Primus NRV M8 thread ?

Dear fettling team
I struggled with a Primus NRV removal like a lot of you, eventally de-soldered the pump and had to de solder the pump rear cap and cut the NRV into pieces to remove it. Before inserting a new valve, I checked and feel some resistance, and think I may have to clean up the pump tube rear cap. Does not look like a M8 although it is 8mm outside diameter ?

 

Hi @jean francois lutz Classic Primus stoves were first introduced at the end of the 19 Century before standard thread forms were developed,

Other Manufacturers tended to adopt the same thread form as the early makers to make spare parts compatible and to be able to compete in the valuable spare part market. Special thead forms also made copying difficult for small-scale operators.

Sometimes a modern thread form can be an “almost” fit; but that is chance

Have a look at this post on NRV Thread size:

From this long thread:

https://classiccampstoves.com/threads/warning-nrv.29083/

Best Regards
Kerophile.

 

Hello friends
I use this tool for screw`s lead

https://www.hoffmann-group.com/DE/d...annungsprüfer/Gewindeschablone/p/477000-M@2FG

Have a nice day

 

Dear friends,
Kerophile, to remove the pump, I used a trick which could be good for all, just filled the stove tank with a glass or two of water, before heating up the pump solder, this avoids any problem like de soldering the bottom or the legs. Thanks, Russenjesus, I will get the Gewindschablone looks really useful to measure a thread.
Looks like my new NRVs are fine thread M8, but I will check with the tool once I get it.

 

hello Jean,
next year on 13.-15.April will be a stove- and lampscollectors meeting in Urloffen nearly by straßbourg.
If you are interested let me know.

Markus(RJ)

 

Hi @jean francois lutz ,

Are you checking the thread with a view of purchasing a die to 'tidy up' the threads ? If you are, I'd be interested in how it turns out (whether or not, the tidied NRV actually means you can then use that NRV)
My good friend Kari (@Afterburner ) has spent some time on this issue as you may of read from the above links , but as of yet , I don't think anyone has tried it (been away from the forum so I may of missed something relevant) , I'd love to know how you get on ?

 

@jean francois lutz M8×0,75 thread tap/die is closest fit that you can buy nowadays from a tool shop for NRV. It is not fully exact fit but in most cases it is adequate/good enough.

@Longilily oh yes, various thread research tasks have took some time... You may remember our SRV thread research project (that still waits to be documented )

I looked possibility to 'tidy up' or 'fully cut' unfinished thread of NRV with a die. Problem is that NRV has only few rounds of threads. In die first 1-2 mm's are not making full thread since that part of the die has a bit taper form to get die to 'bite into' material. Therefore standard die will not cut/'tidy up' NRV threads almost at all.

For 'tidying up' NRV 1-2mm from surface of the die should be grinded off. Then the die would have full thread cutting section just on the beginning. Then it could work for NRV thread 'tidying up'.

 

@Afterburner

Apologies if this repeats earlier info and your efforts, and admit not having read the earlier thread. Do cleaning dies fit what you are after?

https://www.threadcheck.com/threading-dies/

Standard dies will have the lead in taper to bite, the cleaning ones should be good from the top.

Appreciate it's a US site but they should be readily available.

Happy to be corrected if I'm repeating old ground.

 

Interesting those are new for me!

Where to get those 'cleaning dies', can you post some link to them or pictures of them?

 

If we are talking about 'Die nuts' here, they are indeed intended for the repair/redress of threads, and are obtainable in the size required here. Regarding the 'lead in' though, they still have a similar lead to split dies, IIRC anyway (as Simes , I'm happy to be corrected here). The back of a split die has less of a lead in than the front (for closer threading such as this topic) but I can't seem to recall much of a difference in lead in, between a split die and a die nut.
I've actually used a die nut a quite a few times (no split die available) to cut a new thread.
Perhaps someone can compare if they can, I'm home now for the evening
Interesting link Simes, never seen a square die or a cone die before. But like they say, every days a school day


Cheers

 

Since standardized Whitworth threads were a product of the mid 19th century is it possible that the NRV is based around a 5/16" thread? 5/16" = 7.9475 mm or. 99.2% of 8 mm.
If 0.75 mm pitch is close, BSW and BSF are too coarse (18 & 22 TPI repectively) but 5/16" Model Engineer's Standard Threads (Whitworth Form) is 32 TPI or 0.794 mm pitch. 5/16" SAE fine is also 32TPI if t'interweb is telling me the truth.

Regards
John

 

I've probably got a 5/16 X 32 ME down the shop, I'll have a look tomorrow
Though it'll still have that lead in that afterburner was referring to (though as afterburner says, it can be ground down to eliminate the lead. If I have one I'll donate a ground down version to the cause

 

I have to admit I've not done any serious metal engineering work for quite a few years, but do remember there are the differing types. There is the equivalent Tap if you need to thread down to the bottom of a hole.

One of our pub quiz team members 'claims' to be a tool maker, we reckon he's a bodger, but I'll make some enquiries.

 

Quite right Simes, these are commonly referred to as 'Bottom Taps' in my neck of the woods, because they have very little 'Lead' thus able to tap further down the bottom of a blind hole.

 

For 'a tool maker' it's easy to make tools that you need. Unfortunately others don't have those nice machines to make special tools.

'Bottom tap' is easy to find but how about that 'to the end die'? (I have cut off the 'lead' from bottom taps also to get thread that goes right up to the bottom (f.ex. spirit can cap needs that 'bottomized' bottom tap.) I tried to search 'leadless die' but not so much success. Here is one photo of die that I am talking about (die on left):


That seems to miss the 'lead in' section. With grinding that kind of die is quite easy to make.

 

Told you it was a while ago, had forgotton the terms 'leadless' and 'bottom'.

Thanks for the prompt. I may bump into my mate a bit later today and may have a bit more of a steer then.

 

Too late to edit, but a quick search has again kicked shoddy memory into action, but are we after 'thread chasers' rather than taps and dies?

http://www.tracytools.com/thread-chasers/thread-chasers2

http://m.autoserviceprofessional.co...ing-Restoring-male-or-female-fastener-threads

 

Down the shop now and found my ME set




Found the 5/16" X 32 ME die, and as you can see it has quite a lead. I'll be happy to grind this down if someone is happy to try it out for me (I don't have the sub-standard NRV's). Of course I'll post it FOC



Cheers

 

@Simes Chaser die is one option but they still seem to have some 'lead in'.

@Longilily 32tpi makes 0.79375mm. Some how think that 34tpi (0.74705mm) would be more close fit to NRV thread.

 

Looks like we're back to your M8 X 0.75 ground down then ?