Mamod wrote:Colours and specifications may change without prior notice by Mamod Limited under its policy of continual product improvement
Some frustrated Mamod owners refer to this policy as 'Work in Progress'. I have not seen a Mark III since April 2016, and although I do not have a Basic Series Roundhouse locomotive, I am familiar with their construction & performance, etc. So here's my views for your request.
The Mark III has a silver soldered brass boiler, but it is still fitted with a plastic/perspex sight glass. At 40 PSI you can get close to the melting point of that sight glass - the one on my Mark I failed, and similarly the one on my TE1A fitted with a Mamod Ceramic Gas Burner scuttle conversion failed, but they are easily replaced (they should provide a spare one with the model, rather than requiring you to contact Mamod [or a trader] for a replacement).
The wheels are still machined mazak castings, but have a better flange profile, and proper splined axles (only heard of one wheel slipping on its axle, but that was a former Garden Rail review model which seems to have had a hard life). Watch my Mark I video to see how the wheel sets are securely mounted to the chassis -
https://www.youtube.com/watch?v=imZwlU3KzXU and my Mark II covers the differences/improvements -
https://www.youtube.com/watch?v=aTH7_uuTiuc
The oscillating cylinders have 'O' ringed pistons, and 'O' ringed piston rod glands -
https://www.youtube.com/watch?v=cXy-bLJsgSQ However, unless they have been changed recently, the gland pocket is too deep for the 'O' ring used so it does not get compressed by the gland collar. Mamod told me this is to allow the 'O' ring to float up & down the piston rod, but how can that then provide any sealing? So you need to add extra packing in order to get that 'O' ring compressed to get a proper seal.
The gas burner is ceramic and thus venerable to contamination from the oily condensate. This is a major problem on the Thomas Telford (which has piston valve cylinders, but Mamod insist in calling them slide valves).
The current achilles heel of the Mark III is the inline lubricator is not the style that we all expect (e.g. the Roundhouse type with a continuous through steam pipe with appropriate sized hole).
Despite having 'O' ringed pistons there was no lubricator on the Mark I. They designed a lubricator for the Mark II, which we refer to as a total displacement lubricator. The steam pipe from the regulator housing terminates into a threaded fitting on the top of the lubricator. The lubricator has a second steam out pipe which is like the overflow pipe in a cistern (i.e. it is not directly connected to the steam in pipe fitting). Thus the lubricator has to be filled with steam first and that tends to displace much of the steam oil at the beginning of a run, flooding the running plate under the chimney. And of course where is some of that oily exhaust going to flow back to - Yup, the ceramic gas burner which does not like being contaminated! This process was further developed as the Mark II filler/drain plug was fitted on the lubricator's side sticking out the cab side and most owners would fill the lubricator with the loco on its side thus priming the steam out pipe - Catch 22! On the Mark III's lubricator the filler plug has been relocated into the cab, requiring the use of a syringe (supplied) for filling, and has a separate drain plug below the cab floor, but as far as I'm aware it still does not have a continuous through steam pipe - Mamod state that it is the same style of displacement lubricator as used by other manufacturers?
As you are going to fit a new body, etc. perhaps the Mark III fits your budget allowing for corrections of the above faults. The exhaust either needs some form of separator tank to stop contaminating the ceramic burner plus a proper inline displacement lubricator fitted, or instead a metal style pot boiler burner fitted (as Millie).
If your budget allows I'd go with the Roundhouse option - although 2nd hand prices have risen dramatically in the UK following the Accucraft Chinese & Brexit factored price hikes, 2nd hand 45mm locos can go for lower prices than their 32mm equivalent.
I would not recommend the Thomas Telford (still work in progress), and it is not easily convertible to 45mm gauge.
If you've read down to here - Well Done!
Chris Cairns
but I now know why. The boiler is almost clogged with sludge and stone. So much for the regnerboiler. 
)
