Wrong threads

[Mike Farmer]

 

Happy Sunday Morning

The other day it came up about rethreading 26 CEI/26BSF. I have since spoken to a metallurgist (Clever guy) If you either re-tap or re-thread the difference being 5Degrees angle. You either: - Carve away approx 6% of the thread metal or bend the thread approx the same 5Degrees in either direction. Thus stressing the thread base. Expert opinion----don't do it, especially for anything important.

Mike

[AltcarBob]

Just use an air powered impact gun and 5° of angle makes no difference. 

[Mike Farmer]

 

Good point, well made and no other explanation required.

Mike

[chaz]

Mike,
as a machine shop inspector , machinist, gauge setter, standards room inspector , final view inspector, manual and cnc machinist, etc etc for over 40 years,

I have been saying this for years BUT those who know best will tell you otherwise, "it dont matter" therefore, Ive given up, so do what you want, after all, whats the worst that can happen?? something snap or fall off, what consequences?? life or death, loss of limb??

As a motorcycle restorer/repairer/service workshop as well, I have to use the correct parts OR if the customer has an accident who is liable?
I cant take chances on railway brakes with multiple occupants, so cant take the same chance with a solo rider.

as I said, I know best but others know better, so do what you think... I know what Im doing..

[Spaceman]

Happy Sunday Morning

The other day it came up about rethreading 26 CEI/26BSF. I have since spoken to a metallurgist (Clever guy) If you either re-tap or re-thread the difference being 5Degrees angle. You either: - Carve away approx 6% of the thread metal or bend the thread approx the same 5Degrees in either direction. Thus stressing the thread base. Expert opinion----don't do it, especially for anything important.

Mike

When talking about internal and external threads, you have to take into account tolerancing. If you made the nut and stud threads exactly the same dimensions, the nut would not be able to screw onto stud - there has to be a gap between the threads. When you also take into account the accuracy with which the threads can be cut, you then need to leave an even bigger gap to take this tolerancing into account.

I haven't looked at the tolerancing data for 1/4" BSC/BSF threads but it would not surprise me if the normal tolerancing applied to these threads would allow a BSF nut to screw easily on to BSC stud - in other words, the normal gap between the threads would tolerate one having a 55 thread angle and the other one a 60 degree thread angle.

[scousebantam]

I don't get this thread (pardon the pun), why can't you just use a 26 cycle thread tap or die?

[Spaceman]

I need to row back slightly on what I originally said about the interchangeability of 1/4" BSC and BSF threads. Having just cut a couple of new threads, the conclusion is that a 1/4" BSC nut will screw easily on either a BSF or BSC thread - no force is necessary. However, a 1/4" BSF nut will only screw easily on to a BSF thread - it will not screw easily on to a BSC thread because of the interference. Given the thread angle is 60 deg for BSC and 55 deg for BSF, this result is fairly easy to understand.

[chaz]

I don't get this thread (pardon the pun), why can't you just use a 26 cycle thread tap or die?

reason being, its easier to get a BSF tap/die than a BSC one.
I only use old stock tooling, I would possibly think that many modern tooling being offered are likely to be made to different tolerances as used in different countries. Using old copies of machinery handbooks there are pages of limits and fits, todays standards are likely to be smaller books.

[scousebantam]

OK, understand. In the 60's when I was rebuilding my Road Rocket I worked as a Tool Setter so when
I required a tap or a die for a certain application I purchased my own with my own money and kept them
when I left the company, and still have them all.
I haven't started my Bantam restoration yet so not aware of the thread problem, as I say just got a bike
the first since 1976
This is the reason I joined the Owners Club so that I can catch up with modern problems, and of course
how to overcome them with advice from the experts on this forum, I am not taking the p-ss I genuinely
mean it.   

