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robs ss

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What is there thread type for the three studs into the underside of the head?
I know it is 3/8" 16TPI - but is it UNC or Whitworth? (EDIT: sorry - I was wrong here - 20TPI)
Cheers
 
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It should be 3/8" - 20 BSF.

"Stud, 3/8″ x 1-7/8″ x 20-26, Cylinder Head"
Thanks for that Les - My friend is about to purchase a threadsert kit for his failing studs. The LH one below the exhaust port will only just accept 20 f/lb.
Is there any uncertainty that the stud thread that goes into the head is 3/8" BSF?
It is a Fullauto head installed by someone unknown.
Cheers
 
 
Is there any uncertainty that the stud thread that goes into the head is 3/8" BSF?
It is a Fullauto head installed by someone unknown.

Is it 20 TPI?
According to Time Warp the Fullauto head thread is 20 TPI...:
The FA cylinder head has the same 20 TPI as stock, we will hope the new US based manufacturer of that head will change the TPI to 16/18 tpi and include three studs to suit.

Edit, CORRECTION:
...and, as far as I'm aware, the original head and stud upper threads are 55°, 3/8" - 20 BSF (the same part number studs used well before the Commando so unlikely to be 60°, 3/8" 20-UN). Edit: (Incorrect information deleted).
 
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Is it 20 TPI?
According to Time Warp the Fullauto head thread is 20 TPI...:


Edit, CORRECTION:
...and, as far as I'm aware, the original head and stud upper threads are 55°, 3/8" - 20 BSF (the same part number studs used well before the Commando so unlikely to be 60°, 3/8" 20-UN). Edit: (Incorrect information deleted).
Thanks Les - so we'll go with 3/8 BSF (not apart yet) and buy/make studs to fit if needed.
Lead time on Threadserts is such that we don't want his bike off the road for that long.
Cheers
 
Thanks Les - so we'll go with 3/8 BSF (not apart yet) and buy/make studs to fit if needed.
Lead time on Threadserts is such that we don't want his bike off the road for that long.
Cheers
Lead time getting British Helicoils was the issue a few years ago while putting my Vincent motor together. I threaded the case to take a 5/16" UNC Helicoil and made up a new custom stud with a 5/16" BSF outer thread. No worries at all.
 
Thanks Les - so we'll go with 3/8 BSF (not apart yet) and buy/make studs to fit if needed.
Lead time on Threadserts is such that we don't want his bike off the road for that long.
Cheers

As per the thread linked to, Jim Comstock did not recommend that. ( using a 20TPI Helicoil )
Post #12.

I had also noted the lack of thread engagement of the stock Commando stud, JC posted the stud had been longer on earlier models (Atlas perhaps)

I had posted long ago about my concerns of the 3/8-20 threads in the FA head and was told here by the maker there was no problem with those threads and was a trouble maker.
That same person some time later (iirc) posted having the threads inserted.

What I found was new 3/8-20 studs were a perfect fit in my Jim Comstock Al/Bronze inserts 750 head but those same studs in the FA head were a wobbly fit and was in no doubt they would pull out.
What I did in the end was make new studs from 750 head bolts, each 20 TPI thread had to be cut to suit each thread in the FA head and were full engagement in the head.
Each stud was longer externally also.
The difference compared to a stock stud in the picture.

Will it last, I hope so, if not I will use a 3/8 UNC helicoil.
It was also noted the FA head has less cooling area (fins) than a stock head.
Lets hope the current manufacturer did not do that also.


thr1 - Copy.jpg
std.jpg

std8 - Copy.jpg
nt.jpg

4a.jpg
 
It might be worth noting, if a 3/8-20 coil insert fails (if a coil is the intention) then what, a Bigsert as the only repair ?

It might be also noted I did not go to the trouble of making tools adapters to torque the cylinder head to fill the day in.
Cranking on those fasteners on a FA with abandon will either pull the threads over time or the nuts/sleeve nuts will have loosened.

Thread type question
Thread type question
 
Uni-Thread
Universal threading system
Thread Repair Kit
3/8X20 BSF are available from Waldridge Motors
I have installed them.
You can also go cNw: 3/8X16 if you want to use their studs / and nut system. I have installed this system. Good results.

"I'm not knocking or promoting products here", either system works, but options are available.
Both of my 850 heads needed the inserts due to stripped holes. I am doing my second one now. One failed but all three get the treatment.
IMHO, I would do it automatically if the head was "off the bike". Do it! But don't drill too deep.
These threaded hole are going to fail eventually. Bad design IMHO but that's what we got for an almost 50 year old machine. Nothing stops us from making things better. Right?

Comstock did a torque test to failure, on heli coils and other inserts.... you should check that out.
Why cry in the garage, after your third heat cycle torque up and it strips before torque up to spec. Or the front nut #7 or #8 will not get tight. Wonder why you got oil stains between the head and the cylinder and that damn nut will not tighten up but keeps spinning. Been there!
Cheers,
Tom
 
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If the three holes were drilled all the way with a 3/8" drill you could use bolts and nuts.
You won't be pulling those out.
 
If the three holes were drilled all the way with a 3/8" drill you could use bolts and nuts.
You won't be pulling those out.
Two of them would go all the way into the exhaust ports - not really an optimal solution.
 
It might be worth noting, if a 3/8-20 coil insert fails (if a coil is the intention) then what, a Bigsert as the only repair ?

It might be also noted I did not go to the trouble of making tools adapters to torque the cylinder head to fill the day in.
Cranking on those fasteners on a FA with abandon will either pull the threads over time or the nuts/sleeve nuts will have loosened.

View attachment 96711 View attachment 96712
Very nice!
Mine not quite so - but equally effective. If others are going this route, just remember to use them at right angles to your torque wrench or you will have to recalculate what the reduction in torque should be due to the extended length.;)
Cheers
Thread type question
 
When you do this thread repair of this 3 critical studs i can warmly recommend Keenserts thread inserts with key locks for this job.
Had tried several types of inserts......best in my technical understanding and own experience are this Keenserts.
I only could buy M10 standart metric thread Keenserts here in Europe. Outside thread is M14 s.m.t. ...this is about 17tpi. sounds better for me for alloy cast than 20tpi. I ordered at my local machine shop 3 studs with M10 thread on one side that goes into the insert and 3/8" BSC for the nut side. Works great......good feel when you torque head down. Bulletproof.
BTW saw last december on a facebook post that John Snead, manufacturer of the Fullauto STS, also uses these Keenserts......
 
It might be worth noting, if a 3/8-20 coil insert fails (if a coil is the intention) then what, a Bigsert as the only repair ?

It might be also noted I did not go to the trouble of making tools adapters to torque the cylinder head to fill the day in.
Cranking on those fasteners on a FA with abandon will either pull the threads over time or the nuts/sleeve nuts will have loosened.

View attachment 96711 View attachment 96712
Here's a commercially available tool that does the same thing

 
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