5" Gauge Hunslet


Just when I thought I could get on with my own projects, another outside job landed in my lap. It's a nice 5 inch gauge Hunslet to the Don Young design. The story I was given was that it had been sent to a professional model engineering company for some work to be done but when it came back it didn't run properly. I don't know whether it has been run since then or has just sat around waiting for someone to sort it. It was suggested that it just needed the valve timing sorting out but judging by past experience I was sure that it would turn out to be a lot more than just that!

The chap that has it at the moment had tried to run it on steam and said it ran but only just so asked me to have a look at it.

I'll have to put up some photos of the complete loco later as the first thing I did was strip the boiler off it to make access to the valve gear etc much easier. It's no fun trying to work on inside valve gear from the underneath! It's always quicker in the long run to remove the boiler so that you can work from the top.

Also, the loco is extremely heavy. The chap said 'it's only a small loco, it's not very heavy'. It took me and my brother all our efforts to get it out of the car! It's a good job that I built that rolling stand for the Britannia otherwise I would have been really struggling.

The boiler etc. came off quite easily. I just had to disconnect the injector pipes etc., the steam pipes in the smokebox, undo a few screws and it just lifted off complete with the smoke box.

I did take a couple of photos to remind me where all the pipework goes when I come to put it all back together.





You can tell from the dust on everything that it hasn't been run for some time!

The inside valve gear looked as though it hadn't seen any oil in ages and the whole chassis was in dire need of a good clean. I gave everything a good oiling, connected up the compressor and tried running the chassis on air. It would run for about 1 revolution and then stop so something was amiss. I tried fiddling with the valve timing but with not a lot of success. I opened the draincocks to listen to the air escaping when the valves opened but there didn't seem to be a lot coming out. A close inspection of the draincocks showed that one on each cylinder wasn't opening at all. It turns out that the draincocks have got mixed up and one of the lefthand cocks is on the righthand cylinder and vice versa. The righthand and lefthand cocks open in different directions so have to be fitted to the correct side. Anyway, that is a minor problem compared to what I found next. The righthand driving wheel was loose on the axle and spun quite freely on it's own! I wonder how long the loco has run with only three of the four wheels doing anything?

The only thing to do was to remove the driving axle to fix the wheel problem so it's a good job that I did remove the boiler etc. Removing the axle will also allow me to check the settings of the valve eccentrics so it wasn't too bad of an inconvenience.

My idea was to Loctite the wheel with 638 retainer which meant getting everything as clean and oil free as possible. I've started to use brake cleaner for degreasing after seeing people on YouTube using it and it does seem to work quite well. It's available on Ebay quite cheaply which is where I got mine from.

Fortunately, the axleboxes are of the split type held together with a couple of pins so you can get them off the axles easily. By removing the one on the loose wheel side I was able to move the wheel across to make sure that the axle seat was nice and clean. A liberal dose of 638 and the wheel was firmly glued to the axle again (I hope!).

I reckon this has been a recurring problem with this loco as a couple of the wheels have had grub screws fitted through the bosses presumably to cure loose wheels.

I then set the axle up between centres on the ML7 to see what sort of angles the eccentrics were set to. I used my favourate method of setting the crankpin on centre height and then measuring how far the eccentric stuck out from the axle using a Tee square clamped to the crossslide.




The crankpins were set at centre height by resting them on a stack of gauge blocks and the distance between the front edge of the eccentric and the axle measured using the cross-slide dial. A bit of CAD enables you to work out the angle of advance of the eccentric. Doing this in reverse i.e. setting the distance of the square from the axle and then rotating the eccentric until it touches the square enables you to set the eccentrics very accurately. This is by far the best way to do it. Faffing around by setting the eccentric to open the valve at top dead centre when everything is assembled is a very tedious way to do it and not very accurate.

The problem is that few, if any, drawings for locos with Stephenson's give you the angle of advance for the eccentrics so you have to calculate it using one of the valve gear simulation programs.

Measuring the angles of these eccentrics showed that they were far from set properly! The righthand eccentrics both gave an angle of about 92° but one of the lefthand ones was 95° and the other 99°!

I had put the Hunslet valve gear into the Wallace valve gear simulator some time ago and had worked out that the best angle of advance for the eccentrics would be 96° so all the eccentrics needed adjusting.