[chaz]

I wasnt being funny, but our senior/top/head difficult to know what to call him nowadays, 3 years ago he was a goods inwards inspector at main building, now titles himself as metrology inspector (which we have no use for)at our building thinks he knows everything, has been an inspector in 3 previous jobs, he has never had the experience in our business or in using "classic" threads. As we only use metric and the odd  BSP thread, he knows little of what I know regarding UNC/F UNEF , BS threads, Zeiss, Panzerghwinde , Acme etc yet he has an opinion on all.
Metalurgists, tend to specialise in materials composition, neither has much experience in engineering from the past as their age has not given them experience. My experience comes from over 40 years handling the different threads and processes, through reclaiming to calibration.
to be offered M5 bolts for 2BA by reputable sellers does make you wonder.
the worrying part about these discussions is that everyone with an opinion quotes figures which is all well and good, but these figures are for new cut threads. What we are dealing with is 50plus year old threads that have been "used" (ie screwed in and out) many times over their past. These threads have worn, wear to the major diameter, minor diameter, effective diameter, flanks weather it be metal to metal (steel/stainless into softer aluminium), or dirt/abrasive detritus not to forget cross threading, incorrect threads, all of which knocks theoretical sizes out of the form book.

[scousebantam]

Chaz, I agree, I would recommend everyone to purchase a Zeus book, they are cheap enough on ebay.
I bought my first one in the 60's and I think I have used it more than every other book that I have.
Tells you almost everything about threads.

[Spaceman]

The Zeus Book looks to be very useful and, for less than £10, I think it is worth having and have now put in an order. That said, I already have all the thread data printed out on laminated A4 sheets hanging from a hook on my workshop wall as they are indispensable when restoring motorbike whether classic British, Japanese or more modern. I also have all the torque settings printed out in the same way for when working on my more modern bikes since trying to find them quickly in workshop manuals can be very frustrating.

[BILL NELSON]

...everyone with an opinion quotes figures which is all well and good, but these figures are for new cut threads. What we are dealing with is 50plus year old threads that have been "used" (ie screwed in and out) many times over their past. These threads have worn, wear to the major diameter, minor diameter, effective diameter, flanks weather it be metal to metal (steel/stainless into softer aluminium), or dirt/abrasive detritus not to forget cross threading, incorrect threads, all of which knocks theoretical sizes out of the form book.

Chaz talks a lot of sense here. There were a number of A65s reimported from South Africa some years ago with metric threads forced into the imperial (BS or Unified) holes. I saw one being restored, where some over-size retapping was required, some helicoils, some welding & re-drilling - all horrendously expensive for anyone without the facilities to DIY.
I've done a great deal of work with smallish Precision Engineering companies over the years & all have told me the same as our own Old-Hands, when cutting a thread a 100% engagement might seem desirable, but the extra stress on taps and dies will accelerate tool wear and increase incidence of tap breakages. New threads are like the clearance on timing side bushes (controversial? ;-} ) in that a bit of slack is good, but not likely to cause failure. The general rule I work to for my own stuff is 75% to 80% engagement, which maybe Chaz will confirm is a good compromise offering tiny loss of strength (If not I need some better guidance!)

[chaz]

as Bill says, 100% engagement would be wonderful, however it would be unobtainable. how many times would you put a bolt into the same hole, how many times has that bolt been in and out not necessarily in the same hole. I would look at the feel/touch guide as to whether acceptable or to be repaired. try the screw/ bolt into the hole and see how it feels, if it has too much side ways movement it probably wants repairing, possibly to avoid a repair, try a brand new screw.
One thing to consider is the material of the fastener. many people like to use stainless steel now instead of mild steel chrome plated. cheaper and easier to get. Stainless is a harder material than mild steel so less forgiving and will wear the hole not the thread. Chrome plated screws will after a while wear through the chrome, this will also cause a baggy thread.
with a worn fastener you are tempted to tighten up probably tighter than you should because it does not feel tight enough, trying to o btain a correct torque might result in stripping the thread.
the next thing to worry about is if you have to fit a helicoil insert, these are stainless steel, if you fit a stainless screw into a helicoil, there is no lubrication and more likely nearer the 100% engagement, there is no give and can in many cases result in the screw sticking in the insert, resulting in the helicoil moving and having to be replaced, the problem with that is that removal may result in the tops of the body thread being removed, this then means the helicoil will not sit correctly.