Materials for pistons

[nx06563]

I am building a Howell V-Twin and I am at the piston and ring stage. Has anybody made their pistons with 12L14. The prints call for cast iron cylinders, rings and pistons.

 

[gbritnell]

I have aluminum pistons in my V-twin and I've had no trouble with them. My bore diameter is 1.00 and I started out with .0015 piston to wall clearance. I say started out because it has many hours of running and I haven't remeasured them. I use 2 cast iron rings per piston.
gbritnell

 

[b.lindsey]

I'm with George. I can understand the cylinder and rings being cast iron, but not the pistons themselves. Since the don't actually "touch" the cylinder wall, aluminum with C.I. rings should be enough. Its too bad Jerry is no longer with us though as he could have shed some light on his reasons for selecting C.I. for the pistons. Can anyone think of a reason why the extra mass of C.I. would be of benefit?

Bill

 

[Dave G]

I'm with George and Bill on using al. for pistons. The clearance between piston and cyl wall should be dictated by how warm the engine will be when up to temperature. Air cooled engines sometimes will run hotter than liquid cooled engines. 6061 al. has an expansion rate twice that of steel or cast iron, the al piston will grow when running. My 9 cyl radial has 6061 al. for pistons at 1" bore with .0025" clearance. After running up to temp and then shutting off the prop is hard to turn. When it cools for a few minutes it will rotate much more easily. I suspect the piston has grown reducing the clearance.

Automotive pistons are made from a low expansion rate al. I have found some discarded pistons from a large diesel engine to cut some pieces from to try to make pistons out of but I haven't had time to try this yet. I think one could reduce the clearance this way. Something to think about.

I normally make my cyls from 12L14. Have had no problem using this matl. Rings are made from gray cast iron.

Good luck with your build, Dave

 

[Charles Lamont]

Can anyone think of a reason why the extra mass of C.I. would be of benefit?

Early IC engines were slow running and generally had iron pistons. They can be made a much closer fit because the coefficient of expansion is much less, and they will generally wear a lot better. The extra mass is no benefit, and becomes a problem at higher speeds, which is one reason why alloy pistons are used in faster running engines. Another important consideration is thermal conductivity, and an alloy piston will conduct heat away from the crown better than iron. Again, with a slow running, low compression engine this is less of a problem. For full-size engines, I would guess a reasonable limit for iron pistons would be somewhere around the 2000 rpm mark, but this just a slightly educated guess.

 

[NickG]

Good points there, I know some people also use cast iron with a very close fit which negates the need for any rings. Or could use a viton o ring with a close fitting cast iron piston too. Some people seem to be having good results using graphite pistons these days ... light, low coefficient of expansion similar to cast iron, easy to machine and obtain a good fit, no rings needed, self lubricating, withstands heat well etc etc.

 

[lensman57]

I am building a Howell V-Twin and I am at the piston and ring stage. Has anybody made their pistons with 12L14. The prints call for cast iron cylinders, rings and pistons.

Hi,
You could make the piston from Al, or cast iron. With cast iron piston, rings and the cylinder the running may prove to be smoother but the acceleration will not be as quick as the one with Al piston due to the weight of cast iron. The expansion due to heat will also be the same for all components, if made from cast iron, which will make for a longer engine life.

Regards,

A.G

 

[nx06563]

Thanks for the great responses. Sorry for the slow response from me But I travel in a 5th wheel about four months a year and have been gone. Plus in the summer I don't do a lot in the shop.
Not to question Jerry's wonderful designs, I think I will be using cast iron to start.
The reason for the question was because I always have 12l14 arround and don't keep a lot of cast iron. The COE and weight is closer to cast iron than aluminum.
By the way nobody commented that they have used 12l14 for pistons,
Gee, I guess I have just talked myself into trying it...

 

[nx06563]

Just got the V twin running and am still debugging. The 99% of carburator problems that turn out to be electrical were actually carburator problems. Chased the wrong dog for a while.
The 12L14 pistons seem to be working fine even if throttle response is a little slow.

 

[Till]

The slow response is more likely a carb issue. When you open throttle, the rate of airflow changes very quickly. Due to fluid viscosity, the amount of fuel drawn through the nozzle orifice changes much slower than the rate of airflow. So the mixture gets lean for a short period of time, which causes temporary power loss (slow response). An acceleration pump actuated at full throttle, such as a small plunger pump with checkvalves is the best solution.
When the throttle opens rapidly, pressure in the pump's pressure chamber rises rapidly wich causes the pump's inlet checkvalve to close and the further pressure buildup causes the outlet checkvalve to open and a small amount of fuel leaves the outlet valve right into the carb, creating rich mixture for a very short moment (very vigorous response).
If you open throttle slowly, the pressure wont rise fast enough to close the inlet valve and the fuel flows back through the inletvalve into the fuel line.