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Calculating piston length

calculating piston length.

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3404613/03/2019 10:11:27
374 forum posts
2 photos

I have some Cheddar Puffin cylinders less pistons.

Bore and stroke is 11mm by 11mm

Bare cylinder length is 19mm

The steam holes are 12 mm apart - centre to centre

The steam hole centre is 3.5mm in from each end - I think these are double acting cylinders.

Would like to know how to calculate length of piston please ?



Hopper13/03/2019 10:57:59
3527 forum posts
68 photos

No idea what the original Puffin drawings specify, but from your measurements, I'd be thinking along the lines of 19mm cylinder minus 11mm of stroke leaves a space of 8mm. Leave 1mm clearance at each end, so deduct 2mm total to leave 6mm.

That's your maximum piston length. Now to accommodate the steam ports you can file or machine a small notch from each steam port up to the cylinder head gasket surface. This can be done by dropping a small slot drill down the side of the bore, or with a Dremel tool or similar. It will let the steam up past the piston side to the head space.

That would be my thinking to get it running anyway. Puffin's designers may have done it differently.

3404613/03/2019 11:03:38
374 forum posts
2 photos

Thanks Hopper - prompt reply is much appreciated.

Nice and clear so fully understand so 6 mm it will be.

Paul Kemp13/03/2019 11:10:10
261 forum posts
9 photos

Don't forget to allow for any spigot protruding into the bore from the cylinder covers. As above will be fine for flat covers but if spigoted the piston will clout em if deeper than the 1mm allowed clearance.


3404613/03/2019 11:31:34
374 forum posts
2 photos

Thanks Paul

Need to make those so flat ones it will be.


Brian Sweeting13/03/2019 14:27:06
345 forum posts
1 photos

Perhaps machine a Delrin piston first to see how it fits before the final metal version.

JasonB13/03/2019 14:51:04
15175 forum posts
1548 photos

Looking at a couple of exploaded views on the net, I would say 5-6mm looks in proportion to the bore.

3404613/03/2019 15:00:37
374 forum posts
2 photos

Thanks for confirmation Jason.


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