For those that are not versed in the intricacies of valve gears, the angle of advance of the eccentrics determines the amount of lead that the valve gear has. In normal Stephenson's valve gear the lead increases significantly as the gear is notched up. If you have a lot of lead when the gear is in full gear then when you notch up to say 50% cut off then the lead can become excessive. What you need to do is set the eccentrics to give zero lead, or even negative lead. in full gear as in some Great Western Locomotives. This will mean that the lead at say 50% cut off will be ok. I usually set the lead to be about 0.010" at 50% cut off which means in the Hunslet's case that it has negative lead in full gear. This is no problem as it's debatable whether lead in model locos is necessary at all. But that's another story.

I wasn't sure if the eccentrics had been Loctited as well as having grub screws fitted but fortunately they came loose after the grubscrews were loosened off. I did give everything a really good clean again in case I decided to add a bit of Loctite once I was happy with the eccentric setting.

It was interesting to see all the marks on the axle that the grubscrews had made. It looks as though there have been quite a few attempts to get the eccentrics set!




I was a bit concerned that all these indentations wouldn't allow me to lock the eccentrics in a different position so I carefully removed the high spots with a fine file and hoped for the best. As it happens I had no problems at all when resetting the eccentrics to the new angle of 96°.

After I'd done it, I looked at the valve gear drawing just to check that I had set the eccentrics correctly. After a bit of thought I reckoned that I had set the lefthand set 180° out. I had set them the same as in the photo that I took showing the setup in the lathe which shows the eccentrics before I touched them but I think they were originally set up wrongly. The outside eccentrics should be the forward eccentric which drives the top of the expansion link and the inside eccentrics are the reverse eccentrics which drive the bottom of the expansion link. I am pretty sure that the lefthand set were 180° out. That wouldn't help the running if they were wrong! Anyway, I'll find out whether I am right or not when I try to run the chassis on air again.

When I first tried the chassis on air I noticed quite a bad blow from the exhaust and I wondered whether one or both of the valves were not sealing. The cylinders have balanced slide valves fitted as per the Don Young design so there are two faces that need to seal - the valve face on the portface and the balance piston against the steamchest cover. It's also possible that the pistons are blowing past. I don't know yet what sort of seal the pistons have but whatever it is could be well worn by now.

I decided to remove the valve chests to have a look and found that they were full of grit. I don't know where that has come from but it won't do the valves etc. any good. Some may have got into the cylinders so I will have to have a look at the bores. Fortunately, from what I've seen so far, the port faces and valves look ok so we may be lucky.



Steamchest and balanced valve:




The balanced valve is hollow and the exhaust steam exits through the valve and through a hole in the steamchest cover rather than through the usual third port in the cylinder block. The valve has a hollow balance piston that presses onto the underside of the steamchest cover and seals the exhaust passage from the steamchest. For more details see the Simply Longer pages where I describe a design that I hope to fit to that.

The DY drawings show a spring to press the balance piston against the steamchest cover but these have a couple of silicon O rings instead which act as a spring and do the same job.

The advantage of the balanced slide valve is a more direct route for the exhaust steam and less force is required to move the slide valve and hence less load on the valve gear.


Progress has been a bit slow over the last few days as I've been feeling a bit under the weather. Also. I've been sorting out a problem with a 2½" gauge Bulleid Spamcan that a friend is building. It had a strange blow up the chimney but not all the time, only at a certain position of the wheels. He didn't build the chassis but was given it to finish off. I suspected a problem with one of the valves but it turned out that the chap that did build the chassis had machined one of the cylinder ports too deep and it had broken through into the exhaust passage. Consequently, when that particular port opened to steam the steam went straight into the exhaust passage and up the chimney! I sealed it up with JB Weld and it's fine now.

Back to the Hunslet - I refitted the driving axle and the steam chests etc. and ran the chassis on air. It runs quite well now so I must have got the eccentrics set correctly. That means that one set had definitely been set incorrectly by whoever did it last. No wonder it ran like a lame donkey.

I noticed that the cylinder ports only open about halfway and I'm wondering if the valve travel is lacking so I need to check the throw of the eccentrics. I had assumed that they would be as per the drawing i.e. 0.4375" throw but maybe they are not.

There are a few other things that need sorting as well. The lifting arms are loose on the weighshaft so that needs fixing. Also the gudgeon pins in the crossheads are loose and wobble about when the chassis runs.

Hopefully, I will get it all sorted next week.


I decided to have a look at the loose lifting arms on the weighshaft next. These have been made differently to that shown on the drawings. The drawings show four individual arms (two per expansion link) but these have been fabricated as a single unit per expansion link.



