every thread table I can find and apart from BSP there are none others specified at 19 TPI so am flummoxed.

Could it really be that Autohelm were really so arrogant that they developed their own thread and Raymarine kept it alive ?
 
winsbury said:
So, I was excited this morning when the postie delivered 3/8 bsp tap and die from Tracy Tools. The tap screws easily into the autohelm 800 pushrod, but it is loose on the threads with some lateral play too. Next, trying the die on the thread of the extension, it simply does not fit and is attempting to cut a new thread. This result implies the 19 TPI is indeed correct ( there no binding up at all ) but the profile of the threads is slightly different.

Between the measurements and this experiment it seems that the Simrad gear probably is indeed 3/8 BSP but the Autohelm/Raymarine is something slightly different.

I have hunted through every thread table I can find and apart from BSP there are none others specified at 19 TPI so am flummoxed.

Could it really be that Autohelm were really so arrogant that they developed their own thread and Raymarine kept it alive ?
Click to expand...

Yup, I was fiddling with BSP fittings and have come to the same conclusion, and I'm not yet convinced that Simrad is BSP either, even if it is closer. I'll drop into a couple of the engineering shops that I use and see if anyone there recognizes them - the A/helm thread reminds me of something that I've used recently, but can't for the life of me remember what it was...
 
Would be surprised if it’s not a metric pitch, may be worth taking it to hydraulic hose fitting trade counter and asking them to try with their fittings, they would have more options
 
According to my old Kemps Yearbook (1996), 3/8 BSP has 19 tpi.

I've also checked my metric Zeus Data chart and this also confirms 3/8 BSP as 19tpi. I'm a bit sanguine about the tapping drill reccomendations in the Zeus chart so won't add them here.
 
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Topcat47 said:
According to my old Kemps Yearbook (1996), 3/8 BSP has 19 tpi.

I've also checked my metric Zeus Data chart and this also confirms 3/8 BSP as 19tpi. I'm a bit sanguine about the tapping drill reccomendations in the Zeus chart so won't add them here.
Click to expand...

It's definitely not 3/8 BSP but it is very close.

From my own measurements (which are unlikely to be perfect as I am measuring worn threads) comes out at:

Tpi 19
Major thread dia 0.670"
Minor thread dia 0.630"
Flank angle 55°
 
What’s it measure metric, as said unlikely to be imperial, could be M10 metric fine
 
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winsbury said:
It's definitely not 3/8 BSP but it is very close.

From my own measurements (which are unlikely to be perfect as I am measuring worn threads) comes out at:

Tpi 19
Major thread dia 0.670"
Minor thread dia 0.630"
Flank angle 55°
Click to expand...

With a micrometer rather than the vernier I used previously I make it 0.675" measuring an undamaged and unworn thread.

Daverw said:
What’s it measure metric, as said unlikely to be imperial, could be M10 metric fine
Click to expand...

No way . It is much bigger than M10 About 17mm and a pitch of 19 tpi does not convert to any likely metric thread pitch
 
Raymarine has US provenance, so imperial can't be ruled out. There exists a UN-20 constant pitch thread standard, which includes 11/16" dia. This would be very close, but measurements here seem sure that it's 19 tpi.
 
FWIW I was able to clean up the thread in the pushrod using the tap by pressing it hard laterally into the thread, the result is very good and smooth so it absolutely definitely is 19tpi but the 3/8 BSP tap is definitely under sized by a fraction. There is no way to use the die to repeat the process to clean up the threads on the extensions.

I think VicS measurement sounds about right. Perhaps this is simply a non standard thread that some machinist knocked up one day and it has stayed the same ever since ?
 
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