Replacing a Sight Glass on a Mamod SL or MSS Locomotive

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Chris Cairns
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Replacing a Sight Glass on a Mamod SL or MSS Locomotive

Post by Chris Cairns » Mon Nov 05, 2012 7:23 pm

Whilst emptying out the packaging that came with some old Mamod spares I bought I came across these instructions from Brandbright for replacing the sight glass on a Mamod SL locomotive (equally applies to the MSS locomotives).

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When I refurbish a Mamod boiler which had its back plate riveted on I do solder in 6BA screws as these instructions suggest.

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However going back through postings on this Forum, and my recent experience with the MSS Locomotive kit build, there is a recurring problem of these sight glass's leaking after replacement/fitting.

Whilst I do have a solution that works I thought I would look into the design first, as I've just replaced a sight glass on a riveted SL3 but using the original seal. It would appear that the seal has changed to a thinner 'O' ring over the years, probably from 1983 when the SL1K was introduced which also brought the introduction of screw fitted back plates.

Measuring the recess from the top of the boiler screw fitting to the deepest part of the recess comes out at 15/128". The sight glass plastic measures 7/128" thick. The 'O' ring uncompressed measures  8/128" thick.

So if we fit the back plate to the boiler the recess measures 15/128" deep and our 'O' ring and sight glass plastic also measures 15/128" deep. So no compression of the 'O' ring leading to a leak under pressure.

So the solution would be a thicker glass plastic, or a thicker seal, so that there will be a good seal under compression.

I use the oval washers which Mamod use on all their current engines as the sight glass seal (interestingly the sight glass has been available since 1978 when Malins (Mamod) Limited introduced them as standard - was the oval seal introduced then or later?). These are 11/128" thick so fitting them to our locos allows for some compression and so no leaks - Simples!

Other alternatives include fitting the sight glass with gasket sealant or silicone sealant as well, which if done correctly should compress that 'O' ring sufficiently to form a good seal. Or fitting the current Mamod sight glass plastic which is thicker but needs trimming to fit into the boiler recess (these sight glass plastics are rectangular).

Only problem is these oval seals are not sold on their own (closest I've seen is the oval washer, sight glass plastic and 2 screws or copper rivets). So they are a more expensive option than the cheaper 'O' ring and plastic loco sight glass replacement kits.

Chris Cairns.

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dougrail
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Post by dougrail » Tue Nov 13, 2012 2:50 pm

If Imay say, the New Mamod 'super gaskets' are sold separately - if you ask Mamod in Smethwick very nicely...;)

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Boustrophedon
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Post by Boustrophedon » Sun Jun 30, 2013 6:37 pm

Rinse and repeat as they say.

As for sealing them I have found that "Blue Hylomar" works very well, and being non drying is easily removed when you have to do the job again... and again...

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Post by METHSSNIFFER » Fri Sep 06, 2013 1:11 am

Been here before. The leaking is not in the fitting of the seal nor the sight glass, its the facing plate and ensuring this is fitted  properly to the boiler. I drilled a further two holes and used four rivets in all and I do remember I cut my own sight glass from perspex.

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