The fabrication consists of a tube that fits on the weighshaft, two side plates and a pin that goes through the end of the side plates and takes the lifting links. The tube is held to the weighshaft with a tapered pin.

The problem is that in order to get full reverse gear the bottom part of the tube and part of the weighshaft have had to be machined away to clear the eccentric rods.



Having studied the drawings it is obvious that this will need to be done. I wonder if this problem has been reported before?

This means that the taper pin only bears on the top part of the tube which is quite thin and the hole in the tube for the pin has fretted and worn oval, allowing the lifting arms to move on the weighshaft. The taper pins in the photographs are temporary ones put in after removing the original ones. It was also noted that the two lifting arms were not in line!

The cure that I am thinking of is to refit new pins to locate the position of the lifting arms and then silver solder the lot together so nothing will move again.

I have checked and it is possible to refit the weighshaft complete with lifting arms by removing the righthand bearing. This leaves a larger diameter hole in the frames and the weighshaft can be fed through this hole after the reversing arm is removed and then slid into the lefthand bearing. The righthand bearing can then be refitted followed by the reversing arm.

The reversing arm was originally fitted to the weighshaft with a roll pin but I've removed that and replaced it with a tapered pin which will make reassembly and any future disassembly easier.


I've repaired the weighshaft assembly by silver soldering the lifting arms permanently in place. Before the soldering I fitted new taper pins and made sure that both sets of arms were in line. I did recut the holes for the taper pins to make them a snug fit again and had to tweak them a bit to get the arms in line as they were quite a bit out before.



After cleaning the assembly up I trimmed off the excess bits of the taper pins and filed them flush underneath the weighshaft where the arms and shaft had been cut away. I don't think that the arms will move again!

Next was a bit of repair work on the lefthand expansion link. The link trunnions are rivetted onto the expansion link and the rivets (actually lengths of steel bar) had become very loose and the whole lot flopped about. I don't think the 'rivets' had been a very good fit in the holes in the first place which would not have helped.

I drilled and reamed the holes in the trunnions and the expansion link to take new pins made from 0.125" silver steel and then Loctited the lot together. The trunnions won't come loose again.

The pivot pins for the eccentric rods were also loose in the ends of the rods so these were re rivetted to make them a tight fit again. They were rotating in the ends of the rods instead of being fixed and not rotating in the bushes in the ends of the expansion links like they should do.

All the valve gear has now been reassembled into the chassis so the next job will be to run it on air again and set the valves properly. Hopefully, there will be nothing else wrong.

I did check the throw of the valve eccentrics and fortunately it is correct. If it had been wrong then I would have had to reset the eccentrics again!


I've run the chassis on air again but I'm still not happy with it. It still seems a bit 'lumpy' and the balance pistons seem to leak straight to exhaust at certain points in the valve travel, the lefthand cylinder being quite bad. I've tried polishing both the balance piston face and the inside face of the steamchest cover which had a few scratches on it but with little improvement.

I did find that the O ring seal on the righthand balance piston had got damaged, possibly when I refitted it, and, of course, it was a size that I didn't have in stock so had to order some.

I decided to take the complete steam chest off the lefthand side to give it a good examination but everything looked ok. While I had it in bits again I decided to check the dimensions of the ports and the slide valve to see if they matched the drawings. It's a good job that I did! The ports seem close to the drawing dimensions but the valve is way out. The exhaust cavity is 0.015" oversize which is not too critical. It just means that the valve has a bit of exhaust clearance which is not a problem. However, the valve is 0.080" too long which IS a problem. It means that the lap is 0.040" more than it should be. I did notice that even when the gear was notched right up there didn't seem to be any visible lead on the valve whereas there should have been. I had set the eccentrics to give a slight negative lead in full gear but the oversize valve was giving another 0.040" negative lead on top of that. This means that the valve is opening far too late in the stroke. It would also explain why the ports don't seem to open enough.

Putting the actual valve dimensions into the Wallace valve gear simulator seems to completely screw up the valve events so no wonder the chassis runs a bit rough!

Fortunately, I can just take 0.040" off each end of the valve which will be a simple milling job. If the valve had been too short it would have meant either making a new valve altogether or silver soldering a narrow strip on each end.

I'll sort the lefthand cylinder out first and then check the valve on the righthand one. That may well be wrong as well.

I managed to sort out the problem with the lefthand draincock that didn't open. I think the operating lever had come off at some time and it had been refitted 90° out on the plug cock which meant it stayed closed when the operating lever was set to open.


