Fettling a Primus #4 Inner Cap

Hola stovie brethren!
I have a cute little Primus #4 that I acquired from a nice seller who resides in the UK (affectionately known as the Eden of Stovedom!).
Oh. So sad. It needed an inner cap which are as rare as the proverbial "hen's teeth". On a tip from the legendary guru of all things stovie, kaw550red, I thought I'd see what was available at my local plumbing supply shop. I came home with various candidates but settled on the nominal 1" copper plumbing pipe cap.




I suppose that the following series of pics ought to explain the procedure. Mark it carefully. Drill a pilot hole. Carefully use a spade bit (a 5/8" twist drill would be much safer) to drill the center hole. Make it shorter. File and sand with some abrasive paper. Voila!




Fire it up and.... "Wow! It Works!" That's what you would have heard me exclaim had you been with me in the shop that day. In fact, you would have been able to hear me go on to say, "Wow! And it works pretty good, too! Wow!" (I guess I say "Wow" a lot) It even passed kerophile's "Tea Test". Wow.



After burning the inner cap is covered with a fine film of oxidized copper "ash". This wipes off easily with a rag.



I've since done a heat-and-quench clean on this burner and it performs much better than in the "high power" photo shown here. The simmer is quite nice and does not tend to flash into underburn at all. The Primus #4 with fettled inner has become one of my favorite silents. Very powerful for it's size. And, cuter than a bugs ear!




This picture shows from left: #5 outer cap, #4 outer cap in brass, fettled #4 inner cap, and #5 inner cap.



-----Sort of an Update-----
Later in time, I noticed that there were some dimensional similarities between the #4 silent outer cap and a #96 burner basket, So another little project was initiated - the Primus #96 Silent Burner Conversion Project™.














This picture shows the copper plumbing pipe caps and their respective stove part analogs.





From the left: a 1" copper cap as a #4 inner, a 1 1/4" copper cap as a #5 inner and a 1 1/2" copper cap as a #5 outer. The sizes are not dead-on, but they're close enough to work.

That's it. I'm really glad all you posters down below here liked this so much. And, I'm flattered to be in the Master Class. Thanks!

Cheers,
Gary

 

Well done you dont get a green flame from the copper.Man i just got to make room up in the brain for that fettle

 

Brilliant

 

That fettle is really, really good!

Well done to you and Bryan.

 

Excellent work

 

Nice to see it working again.
Put it in me bag of tricks to try and remember.
Thanks
Barra

 

Those innercaps are sure difficult to find sometimes, and I am pleased that you showed us how you managed to make them yourself. Maybe one or two stoves more can be saved for the future in this way.

Thank You!

Anders

 

I think this would qualify for the Master Fettler section!

 

Great job!
I am wondering which I should do: buy the drill press, or buy more stoves with the money?

 

Agreed & actioned

 

That is brilliant, any suggestions for outer caps ?

I've got three outer caps for a No 5, and they all burn differently, can't see any difference between the three, but only one of them burns with a nice blue flame. The other two start off well, but soon end up with a yellow flame.

 

Hi Gary, Great Work!
Here is a post of a Primus No.4 operating, so that you have something to compare flame patterns with:
https://classiccampstoves.com/posts/49932

In the past I also posted some dimensions of inner and outer caps of three silent burner types. It was in this tread:
https://classiccampstoves.com/posts/52571

I believe that your inner burner cap is of a smaller diameter than the Primus original. It is good that it works, as we all know silent burners can be very fussy about which combination of inner and outer caps they will operate properly with.

Best Regards,
George.

 

Hi, I got the job of doing the "Road Test" on Gary's Custom-made Primus No.4 inner caps. I received a shipment this morning, so here is my report:

Firstly I compared them with the original Primus parts:

Here is a photo of original Primus Inner and Outer Burner Caps for a No.4 stove.



This is a close-up of the inside of an original Pr.4 Inner cap:



Next we compare the Gary-made inner cap (on the left in each photo), with the Primus original:






Below you will see the inside of the burner on a Pr.4, showing where the inner cap has to locate:



This photo shows an original Primus inner cap in place:




Below you will see the corresponding shot with the Gary Custom-made inner located on the burner:




Next we see a Primus No.4 stove assembled, using one of Gary's inner caps:



Now some photos of the stove operating:





Here is the stove at maximum power and you can see the flamelets almost pushed off the burner:



This shows the stove at low power. It is very controllable, and Gary's inner cap is obviously doing its job.




