1/5 Chenery Gnome Rotary: Piston fit with cylinder

[albd5a]

This is a general question; but before I fabricate nine pistons to the plans dimensions: how should the fit within the cylinder walls be - both with the rings and without rings in place? (Photos show ‘slave’ fixture for aluminum piston fabrication).

 

[Jasonb]

With 1thou clearance the piston should slide freely within the cylinder, you will feel resistance when the rings are fitted particularly when new before they bed in.

 

[petertha]

Not sure if it is directly applicable to your build but Terry typically includes a nice dimensional sketch of his pistons within the bores and he directly relates them to shop made piston rings using Trimble method originally published in SIC magazine. Attaching sample pics here, but worth reading the build details. For example link to his Offy engine (post 453). Now people make rings using various methods other Trimble's method, which is a whole subject unto itself. Do a forum search & you will get LOTS to read. I'm just trying to say designing the piston will typically include considerations to intended compression rings above & beyond annular clearance.

https://www.homemodelenginemachinist.com/threads/270-offy.31486/page-23

 

[rklopp]

One thousandth clearance may not be enough for an aluminum piston in an iron or steel bore. I built a Breisch Hired Man with about that much clearance, and the piston started to seize when the engine got hot. I turned another 0.002" off the diameter, and it has run fine ever since. Aluminum expands at about double the rate of steel. The piston rings do all the work of sealing the gases while the piston/bore fit is just a guide.

 

[albd5a]

One thousandth clearance may not be enough for an aluminum piston in an iron or steel bore. I built a Breisch Hired Man with about that much clearance, and the piston started to seize when the engine got hot. I turned another 0.002" off the diameter, and it has run fine ever since. Aluminum expands at about double the rate of steel. The piston rings do all the work of sealing the gases while the piston/bore fit is just a guide.

 

[albd5a]

Thanks, rklopp. This answer/explanation makes sense to me. Sounds like about 3 thous total. The larger the tolerance the easier… There will be two cast iron rings. There’s a guy on YouTube video that shows a very straightforward way of making these. I’ve read a lot of the articles on how to make rings. I found that many of them were poorly written and they just refer about each other’s work with no substantial details.
It almost seems like I should custom make each piston fit each individual steel cylinder liners… There are nine cylinders total.

 

[petertha]

Personally I think its better to lap all the bores 'the same' & whatever that dimension happens to be is how you make the pistons & particularly the rings to suite. You will have all the messy lapping apparatus set up, so just circulate the cylinders on the same lap tool setting, rinse & repeat. Actually even this isn't 100% precise because the lap will wear a bit from cyl-1 to cyl-9, so alter the order or keep track of the finishing lap, but hopefully you get my point. After break in for sure keep the rings mated to their cylinders, but making a custom ring per cylinder sounds like a lot of unnecessary work to me.

 

[albd5a]

Not sure if it is directly applicable to your build but Terry typically includes a nice dimensional sketch of his pistons within the bores and he directly relates them to shop made piston rings using Trimble method originally published in SIC magazine. Attaching sample pics here, but worth reading the build details. For example link to his Offy engine (post 453). Now people make rings using various methods other Trimble's method, which is a whole subject unto itself. Do a forum search & you will get LOTS to read. I'm just trying to say designing the piston will typically include considerations to intended compression rings above & beyond annular clearance.

https://www.homemodelenginemachinist.com/threads/270-offy.31486/page-23

Terry’s info was great and interesting.

 

[albd5a]

Personally I think it’s better to lap all the bores 'the same' & whatever that dimension happens to be is how you make the pistons & particularly the rings to suite. You will have all the messy lapping apparatus set up, so just circulate the cylinders on the same lap tool setting, rinse & repeat. Actually even this isn't 100% precise because the lap will wear a bit from cyl-1 to cyl-9, so alter the order or keep track of the finishing lap, but hopefully you get my point. After break in for sure keep the rings mated to their cylinders, but making a custom ring per cylinder sounds like a lot of unnecessary work to me.

Is the purpose of lapping to smooth the steel cylinder liners or two size the steel cylinder liners with more precision? Obviously it would be easier to work the aluminum pistons rather than the steel liner… Which is basically what you just said.

 

[Jasonb]

It is usual to hone the cylinder to remove cutter marks and refine the surface for the rings to run against so as you have to do that anyway may as well try to get the cylinders the same.

The conical shape of the crown of the piston will take most of the heat and expansion so the drawings 1thou clearance may well be fine.