The valve on the righthand cylinder was indeed too long, this time by 0.065" so that was reduced in length as well.

After reassenbly the chassis was run on air again and it ran noticeably better than before. However, there was still a very bad blow from the exhaust on the lefthand cylinder. I was still thinking that it was the balance piston not sealing to the steamchest cover properly so took it all to bits yet again. This time I lapped the steamchest cover on a piece of plate glass using some fine grinding paste and it did seem to have a hollow spot in the centre.

Back together it went and it still leaked as badly as before. By this time I was starting to get really fed up with it!

If it wasn't the valve leaking then the only thing left was the piston seal. If the piston was leaking in the bore then air would leak past the piston into the other half of the cylinder which would be on the exhaust stroke. To cut a long story short, it did turn out to be the piston ring that was not sealing.

The pistons are fitted with PTFE rings but I hadn't actually removed the pistons from the cylinders to see what they were like. I had assumed that they would probably be ok. Never make assumptions!

I had a look at the lefthand piston first as that was the one that was leaking badly. I managed to get the piston out by removing the slidebars and undoing the rear cylinder end cover and sliding the piston out of the bore. The piston ring was just a plain PTFE one split to go over the piston but it didn't have an O ring underneath to hold it against the bore. I know that I rely on just the PTFE ring on 2½" gauge engines and have never had a problem with them sealing but I think it is pushing it on a 1.75" bore. I think anything that size definitely needs an O ring underneath it.



The ring had been cut at 45° but on the edge face of the ring. It would have been a lot better to cut it across the outer edge. The inside of the ring had quite a bit of clearance to the bottom of the groove in the piston and consequently the top part of the joint was riding up over the bottom part and pushing the ring away from the cylinder bore leaving a big gap between half the ring and the bore. That's why it was leaking so badly. If the ring had been split across the outer edge, this wouldn't happen.

I didn't have any PTFE rod of a suitable size to make a new ring so decided to fit an O ring underneath the original one. I had some suitable O rings in stock so just machined another groove for the O ring in the bottom of the piston ring groove.



It wasn't quite that simple as I needed to remove the crosshead first so that I could hold the piston rod in the lathe chuck to cut the groove with a parting tool, The crosshead was held onto the rod with a taper pin which was very tight but I also found that the crosshead had been Loctited on as well!

It was obvious that the piston hadn't been finish turned after fitting to the rod as it was pretty eccentric when I spun it in the lathe! However, the bottom of the piston ring groove was concentric to the rod. There was nothing I could do about the outer edge of the piston so it had to stay as it was.

I had already tried the PTFE ring in the bore to see what the fit was like and it was ok but the gap was completely closed with no allowanace for expansion. I decided to open it up by 0.020" just in case. Any slight leakage through this small gap should disappear when everything gets hot and the ring expands.

I had to make an improvised piston ring compressor to get the piston back into the cylinder bore and it was definitely a much better fit than before.

Running the chassis on air again showed that the blow from the exhaust had completely gone and it definitely ran even better now that the piston wasn't leaking. Thank heavens for that!

The process was repeated on the righthand piston even though the leakage was only slight. This may have disappeared under steam when everything was hot but it seemed sensible to have both pistons the same.

The chassis seems to run quite well on air now so hopefully I can put the rest of the loco back together and call it done. I'll check the valve timing again though as it may need some fine adjustment now that the cylinders are doing what they should do.


Well, just when I thought that I was on the home straight, another major problem rears it's ugly head!

I was running the chassis on air tonight playing with the valve timing to try and get it to run slowly without being jerky at all. For no particular reason I looked at the main cranks and thought 'they don't look right'. I looked a bit closer and thought 'they are not at 90°!' In fact they are nothing like 90° and a bit of rough measuring showed that they are actually at about 120°. What the @#**! Did someone think that it was a three cylinder engine instead of only two! A few degrees out on the quartering won't make a lot of difference but 30°! No wonder that I'm struggling to get it to run nicely! I'm surprised that it runs as well as it does.

I may be being a bit dumb but I would never have thought to check the crank settings. Was it wrong from the day that it was built or has someone had the cranks off the axles and put them back wrong? Who knows. Anyway, it's a real bummer as I've now got to dismantle it all again, take out the wheelsets and reset one of the cranks on each axle. I will also have to reset one pair of eccentrics again. What a nightmare!


To be continued


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