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#250460 - 04/27/09 2:27 am 650/750 triumph, what type of threads?  
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rusty Offline
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what are the threads for all the engine covers (primary,timming, gearbox) are the bolts to hold them on? from 1963 to 1979? are any of them 26 treads per inch?

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#250476 - 04/27/09 9:34 am Re: 650/750 triumph, what type of threads? [Re: rusty]  
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The cover screws from 1963 to 1967 are certainly 26tpi. 1967 was the start of the change to unified threads.

#250480 - 04/27/09 11:28 am Re: 650/750 triumph, what type of threads? [Re: dave jones]  
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then why do the 20 tpi fit both my 1966 motors ? why does triumph part numbers are the same for 63-early 68 . and late 68 to 72 650cc? except for the 2 longest bolts are 1/8" different length from 63- 68 and 68 to 72.

#250519 - 04/27/09 4:53 pm Re: 650/750 triumph, what type of threads? [Re: rusty]  
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Originally Posted By: rusty
then why do the 20 tpi fit both my 1966 motors ?


Because the fine thread stripped out with unerring frequency. the cure was to Heli-Coil with 1/4-20 UNC.


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#250523 - 04/27/09 5:33 pm Re: 650/750 triumph, what type of threads? [Re: RF Whatley]  
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Originally Posted By: RF Whatley
Originally Posted By: rusty
then why do the 20 tpi fit both my 1966 motors ?


Because the fine thread stripped out with unerring frequency. the cure was to Heli-Coil with 1/4-20 UNC.



mine have not been heli coiled.

the bolt kit i ordered are 20tpi and fit for my 1966. all the part numbers are the same for the years, if different years had different threads wouldnt there be more then one part number? this is what im trying to figure out for a friend.

#250525 - 04/27/09 5:44 pm Re: 650/750 triumph, what type of threads? [Re: rusty]  
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the thing is if you have never owned the engine from new you dont know whats been done to it. my motor has played me a right merry dance with regards to thread types but its all done now smile

#250540 - 04/27/09 6:28 pm Re: 650/750 triumph, what type of threads? [Re: mick2]  
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Dealer fitted crankcases stamped with orginal numbers but 68 cases maybe.

#250543 - 04/27/09 6:33 pm Re: 650/750 triumph, what type of threads? [Re: mick2]  
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While the threads used to bolt steel to steel (e.g. fender brackets) were fine threads as these resist vibration better the threads in aluminum castings are almost always a coarse thread to resist pulling out of the threads in the soft aluminum.
Older bikes have BSW (British Standard Whitworth) while newer machines had UNC (Unified Coarse) threads.
If you are pedantic you need the correct thread.
However it has always been the case that UNC bolts/screws are used in Whitworth threads as in the most common sizes the threads per inch are the same as per:
1/4" 20
5/16" 18
3/8" 16
There are differences in the detail thread form but in practice it makes as much difference as an ice cream in hell.
HTH.

#250571 - 04/27/09 10:30 pm Re: 650/750 triumph, what type of threads? [Re: Tridentman]  
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Hi,

Originally Posted By: Tridentman
Older bikes have BSW (British Standard Whitworth) while newer machines had UNC (Unified Coarse) threads.

'Fraid this isn't correct. While I used to say that Triumph (and BSA) never used Whitworth- (BSW-)threaded components, I recently found one (can't remember what it was). Nevertheless, certainly the BSA Group Automotive Division standard for in-house-made components was Cycle threads on/in iron and steel (including bolts and screws into nuts) and BSF on/in aluminium; in general, at any given diameter, BSF is a coarser thread form than Cycle but, at a 1/4" (most engine cover screws) and 3/16", the tpi are the same. Only when Unified forms were adopted were UNC-type tpi seen.

Originally Posted By: rusty
why does triumph part numbers are the same for 63-early 68 . and late 68 to 72 650cc?

"63-early 68" will be BSF, "late 68 to 72" will be UNC.

Originally Posted By: rusty
why do the 20 tpi fit both my 1966 motors ?

Parts books were intended by makers to help retailers' parts' personnel find parts for owners, not as a restoration bible nearly fifty years later. Moreover, in the 1960's (and earlier and later), parts books were produced by 'hot metal' printing, so the pages went off to the printers weeks before the start of a new 'season'. However, manufacturing decisions continued to be made and changed subsequently, sometimes rendering a parts book incorrect, so loose sheets of typed corrections would be sent to dealers either with a book or later; it's highly unlikely that such sheets have survived.

And/or your engines could've been made towards the end of the '66 season, when cases stockpiled for the start of '67-season production were used because 'correct' '66-season cases had run out.

Or, to agree with Mick, that your engines are sitting in front of you means they've survived the thick end of fifty years, including through the 1970's when there was no such thing as 'classic' bikes, just cheap old bikes, when they were 'maintained' by the ignorant and the impecunious. Do you know all the engines' previous owners? How many other near-fifty-year-old assemblies do you know of that still have all their original parts?

Hth.

