Comparision of Mamod/MSS/PPS Reverser Valves

[Chris Cairns]

Doug's enquiry about transplanting a new Mamod reverser valve into the old Mamod/MSS locos got me motivated to provide an answer.

Here we have the reverser/regulator valve that is fitted to the Mamod SL locos (actually from a 1983/84 SL1K kit).





Note that the centre pivot hole is not vertically central to the block - I've found this on a few old Mamods and you must make sure you mount the block vertically correctly (pivot hole towards the top) so it does not foul the smokebox casting. The holes where the steam inlet pipe and exhaust steam outlet pipe are fitted are a slightly larger size on the front face than the other 2 steam/exhaust steam path holes.

Here we have the reverser/regulator valve that is fitted to the MSS side tank locos (from a recent MSS Side tank loco kit).





Note the centre pivot hole is vertically central. Only other difference is this is supplied with a lever cover. The 4 holes on the front face are the same size.

Here we have a PPS reverser valve (actually fitted to my early IP Jane).





The block is the same size as the Mamod/MSS ones above, although the 4 holes on the front face have been countersunk. The rotary valve is a one piece block (the old Mamod & MSS ones above are 2 pieces solder pasted together) with a finely machined cutout which is smaller in overall size to those in the Mamod & MSS ones above.

Here is the reverser valve fitted to the new Mamods (from my Mark 1 loco).





The block on this is smaller in width than the 3 others above, and in addition the 4 holes on the front face are closer together as well. The rotary valve is a one piece block which is fitted with a gasket to provide the steam/exhaust steam path control.

A comparision of the 4 rotary valves.



L to R, Old Mamod, MSS, PPS & New Mamod.

So you have a Mamod SL loco or an MSS Loco that you wish to have better regulator & reverser control of, so what to do (without fitting the cab regulator)? The new Mamod reverser valve will not fit so that leaves the PPS valve, now produced by Roy Wood. You should be able to countersink the block on your Mamod/MSS loco so all you really need is the PPS rotary valve. Wonder if Roy would sell these separately as £20 is a bit much for the full assembly when all you need is this rotary valve.

In the 16mmngm Mamod handbook there is the following recommendation.

The recommendation is to drill out the steam holes in the loco frames to 1/8”, although 3mm seems adequate. The valve block holes should be chamfered out similarly, to a depth of 2-3 mm.

I do this and also find that the mounting holes on the MSS block need the thread cleaned up with a 6BA tap.

See here for my previous discussion on the poor quality control on the old Mamods - http://gardenrails.myfreeforum.org/about423.html

I have also suffered from a slightly different problem with MSS reverser valves, where the solder paste process has not completely sealed the rotary valve.





In this example you can see my feeler gauge actually goes through the holes in this rotary valve. This example was replaced by Dream Steam, and I now tightly clamp the 2 halfs of the valve together and run solder down the outside to positively seal the seams.

Chris Cairns.

[Superbiker_uk]

Thank you for going to the trouble of investigating this matter and posting all this information - a very interesting and useful thread. Brilliant.

[Dr. Bond of the DVLR]

Should we move this to the reference library?

[Narrow Minded]

Should we move this to the reference library?

I would think that the content and quality would merit the move Mr B.

[dougrail]

I also support this motion as it's something that's ben making me have a think for a while - thanky you very much Chris Cairns!

A shame that the New Mamod bit can't be used on our SL/IP/PPS engines but ohwell. Still, I always said the PPS blocks were best.

[Spule 4]

More great info from "Dr. Cairns"

I must admit, my two Mamod (SL3 and SLK1) reversers are VERY different. THe SLK1 reverser looks like the MSS one, and even the passeges in the rotary valve are not uniform.

The obvious difference is the SL3 reverser rotary valve looks as if it is solid, not two parts, and it is not threaded, the bolt is heald with a nut. Where the SLK1 reverser is threaded, sans nut, and of course it works loose as you reverse it. I will be adding a "captive" nut.

Otherwise, the SL3 reverser works well, but makes me wonder if I should hit up Roy Wood for the SLK1 rebuild...

[dougrail]

Yes, yes you should Garett!

[Chris Cairns]

Garrett,

Not come across any variations of the old Mamod SL reverser valves (all mine are as above although some are fitted with 2 springs - see below), and if I recall correctly you have had Jan-mod since new, so your SL3 reverser valve is a mystery.

One trick I've picked up is to double up the springs on the valve pivot bolt, spaced with a small size washer. This reduces the steam leaks on a well honed valve and also stops the pivot bolt from unscrewing.

Chris Cairns.

[Spule 4]

Garrett,

Not come across any variations of the old Mamod SL reverser valves (all mine are as above although some are fitted with 2 springs - see below), and if I recall correctly you have had Jan-mod since new, so your SL3 reverser valve is a mystery.

One trick I've picked up is to double up the springs on the valve pivot bolt, spaced with a small size washer. This reduces the steam leaks on a well honed valve and also stops the pivot bolt from unscrewing.

Chris Cairns.

OK, looking at JanMod whilst in the shops...

It does appear in fact that it is a two piece rotary valve, but not the same as the SLK1.  You have to look hard to see this.

Also, there is no "dog leg" the lever, it just comes out from the back at a 45 degree angle.  

The odd bit is that it is not threaded at all, and the rotary valve is retained with a nut.  When new the order of assembly was (from the front of the loco)  bolt head, rotary valve, backing plate, retainer disc, spring, washer and then nut.  What I did do to aid re-assembly was to reverse the bolt to help hold the spring.  

I would take it out and take photos, but JanMod is running too well right now to go about doing that!

Maybe I can get a photo of the top of the valve?

