grassroots

Right now Iam getting 14K on 6x3 cox prop, only mod done thus far is drilling out the venturi.

Will SPI help to pick up the RPM's?

How much do you take off the bottom of the piston skirt to get SPI?

Also how do you guys remove the material?

Do you need to remove the rod from the piston? If so how is this done?

Thanks for the help!

combatpigg

My Feedback: (3)

To get geared up for this project you will need a holding fixture for the piston. I've heard of using brass tubing with a slit, I use an aluminum bushing with a slit, the wall thickness of the bushing is about 1/8". This will allow you to chuck the piston up without distorting it. Some guys use an electric drill in a vise to do the work. The bottom of the skirt can be raised with the back side of a #11 blade held against a tool rest. Improvise with what you got, just don't go hog wild and distort it. After the skirt is raised, then shave the inner wall of the piston with the blade. Hold the rod off to one side with a wire or string.
.008" SPI is enough. Bring the piston up to TDC and scribe a mark even with the bottom of the exhaust port. Then get to work!

icerinkdad

Before you get too excited about SPI what cylinder does your engine have?   If it is two exaust ports and one intake then the fastest and easiest upgrade is a two intake swap out.    Also there were a couple of different two intake port cylinders made.   Most were manufactured with two simple ports but some were also made for TD engines with one or even two boost ports widening up the existing ports on each side.    Also what reed does your engine have?   The old "brass" reeds were slower than both the steel and mylar reeds used in later engines.   Finally the 6-3 prop was never the optimal prop for Cox engines.  They run better on a 5-3.   Before you go hacking on a piston I would look at all the other options to get more power.

Bob

lildiesel

Going to a 5x3 will give you more of the revs you want. Of course, bench numbers are for bragging rights: the in flight qualities are most important. What will you fly this engine on?

BTW, the faster you run a Sure Start the more you really need to reset the con rod top end ball in the socket.

grassroots

CP,
Can you futher explain or draw me picture?

"The bottom of the skirt can be raised with the back side of a #11 blade held against a tool rest."

Thanks,

Dan

Remby

My Feedback: (3)

I have tried and use slit exhaust cylinders with a cut on one lower slit using a hacksaw procedure. It gives that boost you are looking for, and the single cut can provide a bit of ram-air effect, if the cut is facing the airflow. The details are in the reedie sticky thread.

 I have never compared this to the piston mod, but use it on Medallions and Surestarts, and it works good. Just another option to consider or tinker with.

combatpigg

My Feedback: (3)

grassroots, once you get the piston turning with either a lathe [or an improvised lathe], all you need for a cutting tool to shorten the piston is a #11 Xacto blade held against something solid [known as a tool rest]. The hard steel blade will shave the softer piston. Practice on a junk piston first.

At 400 cycles per second, every microgram of material removed from the piston saves wear and tear on the moving parts and improves rpm.

BMatthews

Grassroots, in case you haven't figured it out from CP's description he's suggesting that you fake a lathe using a hand drill and this Xacto knife method.  If you have a lathe or even a drill press available then you're far better off.

The holder bushing fixture can be made using a lathe, drill press or even a 1/2 inch chuck equipped hand drill.  You don't need the big chuck to MAKE the bushing but you will for using it..

Need more ideas?  Start by telling us what tools you can access.