Head spacing or clearance

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Head spacing or clearance

Post by JohnR » Sat Jan 08, 2022 4:48 pm

I am designing a 1.5 " x 1/5" two cylinder engine. Is there a formula or rule of thumb for determining the
clearance between the end caps and the piston face.
I will be incorporating cylinder drains on each cylinder.

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Re: Head spacing or clearance

Post by daves1459 » Sat Jan 08, 2022 7:32 pm

Any space is a waste of steam that has to be filled and emptied each stroke. Therefore the closer the better. However, stack up of part dimensions and expansion/contraction for a cold to hot engine needs to be considered. I use .050" for engines the size you describe.


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Re: Head spacing or clearance

Post by VGC » Sun Jan 09, 2022 1:44 am

If you do not have any space at all, you cannot have low degrees of cylinder filling because of the outlet closing early which leads to too much compression work then. That is also a waste of steam.
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Re: Head spacing or clearance

Post by FKreider » Sun Jan 09, 2022 11:50 am

When calculating proper head space in the past I have always been surprised how little clearance is really needed.
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Re: Head spacing or clearance

Post by Soot n' Cinders » Sun Jan 09, 2022 5:28 pm

Also need to account for changes in piston position over time due to wear. The piston will have to travel more to take up slack in the wrist pin and big end bearings.
On my shay, the new clearance is 1/16in at dead centers.

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Re: Head spacing or clearance

Post by Asteamhead » Sun Jan 09, 2022 6:06 pm

Changing of dimensions cold/warm, wear/backlash of main rod and -axle and movement up/down of the main drivers demand for sufficient space :!:
Otherwise volume of "dead" space will used during expansion and help for smoother running due to reduced compression.
Steam consumption of our small modell steam engines will be increased by almost nothing yet due to the much more important losses of both condensation and steam leakage :(
The worth of narrow clearance is overrated! Thus I used about 2 x 2 mm ( min. 1/16 ") - crosshead side slightly more than front - on my engines.
Just my two cents.
Best regards

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