Piston to cylinder clearance

[Basil]

I'm building the 30cc Seal Major engine which I plan to install in a boat. Because the engine will be working under load I want to be sure of getting the correct piston clearance from the get go. This is what I have and what my thinking is:
Piston material supplied in Hemmingway kit; Pretty sure from the way it machines its 6061 t6 Equivalent. (will check with them on this to be sure)
Piston size 13/16".
Cast iron liner Material. Water jacketed
I've measured an OS Aero Four stroke piston and liner of a similar size and they use 3 thou. and 4.5 thou above the ring (they use only the one ring). The OS piston is cast Alli.
I am thinking the billet piston will grow more than a cast piston requiring a little more clearance and the engine will run fairly cold with the water jacket.
Numbers I am looking at are 4 Thou. main body with 6 Thou. above the ring.
I have taken the piston up to 160c in the oven and it grows by about 1.5 Thou. from room temperature.
Knowledgeable advise would be very much appreciated on this. Thank you

 

[gbritnell]

I use .002-.0025 for the whole length of the piston and have never had any problems with my water cooled engines. When I worked on motorcycle engines with approximately 2.50-3.50 diameter bores they called for .003 on a new bore so .002 is more than enough for a bore of .812.
gbritnell

 

[Basil]

I use .002-.0025 for the whole length of the piston and have never had any problems with my water cooled engines. When I worked on motorcycle engines with approximately 2.50-3.50 diameter bores they called for .003 on a new bore so .002 is more than enough for a bore of .812.
gbritnell

Thank you for the advice, It does sound like a lot of clearance to me also for such a small cylinder.

 

[Charles Lamont]

I have used even less. My 3/4" bore 'Seagull' is running with a little over 0.001" clearance. It is also a Hemingway kit so the material is likely to be the same. It has not yet run for very long, but long enough to get hot, and it seems to be fine.

 

[Basil]

Thanks Charles. I have your build page bookmarked for reference. The camshaft is next. I have drawn it up in Fusion and it is now resin 3D printing. This should be a great reference to make sure I get the bumps in the right places.

 

[bluejets]

This engine runs in a boat, around 20mm bore, has 2 thou on the skirt and 3 above the piston.
Ring gap usually 3 thou per inch of bore.

 

[mrehmus]

This article was written by a friend who was the go-to guy when someone could not get his model engine to run. George is gone but not forgotten.

 

[Basil]

This article was written by a friend who was the go-to guy when someone could not get his model engine to run. George is gone but not forgotten.
View attachment 123244

Nice information there! Thanks