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Injectors of Peter Cauley

Understanding the drawings

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Speedy Builder512/09/2021 19:18:46
2383 forum posts
181 photos

Has anyone got an accurate drawing of the main body and cones of his horizontal injector for 5" Gauge loco.

The script that I am using indicates that the1/4" water inlet and overflow will just burst the sides of the main block however the drawing (NTS) shows a clearance of about 0.040".

On page 3 it states that the second cone (0.213" long) "....drill through number 63. An accurate hole is required-use a number 62 drill as a pilot." In my book, a 62 drill is larger than the 63. In another script, another author says "drill through number 56"

I am just a little bit confused!!


Simon Collier12/09/2021 21:59:40
415 forum posts
60 photos

If you are building injectors for the first time, get Derek Brown's book. It can't be recommended highly enough.

Paul Lousick12/09/2021 23:58:42
1838 forum posts
659 photos

Recommend Derek Brown's book yes


Stewart Hart13/09/2021 08:39:40
666 forum posts
354 photos

Hi Bob I'm a good friend of Peters and he's seen this post and he's asked me to reply I've successfully made Peters Injectors so I'm a little confused what what drawing your using if you check your personal messages I've sent you a contact email address so that I can send you a copy of Peters drawings

Regarding Dags book you will find the bodies of Peters Injectors far simpler and easier to make and regarding the cone sizes and proportions Peters are very similar to Dags, but with all due respect to Dag some of his methods are overcomplicated Peters method of sizing the cones is simpler.


Stewart Hart13/09/2021 08:44:16
666 forum posts
354 photos
This is a vertical injector I made with Peters help using his cone sizes
Weary13/09/2021 11:23:57
343 forum posts

How and where can I get hold of Peter Cauley's injector drawings and/or manufacturing descriptions please?  I'm especially interested in the vertical injector.

I have tried searching t'net & and the ME index without success.


Edited By Weary on 13/09/2021 11:42:21

Dave Wootton14/09/2021 16:29:19
197 forum posts
51 photos

I too would be interested in Peter Cauley's injector design information, as I'd like to have a go at making injectors. I did make some to Laurie Lawrences design years ago, but I'm afraid they didn't work very well untill the late Jim Ewins took them away and breathed life into them. Well three out of the six were saveable! I won't even mention the one I tried to the LBSC design, known as old reliable- you could rely on it never to work!

I'm also interested in a vertical injector, so any leads would be much appreciated.



I am aware of the injector book mentioned above.

Edited By Dave Wootton on 14/09/2021 16:30:19

Speedy Builder514/09/2021 19:52:32
2383 forum posts
181 photos

This post will give you an idea:

MEM injectors


Edited By Speedy Builder5 on 14/09/2021 19:53:30

Dave Wootton15/09/2021 10:58:45
197 forum posts
51 photos

Speedy Builder 5 thank you for the above link, I hadn't seen that - very interesting, Stew has kindly been in touch, always good to see another approach to something.


Speedy Builder515/09/2021 20:47:13
2383 forum posts
181 photos

I am not here to criticise, but to help and with apologies to Pete and Stuart.

Following the explication and drawings, I found the following may be helpful :

When inserting the combining cone, I inserted the delivery side first (from the steam side) using the setting tool, then insert the steam side cone checking the gap between them with a Number 63 drill poked down the ball chamber as you go.

From Stuart Hart’s link (On the MADMODDER site) :-

1/ The cross section for the injector shows a delivery cone of 6 degrees - this should be 9 degrees ??

2/ “Face the length down so that the gap between the body is 0.094" measured with a No 63 drill, and set the saddle dial at zero, this is important. “…..

This should read:- The body is 0.037” (0.94mm) measured with a No 63 drill …..

3/ Making the steam cone, "Now I say this only once ........ then face off by a further 0.0015" ……

This should read ……. a further 0.015” (0.38mm).

From Peter Cauley’s series :-

Page 3, item 5 “ An accurate hole is required - use a No 62 drill as a pilot” should read No 64 drill perhaps ??

Steam Cone Manufacture and setting

Page 4, Its not wrong, but I scratched my head to understand item1 (perhaps I am a bit thick!)

1- Chuck a length of 5/16” dia brass and face the end. Machine a 1/2” length (with a square shoulder) to be a sliding fit into the injector body. Reduce the gap between the shoulder and the body to 0.037”.

I read it, and again and obviously got fixated into finding this gap… until it dawned on me.

Slide the steam side of the injector body onto the cone and Reduce the gap between the shoulder and the injector body to 0.037”.

All I need now is a boiler to test it with - Fingers crossed!


Dave Wootton15/09/2021 21:52:48
197 forum posts
51 photos


I've got a copy of the drawings and instructions now thanks to Stew, will have a good look , I'll have a go at some when next I get bogged down on my loco.

Out of interest I was doing some googling today and found a short series of articles of tips on making injectors on the Maidstone Model engineering societies website, in their newsletters archive starting in spring 1975. They were written by Fred LaRoche who was a fellow club member some years ago, Fred made injectors on a semi commercial basis,using cast bodies, I've got a few of his and they work very well. All done on a ancient Little John lathe with a top speed of about 750 RPM!

Good luck with the injectors.


Stewart Hart16/09/2021 07:49:41
666 forum posts
354 photos

Thanks for your heads up Bob,

It's not easy to spot your own mistakes it needs someone with the diligence to read closely, I'll revise things accordingly.

Bean in touch with Peter and he's extremely pleased that we have people prepared to give his design a go, and would like to know how you get on so please bring us up to date on your injector builds.


Stewart Hart16/09/2021 09:38:59
666 forum posts
354 photos

Just been in contact with Peter he confirms the sizes he uses for the smaller injectors for 5" G are as follows:-


I see we have some corrections in the computer articles Don’t know how they have cropped up the sizes should be for working pressure 80 to 100lbs sq ins

Steam cone inlet angle 13 degree

Outlet angle 9 degree

Throat no 56 drill 0.046" dia

Combination cone 9 Degree

Throat no 63 drill 0.037" dia

Del cone 13 Degree ( not 12 has pointed out in forum )

Throat no 68 drill 0.031" dia

Steam cone insertion 0.037" annular gap 0.0075"

Can you sort them out please:- they are the sizes I use for my small injectors


Hope this hasn't caused to much confusion


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