Cylinder - piston clearance

[deverett]

What does the team think cylinder - piston clearance is ideal?

In this case the cylinder bore is 1.375", it is made from cast iron, and air cooled.
The piston will be made from aluminium and will have 2 rings.
The engine is IC and will be fairly slow revving, although not hit 'n' miss and will not be expected to work hard (demo running, basically).

Dave
The Emerald Isle

 

[bazmak]

My first 2 stroke motor bike with nom 2in bore to your spec
was about 5 thou. Its not too critical with multiple rings.But not too tight
or will bind with heat expansion. Hope this helps but im no expert

 

[retailer]

I believe about 1-1.5 thou per inch of the bore size is ok, although I did once have a Honda S600 roadster, bore approx 50mm, clearance specified as 1/2 thou, it took some time to locate a machine shop that was able to work to those tolerances, no problems with pistons seizing despite seeing 12,000 rpm on the rev counter.
Cheers

 

[deverett]

Thanks for the advice, guys. I'll try 3 thou clearance.

I'd like to say 2.5 thou, but my machining isn't that good! (Yes, I know about swinging the topslide over to about 6 degrees. 10 thou infeed = 1 thou cut)

Dave
The Emerald Isle

 

[Mechanicboy]

Use thermal expansion to calculate size of piston and cylinder if the working temperature and material is known. The piston will not seize in the cylinder by heat of working temperature if correct calculated. See this: http://en.wikipedia.org/wiki/Thermal_expansion

 

[Krutch]

For Harley engines, which use the material/cooling combinations you have, I set the clearance at .0015" for a 3.4375" bore for street use. For racing use I go for .0025 to .003". These figures are for refreshed bores using cast pistons. Forged pistons require more clearance due to the higher expansion rate of the forged material
.002" should be OK for your application.

 

[bobsymack]

Hi Dave
Why don't you make the piston out of cast and just have it as a sliding fit and no need of rings.
Regards Vince

 

[barnesrickw]

Why does cast not need rings, lower expansion rate?

 

[bazmak]

You always need rings of 1 type or another.Getting a dead fit is difficult
Slight use/wear and your fit is lost and only gets worse with time
Hence rings or seals

 

[deverett]

Thanks for the extra replies. In the end, I went for 2.5 thou (as near as I could measure).

For an IC engine either rings or an O-ring will be needed for long life, especially in the material combination that I am using. I know that Paul Breisch used ringless pistons on some of his engines, but they were quite small bore (less than 1" from memory).

Dave
The Emerald Isle