Time to celebrate by preparing a cup of Turkish coffee!







Finally here are some spare inner and outer cap sets for my Primus No.4 stoves.




So that is the Test report; as you can see Gary's inner caps are excellent and perform as well as, or even better than the originals. It is even more remarkable when you consider that Gary did not have access to drawings, or an original Primus No.4 burner inner cap when he started his experiments.

Well Done.

Best Regards, and Thanks,
Kerophile.

 

and there's that ibrik again. does he make both inner and outer? and does he make them for other stoves as well? well other sizes I guess?

 

Hi Hobo, Inners only. You need expensive, specialised machining equipment for making outers...its all those holes!
Anyway you can buy Pr.No.4 and No.5 outers very cheaply from Base Camp.

Gary hasn't started making Ibriks yet.

Best Regards,
kerophile

 

Hi Gary, I am really pleased that we now have a source of No.4 silent burner inner caps.

I should have said that the stove I used for the Trials is a 1935 (code Z13) model fitted with a "new old stock" burner dated 1938(code AC) which I acquired at Newark in 2007. My burner was unused when I got it so totally free of carbon build-up.
I find it useful to see photos, on CCS, of stoves operating well as it gives you a reference as to what your own stove burners should be capable of.

Keep up the good work.

Best Regards,
Kerophile.

 

Hey George,

Have you ever run your stove without any inner cap? In the past I fettled some stoves and run them but a bit later I noticed I had forgotten to put inner cap. But all the stoves run perfectly without any difference.

I remember this was negotiated highly before but it seems to appropriate time to re-think about the matter as here the subject is home made inner caps.

 

Hi Sefa, In the past I did the experiment of running without inner caps on some No.5-type stoves. I could get them to run but the flame pattern was very uneven.
I seem to remember that the flamelets were larger on the upper rows and very much smaller at the bottom of the burner cap.
On one stove I also had the problem of under-burn when I tried to run at low power.

I reckon the inner caps must be there for a reason. One of these is probably to equalise the pressure of the fuel mixture across all the burner holes.

We sometimes here reports, usually on Ebay, of people who claim to be able to run silent burner stoves withut any caps at all. Now that is clever!

Best Regards,
Kerophile.

 

Gary, great work on the inner caps - just the sort of out of the box thinking we need to keep this stuff chugging along.

Kerophile, great report with your usual excellent attention to detail.

Thanks go to both of you.

It's interesting that we often talk about silent burners being temperamental and that a mixing and matching of inner and outer caps is often necessary to come up with an optimum burn. However in this case Gary's cap is patently different from the original and yet it didn't make a bit of difference and performed excellently. Kerophile also brought up a supposition in his later post about maintaining even pressure from top to bottom of the inner to outer cap gap. This does sound extremely logical.

Soooo, I'm wondering whether when we mix and match caps we are merely treating symptoms rather than actually solving an underlying problem. Are we possibly tuning for either too much or too little pressure beling delivered?

Or is there some other explanation? From Kerophile's pics it seems in Gary's set-up there will be a larger than design spec gap between inner and outer. Is there an optimum minimum gap required between inner and outer and once you have that, within reason is doesn't matter how big the gap is? If you have a smaller than minimum gap is this the case that doesn't work?

Or is it just a fitting issue that some caps fit better than others and that some allow extra air into the pre-mix or allow a back burn that causes the underburn.

Anyone else with ideas? If us lot can't collectively crack it then it'll just be consigned to the "it's just one of those things" bucket and I hate that!

Time for a more scientific play I think - won't be for few weeks though unfortunately.

Complete this well known Kerophile phrase:

"Aren't stoves........!"

 

I have endless trouble with outer caps, in fact, there is only one out of four caps that work well.

I can mix and match the inners and the stoves, (all No 5's), all I like , but only one outer gives a continuously good burn.

The bad caps all start off well, but once everything gets hot they give off yellow flames. More pumping helps, but not much.

The good cap will perform well even if I raise it up with a screwdriver, so I don't think it's a case of bad fitting.

Al of the caps seem to be identical in size, and the holes are all the same as far as I can tell using wire as a guage.

It's one of those problems I keep coming back to, but never seem to find a solution.

Any ideas ?