Mamod Diamond Jubilee Saddle Tank Locomotive

[Chris Cairns]

Rather than add this to the Royal Family Topic where it may get missed I thought a new topic would be better.

So no thanks to myHermes my eBay purchase finally arrive today.

I was aware that the previous owner was new to live steam and this showed. The loco has obviously been stored in a shed or damp loft as the box & paperwork are damp. It also had some gas in the tank, and there was still some water in the boiler. It obviously did not get wiped down properly after a running session as the oil has rotted away some of the polystyrene inner.

And it is No.14 of the 100 Limited Edition - and if you have a good memory that was the one that Mamod showed in their video, so that video must have been old as this loco was originally delivered in March 2012.

So taking it out of the box shows the weakness of the construction method used. Whilst it appears reasonably straight on the left side -



It is not so on the right side -



The cab front is screwed into the boiler and there is a fitting which is riveted to the cab front and saddle tank. The regulator steam pipe outlet is not at 90 degrees like my Mark II so the slightly longer steam pipe into the lubricator is pushing up the cab front a bit causing the distortion. Also the lubricator is not fixed to the cab floor (looks like it was glued on which has failed). By removing that steam pipe and bending things back into alignment it seems to be better looking now.

I have mentioned before that the gas jet holder used by Mamod is a loose fit into the burner tube and has caused me a blowback before. This previous owner had a similar problem as the gas pipe was not bent properly so the gas jet was not all the way into the burner tube and had obviously leaked out gas which burnt backwards, with soot on the chassis frames & rear buffer beam plate, plus a scorched grommet in the cab floor.





So I removed the wheels, burner and gas tank, and as a video had shown this loco leaking steam around the cylinders I decided to remove them for honing. Here is the cylinder gasket -



The previous owner obviously had other problems with the gas burner, either due to gas pulsing or burner flaring, or by turning it up too much, as there is scorched paint on both the chassis and the bottom of the saddle tank -



Also note the slight difference to the Mark II where there is a tab in the middle of the air slot on both sides of the chassis.

Sadly I'm renewing my indoor track trackbed (carpet tiles) before relaying my test loop so no video of it running yet.

However using the burner meant no more than a quarter turn open on the gas valve (when I had it running fast backwards on blocks that fanned the flames out of the air slot so ended up with gas valve at just less than a quarter open). Some of the hot air comes out of the holes in the saddle tank around the safety valve (the biggest hole), false steam dome & regulator body on top of boiler. However that hot air is not as much as my Mark II so guess what - this loco gets seriously heated up all over (even the engraved plaque on the cab roof heats up)! I believe this will cause the gas to pulse causing the burner to be erratic, so I have put a piece of felt pad on the bottom of the gas tank to keep the temperature down (need to get some new masking tape to wrap the tank against the heating effect of the retaining clip). What is not obvious from any of the photos is the Diamond Jubilee decal is not one of the plastic ones as used on the Williams & Mark I/IIs which burn easily, but rather it is a metalled decal (like the water level warning one) so appears to stand up to the heat - only time will tell!

What was very pleasing is this loco is very smooth running, even at very slow speeds, and was running well with the regulator only a quarter turn open so should be a good runner.

And one for Lloyd (steamie1) - whilst it was running on blocks I was getting the odd burst in speed increase. By rocking the loco to & fro sideways these speed bursts increased with a noticeable increase in water being carried over in the exhaust. Perhaps this is a disadvantage of using reheating tubes in the boiler causing some of the water to boil over into the steam pipe outlet?

Hopefully some video soon when I get my track relaid and things tidied up.

Chris Cairns.

[dougrail]

Interesting, I was under the impression that the Saddle was well built. I take it then there's no screw/nut that goes through the jet holder that holds the jet in place?

[Chris Cairns]

With the side tank locos the tanks are either glued to the cab and further held by the front cab handrails (William II), or spot/tack welded to the cab (Mark I & II). Thus the bodywork is generally very square.

On the saddle tank you have one rivet each side holding the saddle to the front plate (2 rivets each side used in the side tank locos above), and another single rivet each side holding the saddle to the cab front bracket. So more room for error with cab misalignment.

With my Cheddar Models, PPS Steam & Bix burners the gas jet holder is a tight fit sliding into their burner tubes. With the Mamod ones the diameter of the burner tube is much bigger than the diameter of the jet holder so they fit very loosely into the burner tubes. They are restrained by a screw but it is possible for gas to leak back around the jet holder, particularly in the case of this loco where the gas pipe was bent so much that the jet was not fitting parallel to the burner and was sticking out a bit (similar to my staged photo above).

Mike Wakefield has recently found out that Mamod have discovered a problem with the design of their burners which was originally based on external profession advice.



That restriction at the end of the burner tube (which you do not see in a Cheddar Models, PPS Steam or Bix burner) I believe may be restricting the gas flow under certain circumstances and the gas tries to find another path. I have certainly had at least one if not two blow backs where the burner starts burning gas from around this loose fitting between the jet holder and the burner tube. By wrapping PTFE around the jet holder I've effectively sealed that gap & stopped a re-occurence of these blowbacks.

Given that the only place to see the burner performance on this saddle tank is by looking through the chassis air slots (on the side tank locos you can see the reflection of the burner on the combustion chamber walls looking from the top of the loco, plus the flames will go over the boiler if the gas is turned up too much) it is much harder to get the correct setting with the loco running on track, particularly outdoors. Thus marking the gas valve is very important to stop scorching the side tank paint as has already happened to this loco.

Chris Cairns.

[Chris Cairns]

Taster video posted here - http://gardenrails.myfreeforum.org/about6874.html

Chris Cairns.