Friday, 15 June 2012

Nut Screws Washers & Bolts!

I've been struggling for some time to place an order for the various nuts, screws, washers and bolts plus engineering studs for holding Sentinel 7109 together. The struggle has been to discover the correct type of steel required in each case.

For run-of-the-mill purposes, the material from which these items are made does not matter; however, when the items being fixed are steam fittings supporting 275psi pressure, I (and the boiler inspector) have to get this right.

Boiler shell fixing studs (EN3B)
As an example, the photo above shows some of the specially made EN3B studs required for holding the boiler together. EN3B is a bright mild steel with properties that enable tighter tolerance threads to be cut and hence provide a stronger engagement between the stud and nut threads. It is not particularly strong in a tensile manner. I'd assumed earlier that it was the tensile strength which was to be important but it seems not. More information on EN3B properties can be found by clicking here.
Outer shell in place with fittings & blanking plates
(for hydraulic testing)
Some of the boiler fittings and blanking plates used during the first hydraulic test are shown above. From this experience, I assumedthat EN3B fixings would be needed everywhere that a high pressure steam joint had to be made. However, on trying to purchase EN3B studs, nuts, screws & bolts, it was apparent that these were not commonly available and that there was some inconsistency in recommendations depending who was being asked!

After some considerable amount of searching, I came to the conclusion that the fixings' material required for steam fittings is more to do with what should be avoided rather than what precisely should be used. The material to be avoided is 'leaded' mild steel such as EN1A. Whilst EN1A is easily machined and welded, it is not for this application. This therefore answers why some suppliers of engineering studs do not specify the precise mild steel their stock is made from; hopefully it is safe to assume that they are fit for purpose and will not be made from EN1A!

Nut and Bolt Inscriptions
Restoration of Sentinel 7109 has not failed to expose gaps in my knowledge. One gap that is not so empty now is the meaning of the inscriptions on nuts and bolt heads. I hadn't until recently noticed that the inscriptions could vary from bolt to bolt; however, they do and they indicate the tensile strength capability. I'm not going to attempt to include a dissertation on the subject here but there are some good references such as this one from 'Bolt Science'.

The '8.8' bolt head and compatible '8' nut pictured above are a medium grade metric type suitable for pulling together steam pipe joints; they are easily available off the shelf.

I think I've got to the bottom of these fixing requirements now and have at last placed an order for 11kg of them. So shortly reconstruction of Sentinel 7109 will be able to commence!


  1. Hi Andy,

    I would advise some changes to purchasing the metric 8.8 bolts and nuts. If your willing to purchase cap head (allen key) bolts they are manufactured by forging and as standard stronger as they are 12.9 grade. 8.8 standard are made by machining or rolling so inherently weaker. I would also advise a standard marine coating to avoid any corrosion. This is easily available and worth the slight increase in price for many years of protection.

    Thanks Stuart Adam

  2. Thanks for your comment Stuart. I'm sure that 12.9 grade or allen key bolts would be stronger; however, I've been advised that the extra strength is not necessary. In fact 8.8 may be more than required but I like the idea of being over specified for steam fittings.


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