Part built Allchin 1.5 inch

Derek, the boiler is definitely recoverable and since a kit alone costs nearly £700 it's worth a good try. I've indicated my suggested solution, which I still favour, although I realise that you will have to arrange the work with someone who can silver solder. The job, to such a person, is not demanding. Even a local engineering fabrication shop might have the expertise.

You'vealready shown some good work with limited resouces. If you carry on this track you will end up with a lovely model.

Hi Clive

i appreciate your kind words, I will ponder and try and decide the next move.

i guess the bolts from the inside would be good if I can get them in, but so far doing one i ruined the thread completely.

regards

Derej

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Derek,

Can't help feeling there is some "over thinking" going on here. Obviously what you have now is not the design intent in terms of depth of thread but;

i don't know the thickness of the boiler barrel or the intended operating pressure. Given that when a screw thread is tightend the majority of the load falls on the first two threads I assume the thickness of the barrel alone will give 2 threads engagement? If you take the area of the cylinder exposed to boiler pressure (12mm hole?) and multiply the operating pressure by that area and divide that by the number of studs you will have the approximate tensile force on each stud. I think you will be surprised by how low that is. Any mechanical forces generated by the piston against the crank will be mainly shear forces so for the purpose of this question can be disregarded. Back in the days of LBSC several of his designs had boiler fittings attached just by a couple of 6ba screws tapped into the wrapper!

So, ideal? No, Compliant with the design? No. Probability of failure? I would say just from the info in your post, low.

Talk to your intended boiler inspector first.

Paul.

Paul, maybe I fix strip thread and shut up about the other 3 holes.

they are probably fine if I don’t overtigten!

As I also said earlier they should be OK with tight studs

For the stripped one tap it out to 4 or 3 BA and make a stepped stud with one end to suit the enlarged hole and the other to match the rest of the 5BA fixings.

Bill, I was thinking of a bronze insert with a very small countersunk head, rather like a c'sk rivet, the head to prevent it dropping through into the boiler but otherwise a small clearance fit in the hole and a fairly light c'sk on the boiler shell to provide a "run-in" for the solder. Drilling and tapping for the cylinder studs would be after blending the inserts and solder to the boiler shell.

One caveat; care would be needed for the initial drilling out the holes in the shell to ensure that the edge of the internal reinforcement plate doesn't deflect the drill. A slot drill might be best for this, but Derek might not have a suitable machine.

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I thought I made that clear by implication! I would judge the load on the studs, look at the existing thread engagement and if 2 full threads or more and the stud loading was suitable wouldn't trouble further. In Derek's case he also has half a thread in the extra thickness so it's going to be better than just the barrel thickness anyway. With the csk insert it relies on pure tensile strength of the silver solder? Putting heat into a completed boiler and given the stiffner is silver soldered inside anyway the joint of which will be affected by nature of heating on the opposite side to my mind is not the best overall plan. But each to their own! Was merely pointing out if the numbers stack up there is no great need for a radical solution and any practical boiler inspector should be capable of assessing the risk and likelihood of failure and adequacy of strength. If it doesn't calculate I would look to use carrot bolts as per full size, thread some fuse wire through the holes and fish out the 12mm hole, wrap in the threads of the bolts and send them through the 12mm hole thread wire through corresponding holes in saddle, seat the block, pull the bolt through, grip with tweezers or needle point pliers, remove fuse wire, thread on nut and tighten, trim off spare thread. If you need to relieve the step caused by the existing stiffener use a dental burr in a dremmel from outside. Good enough for full size, good enough for me!

Paul.

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Internal stud fitted as suggested, only on the stripped thread, the other 3 I will risk and see how we go.

Hopefully this will be ok, there was slight wobble so I had to loctite in but if fails I will send boiler away in the future when I’ve saved up.

What’s the best way to work out how tight to do the nuts/bolts up, again thank you everyone

Derek,

Sadly you ask the unanswerable question! Correct answer is tight enough to do the job but not tight enough to strip the threads! Which you won't find particularly helpful I think?

In definitive terms I only know 2 ways to tighten a threaded fastener to achieve the correct tension. First is using a torque wrench which isn't going to be helpful in these small sizes as friction will play as big a part as anything else. Second is by measuring the bolt stretch, this is a much more reliable method than torque on the nut to achieve the pre load but again impractical in this size / application. There is a third way of torque and turn which is a combination of the two main methods and a bit more accurate than torque alone where a set torque is applied to seat the fastening and reach a predetermined frictional / low tension condition followed by turning the nut or bolt a calculated number of degrees to bring to full tension, this is worked out on extension required relative to thread pitch, again not really helpful for you.

So all you are left with is feel! Getting used to the amount of pressure you can apply to the spanner or socket being used. If you are not confident you can 'feel' how tight you are doing them up than I suggest you get a piece of copper the same thickness as the barrel and drill and tap the same size as you are using (do several holes). Drill a bit of scrap steel clearance and put a piece of the jointing you are planning to use between and use one of your bolts. Screw it in so it is seated firmly and then place the spanner on it and turn until you strip the copper thread, noting the angle it passes through. Half the angle would be a good guide to the maximum you can tighten the bolt without compromising anything. That is about as scientific as you can get in practical terms if you are not confident with your feel. It's worth noting what Jason said earlier and making sure the male thread is a good fit in the hole, if it's too small and there is too much clearance all the load will be on the thinner crests of the threads and they will strip easier than if a good firm fit.

its worth developing a sense of how tight is tight enough on these small fastenings and odd (non ferrous) materials for elsewhere on the engine. Good luck.

Paul.

Edited By Paul Kemp on 25/08/2019 23:13:14

Jason could I ask you how to make a 5ba bolt that’s wider? My die seems to cut the same, I haven’t even tightened the grub screws so I can’t tighten them less.

id like to make 3 more 5BA bolts but with slightly bigger diameter for these annoying holes I have.

kind regards

Derek

If they are a tight fit in your diestock so that the middle screw won't enlarge them then you may have to make a temporary one with a slightly larger hole. Assuming you have a split die to start with.

It is quite possible that your hand held drilled holes will be a bit large anyway so you may have lost some percentage engagement so going for the plus size make thread will help compensate for this.

Thank you Jason, I was thinking about the not from the inside idea.

would it matter that I can pull the bolt fully against the inside of the barrel? Problem is the strengthening plate is in the way and means there is a shelf type situation if that makes sense.

Sorry meant bolt/stud from the inside

I think just good fitting studs from outside will suffice.

The boiler is 13swg so about 2.5mm thick and with 5BA having a pitch of 0.59mm you are getting 4 turns in the barrel and a bit more in the partial thickening plate.

I don't know what size you have drilled the filler plug hole into the barrel but for added holding power you could open up the threaded hole in the cylinder saddle to clearance size and then tap the boiler so the plug also has holding power.

Hi Jason

right I’m going to listen to you and stop worrying, the 3 holes do have slop so I do need to try and make 3 slightly bigger bolts by adjusting the die.

i tapped the boiler filler hole 1/4x40 as I wasn’t sure if this was to be tapped or just a plain hole.

in the area in front of the filler plug would be space for one or two more fixings, it wouldn’t be to drawing but I guess if I get leaks I could add one or two as this would hit the inside plate.

i have a gasket and sealant, foliac I think it’s called and I guess you also add sealant to the fixing bolts.