Thread

[Gareth Brandt]

Hello
What thread for the flywheel centre bolt? Anybody know?


Gareth

[Spider]

Hello
What thread for the flywheel centre bolt? Anybody know?


Gareth

Yes, but it's a secret.






5/8" dia x 16 TPI, it's a special, not UNF or UNC.

[phil.1380]

Hello
What thread for the flywheel centre bolt? Anybody know?


Gareth

5/8" by 16 TPI whitworth.

http://www.minispares.com/product/Class ... 0to%20shop

Phil.

[mk1]

http://www.ebay.co.uk/itm/BMC-A-Series- ... 234be4f1d9

[Vegard]

Hello
What thread for the flywheel centre bolt? Anybody know?


Gareth

5/8" by 16 TPI whitworth.

http://www.minispares.com/product/Class ... 0to%20shop

Phil.

You're very brave, stating that this is a Whitworth thread. I don't think the jury has decided yet.

[Gareth Brandt]

Thanx!! Will get the UNS tap

[ianh1968]

Hello
What thread for the flywheel centre bolt? Anybody know?
Gareth

5/8" by 16 TPI whitworth.
http://www.minispares.com/product/Class ... 0to%20shop
Phil.

You're very brave, stating that this is a Whitworth thread. I don't think the jury has decided yet.

... If you don't believe that it's Whitworth, measure the head...

You will probably find that it's 1.300" and not 1.3125" or 1&5/16".
A Whitworth spanner will fit "properly" and the others sold as
flywheel bolt spanners will more than likely be slack...

(Sorry, Rich...)
http://www.tracytools.com/taps-and-dies ... ct_id=1108
CHECK THE PRICE BEFORE BUYING!

That said, before I found out the correct answer I bought a 5/8" x 16 UN tap
and it cost me OVER £100. This is the one I use, but it being "wrong" annoys me...

Mark's tap is listed as UNS/Whitworth - Go figure...
UN is 60 degree, Whitworth is 55 degree.
Maybe they've had them cut to 57.5 degrees?
... or maybe the listing is just (am I allowed to say) WRONG?

Conclusion: e & oe etc (I think!)
The official/correct thread is Whitworth, but a x16TPI Unified will probably do...

Ian

[phil.1380]

Hello
What thread for the flywheel centre bolt? Anybody know?


Gareth

5/8" by 16 TPI whitworth.

http://www.minispares.com/product/Class ... 0to%20shop

Phil.

You're very brave, stating that this is a Whitworth thread. I don't think the jury has decided yet.

Naaaw not brave, just quoting what the MiniSpares site says !!

As we all know there isn't a definitive understanding of what the thread is called, but someone must know how to make it as there are both taps for internal threads, and bolts which screw into it, so even if we don't know its technical name both parts of the thread can be made with the right tools.

Phil.

[Gareth Brandt]

So, which one is right

http://www.tracytools.com/index.php?rou ... ct_id=1108

or

http://www.tracytools.com/5-8-x-16-tpi

???????

[mk1]

This one;

http://www.tracytools.com/index.php?rou ... ct_id=1108

5/8th 16tpi Whitworth form.

[Gareth Brandt]

Thanx again I'll buy both..........

[dklawson]

As above: "5/8th 16tpi Whitworth form"
That comes from the factory workshop manual.

Years ago and on another forum this topic surfaced. Before the topic concluded I made CAD models of a 5/8-16 Whitworth form tapped hole (radiused thread crowns and roots, 55 thread profile) and a 5/8-16 UNS bolt (truncated crowns and roots, 60 degree threads).

If your purpose is to just clean up the crank hole, the UNS tap will work OK. It will shave about .0005" off the walls of the 55 degree threads (in specific spots) and the top of the UNS tap will cut tiny scratch marks into the crest of the Whitworth thread form. In short, the UNS tap will clean the crank threads with minimal damage. Buy the right tap if you can but don't freak out using the UNS tap if you have to.

[Spider]

Buy the right tap if you can but don't freak out using the UNS tap if you have to.

[Gareth Brandt]

Ok, tried both and my conclusion is the UNS fits better. Tried them on 4 cranks. The withworth one got tighter and tighter.........so I tried the UNS in the other end and it just cleaned up the threads an went all the way in.

[phil.1380]

Ok, tried both and my conclusion is the UNS fits better. Tried them on 4 cranks. The withworth one got tighter and tighter.........so I tried the UNS in the other end and it just cleaned up the threads an went all the way in.

Good to know !!

Phil.

[rich@minispares.com]

I always use a 5/16 uns tap to clean out damaged crank threads.

never had a problem with any of them.

I suspect the whit thread is the one to use if your cutting a brand new thread though

[ianh1968]

I always use a 5/16 uns tap to clean out damaged crank threads.

Rich, that wouldn't even touch the sides...

I think that the bottom line is that there is an "official" thread,
and the "actual" thread... 5/8" "Something Thread"

I am guessing that the new cranks may well be UN[S] threads
but us die-hard perfectionists will always try to use the "official"
original sized tap.

If the cap fits, wear it!

Ian

[rich@minispares.com]

GODDAMMIT

I meant 5/8 uns



ffs!

[Gareth Brandt]

Well I don't know......one of the cranks is a 62 one and if I'd used the "withworth" tap, I'd be cutting lots of material out of the threads. Can't see why BMC would go to a special W thread when a UNS is a standard?

[dutchacme]

Perhaps it is the plastic deformation of the threads playing up. In screw threads the first thread revolution takes almost 40% of the loading, the second close to 30% and the fifth thread takes only 5%. Subsequently threads do not contribute to the load carrying.

The actual force distribution over the threads is dependent on the stiffness of the metal combination, which is why UNF is mostly used for steel steel combinations and UNC for steel light-alloy combinations.

So if the first thread revolution has gone over the yield stress (creep) it will have deformed plastically. Subsequently with time the second thread revolution will deform plastically. Putting a withworth tap in these deformed threads will get noticed. Apparently an uns tap due to its dimensioning does not notice the thread deformation.