Regards,

Last edited by Stuart; 04/27/09 10:31 pm.
#250574 - 04/27/09 11:21 pm Re: 650/750 triumph, what type of threads? [Re: Stuart]  
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Stuart, sorry old boy but I have to disagree with you on this one.
I have never seen the BSA Group Automotive Division Standard and frankly I doubt very much whether the guys at the sharp end at Meriden and Small Heath had either.
Over the last 45 years plus I have had over a dozen bikes from the BSA Group and the older ones all had Whitworth threads in the aluminum crankcases where the covers were attached.
I am thinking now of A7s, A10s, B31s, Speed Twins, Thunderbirds etc etc.
Frankly they were right---26 tpi is much too fine a thread for aluminum.
The guys then who used Whitworth for this application (and UNC later) had it totally right IMHO.

#250575 - 04/27/09 11:26 pm Re: 650/750 triumph, what type of threads? [Re: Tridentman]  
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And to add a quick footnote----the reason that more recent thread forms (UNC) are specified back to the older bikes is in my first posting (UNC = Whitworth tpis).

#250588 - 04/28/09 12:43 am Re: 650/750 triumph, what type of threads? [Re: Tridentman]  
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so what does UH mean when talking about threads/bolts?

#250597 - 04/28/09 1:26 am Re: 650/750 triumph, what type of threads? [Re: rusty]  
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#250632 - 04/28/09 8:14 am Re: 650/750 triumph, what type of threads? [Re: pittss1s]  
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Hey Rusty- I don't know what all this debate is about. As I said in my first answer the cover screws on Triumph twins up to 1967 should be CEI 26tpi. I have a 67 650 and the cover screws are this thread. If you can put 20tpi screws in yours then something has been changed, as noted by Richard Whatley. You can put 1/4 bsf 26tpi screws in but the thread pitch is slightly different.

BSA bikes did have coarse threads at this point.

Last edited by dave jones; 04/28/09 8:16 am.
#250713 - 04/28/09 6:50 pm Re: 650/750 triumph, what type of threads? [Re: dave jones]  
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so whats the difference between BSF and BSW threads?

#250719 - 04/28/09 7:36 pm Re: 650/750 triumph, what type of threads? [Re: rusty]  
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BSW is a coarse thread and BSF is fine. In the US, however, CEI or cycle thread (the fine thread in the early unit Triumph cases) is sometimes known as Whitworth fine.



Last edited by dave jones; 04/28/09 7:34 pm.
#250735 - 04/28/09 11:03 pm Re: 650/750 triumph, what type of threads? [Re: dave jones]  
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Hi,

Originally Posted By: Tridentman
I have to disagree with you on this one.
I am thinking now of A7s, A10s, B31s, Speed Twins, Thunderbirds etc etc.

You are correct here - pre-units did use BSW. However, ...

Originally Posted By: rusty
what are the threads for all the engine covers (primary,timming, gearbox) are the bolts to hold them on? from 1963 to 1979?

... are unit engines and, according to the parts books, '63 to '68, the threads referred to here were BSF, on Triumphs and BSA's.

Originally Posted By: dave jones
As I said in my first answer the cover screws on Triumph twins up to 1967 should be CEI 26tpi.

No, they shouldn't. They should be BSF. It doesn't matter if you put Cycle (CEI) screws in the 1/4" ones because the tpi (26) are the same and the actual threads are likely to be sufficiently worn that differences in things like the pitch angles between Cycle and BSF won't matter. Here, do not be misled by resources like stainlessbits.com/link12.html - this is the stainless equivalent of the original BSF screw, as no-one makes 1/4"BSF screws - 'cos they're the same tpi as Cycle.

However, if a 5/16"Cycle fastener fits in, for example, your 650 primary chaincase, your bike has been changed from the original, which was BSF (refer to the aforementioned resource if you don't believe me).

Originally Posted By: dave jones
BSA bikes did have coarse threads at this point.

No, they didn't. Again, if you don't believe me, get part numbers from a '63 to '68 parts book and cross-refer them.

Rusty: according to all the parts books, unit 1/4"-dia. engine case threads were BSF (26 tpi) 'til the end of the '68 season and UNC (20 tpi) from the '69 season. If your supposedly-'66 engines have UNC threads, something happened in their history, probably after they left Meriden.

Hth.

Regards,

#250803 - 04/29/09 11:47 am Re: 650/750 triumph, what type of threads? [Re: Stuart]  
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Stuart- Thanks for putting me right on those points. I shouldn't have said anything about the BSAs anyway because I haven't got one. I have always been a bit confused about the threads on my bike.As a '67 it has cycle, unf, bsw and now bsf too!

Rusty- Sorry if I confused you- at least the 1/4" threads were 26tpi if not Cycle!

#250863 - 04/29/09 5:09 pm Re: 650/750 triumph, what type of threads? [Re: dave jones]  
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Rusty---if 20 tpi fits your bike then IMHO they were originally BSW but will take UNC with no problems.
HTH.


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