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6th March 2006


Making Pistons For Experimental And Restoration Engines by Stephen Chastain

Reviewed by Humbernut

A little black book yesterday.This book arrived just as my articles on making pistons in a home workshop appeared in RealClassic magazine (issues 17, 18 and 19). Good timing!

As the title suggests, this book is all about making pistons. It is quite a short book of only 64 pages but it does contain some very useful information and data, particularly to anybody making a piston who does not have either a drawing of the original or a worn out one to copy.

The data in the book is presented mainly in tabular form and quotes sizes as a ratio of the bore of the cylinder. This system takes a bit of getting used to but is a good way to describe things and, if followed, ensures correctly proportioned components. It is widely used by model engineers in the design of small internal combustion engines so if you have built any of these little mechanical marvels you will be familiar with it.

The book gives a good insight into the temperature distribution across a piston crown and down the ring groove area to the skirt and also shows, using relatively complex heat transfer calculations, how to determine piston head thickness. I doubt whether any of us would use this complex calculation as we would be unlikely to have all the data required. However the author goes on to describe how to determine critical dimensions by describing them as ratios of the cylinder bore or piston length, etc, thereby making it much easier to determine these dimensions.

The book showed my original guestimate of maximum working temperature of the piston that I made for my Humber of 200 degrees Celsius to be not too far off the mark, and using this temperature for the estimation of piston clearance would ensure that the piston wouldn't seize due to expansion when running.

There is one calculation that departs from using ratios of engine bore or other engine components, and that is the calculation for side thrust, used to determine the skirt length of the piston. If you have determined all the other dimensions required of your piston then I am sure an educated guess could be used to determine skirt length, particularly for an old clunker like the ones we ride! The main thing to remember here would be not to make it so long that it fouls the flywheels at Bottom Dead Centre or that it is so short that it wobbles in the bore and slaps like mad.

Readers may at first be put off, when they first open the book by the appearance of mathematical formulae but these are easily explained and are really useful for anybody designing their own pistons.

I was a little disappointed in that the book failed to tell people how to make their own rings, although some details are given of ring dimensions relative to cylinder bore and optimum gap of the ring prior to fitting. This latter dimension I have not seen quoted before and should come in very handy for anybody who does make their own piston rings. I have to admit that I tend to guess this dimension but I have found that my guesses seem to be about right so far.

Traction Engine Stuff on eBay.co.uk

The book then goes into the description of some of the casting processes. I have never cast my own pistons and to be quite honest I do not see myself doing so in the near future! However I did find this part of the book interesting and it did enlighten me on why there are such things as gates, runners, filters and traps to help the castings to be free from impurities and blow holes in the casting process.

A short section is included, describing the sequence of operations required to make a homemade piston and how to make a set of working diagrams for both the piston itself and the patterns that would be required to make it. The author describes the need for adding on shrinkage for castings, as well as machining allowance before going on to introduce pattern making to his readers.

Casting processes are not just a simple matter of pouring molten metal into a mould. Someone has to produce that mould -- and in this case it is the reader! The process of making the mould is just as complex as the machining of the piston.

Three little pistons, sitting on the bench. And if one little piston should accidentally seize...

The pattern-making process for pistons is described in reasonable detail, covering how to make the body pattern as well as the pattern for producing the core required to enable the inside shape of the piston to be made. If you are a good hand at pattern-making then I see no reason why you should not be able to produce your own patterns using the information supplied in the book. However, the book is part of a series on the home foundry and assumes that you have read the other volumes before starting on this sort of project. If you have experience with the home foundry then I would say go ahead and cast your own. However… beware -- how do you know that any item you have cast yourself is free of any imperfections that would cause failure in service with disastrous results?

If you are capable of making your own patterns for both the cores and the piston then I would recommend that these are taken to a reputable foundry to have cast for you. Machining would then be a matter of following the methods described in the book or adaptation of the methods described in my workshop series.

Overall I would recommend this book for anyone who is involved in vintage motorcycle restoration who may, from time to time, need to make their own pistons. The information on estimation of sizes, etc, is valuable and worth buying for this alone.

Making Pistons for Experimental and Restoration Engines by Stephen Chastain is a paperback of 64 pages, published by Chastain Publishing in 2004. ISBN 0 970 220 340. Cost £11.95 from http://stephenchastain.com/index.html

And thanks to whoever sent the book in for us to look at. (EVGuru, was it you?)

******

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