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Fitting Cylinders

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Rogeman22/05/2022 12:28:53
6 forum posts
1 photos

I am building a 3 1/2-inch firefly. I have reached a point where I have made all the cylinder parts and I am trying to work out how to fit them to the frames. In each of the frames there are two holes for the steam pipes. The steam pipes are screwed into the cylinders and valve chest.

All the threads on the pipework are right-hand threads I am unable to work out how to fasten the cylinders, valve chests and pipework together and fasten them to the frames. The steam pipes are to go through these two holes in each frame. Could someone suggest a solution to this problem, the only answer I have is to cut a slot in the top of each frame and fit the valve chest separately after having first fastened the cylinders in place.


Weary22/05/2022 13:45:02
376 forum posts
1 photos

1) Fasten cylinders to frame

2) Screw exhaust pipes equally and deeply into central block, this should allow the pipes to just fit between the frames/cylinders. Maybe some 'joggling' required. Further extend (cut) threads on pipes and into block if required.  Looks like you have adequate extent of thread on pipes from your pic tho'.

3) Position exhaust pipes correctly, unscrew exhaust pipes from central block and they will engage in cylinder threads. Obviously this necessitates same thread pitch at both ends. When you have adequate thread engagement in both ends and in central block & central block is correctly positioned: - stop!

4) You can make the process easier by silver-soldering a hex onto each pipe somewhere accessible (i.e. midway). Sometimes a lock-nut at one end of each pipe or the other assists with rigidity once pipes are screwed to correct position too.  You may wish to apply your preferred thread sealant to the pipework etc., before performing procedure.

Now that you know process inlet is the same process/arrangement.



Edited By Weary on 22/05/2022 13:48:25

Rogeman22/05/2022 15:02:07
6 forum posts
1 photos


If I understand you correctly you are saying to, for example screw pipe into cylinder twice required depth, then as the other end is screwed into the other side it unscrews half the depth. Both sides then have the same depth of thread.

That sounds like a good method central blocks and pipe work do not get in the way.

I will give it a try as soon as I can and let you know the result.

Thank you for your helpful reply.


Weary22/05/2022 16:42:18
376 forum posts
1 photos

Well you could do it that way, If I have understood your description correctly.

However, taking your exhaust as the example as it is most clearly visible in your pic' I suggest screwing both the pipes 'in' to meet at the midpoint of the central block. This is done first 'away from the field of battle' as a preparatory gambit. . This will temporarily make the overall length of the connecting pipes as short as possible.

Then drop that assembly in line with your cylinders. This may involve some (very gentle!!) wiggling and juggling.

Additional note: You can if you wish drop this exhaust pipework into position before both the cylinders are fitted. In this case the total overall width of the pipework can be the distance between the outside of your frames. This approach may be easier to get the pipework into place BUT it will mean that you will need to remove (or at least loosen) one of the cylinders if you wish to remove the pipework at any time in the future. This can be a royal pain! As thread starts are unlikely to align exactly it can make initial cylinder thread engagement awkward too. Though looseness in thread engagement normally allows for this.

Anyway, to return to the main drama:

Having got your pipework roughly in place and your cylinders secured: Unscrew one of the exhaust pipes from the central block whilst holding the central block 'in place' (i.e. 'centrally'!) and keeping the outlet vertical. As the pipe is 'unscrewed' from the central block it will engage with and 'screw-into' the cylinder on that side. When you have 'adequate' engagement of this thread at both the cylinder and central block - stop. Now tackle the other pipe connection: supporting the central block (and keeping the outlet vertical) unscrew this pipe in turn, once again, as it unscrews it will engage with the thread in the cylinder and screw-in. Some gently 'joggling' may be required to start the thread, there should be enough 'give' to allow this. Once again when 'happy' with the engagement on this side - stop. The exhaust pipework should now be self-supporting - but lock-nuts (gently applied) can assist.

If you have a go I'm sure that it will become clear. Basically your start-point is to make the overall width of the pipework as long as possible when screwed fully into the central connector whilst still allowing it to be joggled into position between the frames. Martin Evans will have calculated this for you in the dimensions and extent of threading that he has stipulated. Unscrewing each horizontal pipe in turn from this central connector engages the relevant pipe with the cylinder.

Your exhaust will be low-pressure so easier to deal with. Once you have established how the system works you follow the same principle with the inlet - which of course is high pressure so will almost certainly need a dab of your preferred jointing.

Be gentle when screwing your pipework in and out - indeed 'gently does it' is the watch-word at every stage right from the beginning of the procedure - keep in mind that you are dealing with soft copper so easy to deform and damage the thread! (I could tell you how I know - but you can almost certainly guess. At least it is quick and easy to make-up new pipes tho'.)

Good luck!


Rogeman22/05/2022 17:22:09
6 forum posts
1 photos


I was simplifying the action but as you say treat each side separately, there is plenty of room in the central block for a good depth of thread so once the outer end is in line with the outside of the frame it can be screwed in.

The same with the other connections.

I am not yet in a position to complete this assembly but just thinking ahead, I was not able to think outside the box so that was why I asked for help. Now that you have explained the procedure I understand what I need to do when I come to assemble the cylinders.

I will report back when it is completed.

Thank you again for your explanations.


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