[Chris Cairns]

When I dismantled my IP Jane to remove the PPS reverser valve the gaskets between the valve & the chassis frames shredded so I had a lot of work ahead to clean up the various surfaces and refit the reverser valve.

So I took the opportunity to do a little experiment with different combinations. I have an SL3 which is currently the workshop hack. This originally had a good Mamod reverser valve but that went onto an SL1K which I was preparing for sale. So I refurbished one of my Mamod/MSS reverser valves (cleaned up the cut-outs, soldered around the rotating part completely, honed the various faces and fitted a 2nd spring on the rotating mounting bolt) and ran the SL3 with this refurbished valve. Whilst it is still capable of runaway speeds with careful setting of the reverser it runs around my test loop under control with barely any steam leaking from the reverser.

I then took the rotating part of the PPS reverser valve and fitted that to Mamod/MSS block. Although this combination worked it did leak a fair bit of steam, and I had to open the valve further to get the loco to start initially, and maximum speed achieved was reduced. There was also a bit of a squeaky whistle with the exhaust steam as well. So no major improvement on the stock Mamod/MSS reverser.

I then fitted the complete PPS reverser to the SL3 for the final comparison. Again I had to move the valve further open than the Mamod/MSS to get the loco to start off and as above the maximum speed achieved was also reduced. There was also that annoying squeaky whistle with the exhaust steam, which I suspect is because the PPS valve has smaller cut-outs in the rotating part. In other words a form of restricting the exhaust steam.

So I'm not convinced I would gain enough extra control to spend £20 on an RWM reverser valve just to replace the Mamod/MSS one.

It is just a great pity that no-one copied the Mike Chaney regulator which although mounted in the cab actually took its steam input from the steam chest nut, and did not involve any cutting of holes in the cab front like the ones Dream Steam & RWM offer.

Would be interesting to hear their experience from anyone who has just replaced the standard Mamod/MSS reverser with the PPS/RWM reverser, as the majority of locos I have seen these valves fitted to also have a separate regulator control as well (i.e. the reverser valve is mainly used to control direction).

Chris Cairns.

[Spule 4]

Again, good information.

I think that Chris Bird @ Sumerlands Chuffer pointed out that he thought the improvement came with less steam leaks, as his model also had the regulator at the boiler.

Agreed on the Chaney reverser, so I am glad I got the unidentified reverser off of eBay.co.uk that I put in the SLK1/Major Hitchcock.

http://www.ebay.com/itm/Model-live-stea ... 1029829167

It actually gives a little more fine regulation than the one on the IP boiler on JanMod. I would buy another one for the bits box based on its performance.

[Chris Cairns]

I agree, as supplied the Mamod/MSS reverser valves are quite rough and can leak steam. However if you get a good one where the 2 halfs of the rotating part are joined correctly, then some honing of all the surfaces, fitting a 2nd or stronger spring to the mounting bolt, cleaning the cut-outs of excess solder paste and fitting the valve to the chassis with gaskets can produce a near leak free reverser.

Of course on locos fitted with separate regulators this reverser valve tends to get set to a position and left there so less likely to leak compared to the standard set-up where speed is also controlled with this valve, and it gets closed prior to raising steam.

Another trick if you are having problems with a loco setting the separate regulator properly is to reduce the amount you open the reverser. I have Samson Locomotive Works loco where I set the reverser to half open which reduces runaways as the regulator has a limited range of movement.

Chris Cairns.

[DolwyddelanLightRail]

On Britomart I use the original regulator (now reverser) as much as the in cab regulator. The reason being is that on a near perfectly flat line I can use the reverser as a proper one as basically notch it. If it's fully open all the time then the loco will drink water quite a lot, but if I have it just open but with more reg then it makes the loco a lot more dosile and doesn't go through as much water.

[dougrail]

I usually just wedge my rev open but use the cab reg to control...I wonder then if your point James might be useful in fuel/water economy...

[DolwyddelanLightRail]

I usually just wedge my rev open but use the cab reg to control...I wonder then if your point James might be useful in fuel/water economy...

It works on Brit very well, and it also means that if needs be and the loco begins to struggle you can just fully open the reverser and you have enough reserve pressure to get the loco and train out of the way

[dougrail]

I think next time I go steam I'll remember this...cheers. I ssumed that the cab reg became 100% regulation of steamflow, but seems not quite.

[dougrail]

I wonder then, hence my thread last night, if a =good= Mamod/MSS rev block might be better as it allows more steam through? The narrow channels of the IP/PPS seem to restrict the amount of steam that can logically pass through to the cylinders and having seen a supercyl'd engine run freely with an old but good block am pondering experimenting. Further to the 'physical proof' (Brit...) Dr Cairn's scientific approach seems to conclude that the PPS/IPE isn't as 'better' as thought.

Hmm.

[DolwyddelanLightRail]

Further to the 'physical proof' (Brit...)

Well, Brit still uses it's original regulator (now reverser). Never had to lap it, and the only mod done to it was to add an extra spring so that it could cope with 45psi of pressure, not 25. Never had any issues with it at all.

(If something now breaks on it because I've said that.....)

[dougrail]

Well yea, exactly, we've seen how Brit works and works good. Brit and SB have the same chassis - DS steelers, DS supercyls, both roll freely when off-loco, but SB struggles a little whereas Brit runs wild. This is even with the gas ramped up.

I wonder if the original block on Brit having wider channels allows for more steam to reach the cyls and thus more steam = more power and better running given how supercyls are stiff needing lubrication just to run.

[Narrow Minded]

Doug, I think you'll find that, after listing the similarities, the difference lies in the amount of running that Britomart does