Gallery, Projects and General > How do I??

Engine question.... bore finish.

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John Hill:
Umm.... if you can lap the two together with toothpaste it maybe that mine are not really close enough, even after second attempt at the piston. :scratch:

bogstandard:
John,

If you are only going to be running it on air as a demonstration model, and because it is only single acting, if you can get it to within about 0.002" (0.05mm), then it should run pefectly well. The average film of oil will easily fill the gap I have mentioned.

It is only when you start to use hot gasses (steam or I.C.) that problems occur, mainly with expansion of dissimilar metals. Only then do you require closer running tolerances.

John

John Hill:
Thanks John, I am not sure if it is a close as .002 and I would have to stop and think how I could measure that however it does make a nice 'pop' when I pull the piston out of the cylinder (dry).

bogstandard:
That should be fine John.

A lot of people go to great lengths to obtain superfine clearances on model engine bores, but in reality, near enough is usually good enough for what we do.

The exceptions are as I have said, engines that have heat in them, steam, IC, sterling, flame gulpers etc.

But there is nothing stopping you going for fine finishes and fits on air driven models, it doesn't do any harm.

BTW, you can measure your bore by turning up a tight fitting plug, and measuring the OD of the plug, you should be able to gauge it to within 0.0005", which in your situation would be more than close enough.

I have had working steam engines thru my hands with bores and pistons worn to more than 0.010" gap, and they still run satisfactorily, though not efficiently.


John

John Hill:
John, I really dont yet have the knack of turning to a tight tolerance, it seems taking off just a shaving makes a tight fit sloppy. :scratch:   Any tips?

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