SPI engine

Another picture -

I might be wrong Matt but I think Joe was after it all connected up.

PM sent Joe.

,,and I think he was after a pic of the cylinder right side up looking down at the piston.

Just a guess.

PV Pilot wrote:PM sent Joe.

,,and I think he was after a pic of the cylinder right side up looking down at the piston.

Just a guess.

That might be TDC in the pic just the cylinder is upside down!

A topside pic of an SPI piston wouldn't show much.

I was trying to see how much of a gap there is. I ran into some Baby Bee's that appear to have SPI. The only Bee I was aware of with SPI were the Super Bee's that came stock on the early P-40's. In fact one of these stamped P-40. A picture with the engine assembled might help.
Joe

SPI should be in the range of .012" - .025". You can measure it with a flat feeler gauge. I'm headed to the shop later and I'll take a picture. I've lost a bit of SPI resetting a piston rod. I shortened one of my Black Widow pistons because of that and it made a noticeable difference.

I have a Babe Bee with SPI. It has one Bypass port. It has a 5cc tank that says Thimble Drome on it. I was told by one of the guys on here that it was from the 70's.

Godsey3.0 wrote:I have a Babe Bee with SPI. It has one Bypass port. It has a 5cc tank that says Thimble Drome on it. I was told by one of the guys on here that it was from the 70's.

One bypass port... what a rip. The old .049 Medallion #2 had one bypass too. But it makes them easy to crank and easy to needle.

It gets really nice pull with a 6x4. Unfortunately, I broke my last one of those a long time ago. Need to find another good size for it. Maybe 5x4 or Three Blade 5x3

Godsey3.0 wrote:It gets really nice pull with a 6x4. Unfortunately, I broke my last one of those a long time ago. Need to find another good size for it. Maybe 5x4 or Three Blade 5x3

It will flat boogie with a 5x4. That's a speed prop. A 3 blade 5x3 has less pull than a 2 blade 5x3. They cavitate.

Rusty: I did not know that. I have a plane that at one point flew VERY nice with Bernies RC Flyer Engine (My first) run on Omega 12% fuel. When that engine died I put an OK Cub on it and it almost flew. That was not good fuel for the Cub. You could screw the needle in and it would not peak. Now it does with Sig Champion 15% and 25%. I know have a standard Sure Start Pylinder (Cylinder and Piston. When talking to my dad I tend to say this. Instead of trying to stop, we just use this term) on a Black Widow crank case and tank. We put a Three Blade 5x3 on it. If that does not fly it then I will try the 5x4 and then 6x3. The Black Widows Pylinder is on my RC Bee which is VERY loud and my fastest running engine. 12cc of Fuel in 1 minute and 30 seconds. I have this on my Combat Wing.

Sorry for straying off topic.

This engine also lost some of its SPI after Ball Socket Resetting. I have been thinking about going the Rusty route and sanding it and the marker and all that, but I worry about messing it up. So I will deal with how it is. I am about to convert it to diesel.

Thanks for posting the picture.

John Goddard wrote:I might be wrong Matt but I think Joe was after it all connected up.

I put the set on an engine, and got it exactly at TDC. The removed it and took the picture.

That is an SPI piston from Bernie.

Here is my theory on the SPI ball socket reset issue. If this has been covered before I apologize.

When the socket is loose before the reset the top of the rod ball slams into the top of ball socket when running and is no longer round but flat. So when you reset the socket not only do you tighten up the ball/socket fit you also take up the play and then some as the flattened top is allowed to squeeze in further. So in effect you just shortened the rod.

That's my theory anyway......

Hmm.. never thought of that. I have a worn out piston that I will gut open. I will see if the ball is flattened. It may be some time till I can get it open.

Time for a Dremel!

Here is the BW with the slightly shortened piston:


This is the one I'm rebuilding with the mouse racers' method. I polished the crank bearings to a mirror finish. When I push it away from the TDC pinch the weight of the piston rotates itself to the bottom. It kept doing it while I was trying to take this picture.

cribbs74 wrote:
When the socket is loose before the reset the top of the rod ball slams into the top of ball socket when running and is no longer round but flat. So when you reset the socket not only do you tighten up the ball/socket fit you also take up the play and then some as the flattened top is allowed to squeeze in further. So in effect you just shortened the rod.

That's not quite how I see it...

The rod wears away at the piston during running and the piston continuously reaches less and less high up at the TDC.
There is no slopping or slamming of these parts as the piston is always under a compression load at regular rpms (below about 22-24000rpm).

When you reset the piston properly, you only alter the position of the rod under tension. But it should never experience tension during running, so in effect you have not altered anything to the running engine and the performance is the same.

It is only engines that are being pushed into high revs that will need a reset of the piston, as there might be tension in the rod at some parts of the engine cycle.

Surfer_kris wrote:

cribbs74 wrote:
When the socket is loose before the reset the top of the rod ball slams into the top of ball socket when running and is no longer round but flat. So when you reset the socket not only do you tighten up the ball/socket fit you also take up the play and then some as the flattened top is allowed to squeeze in further. So in effect you just shortened the rod.

That's not quite how I see it...

The rod wears away at the piston during running and the piston continuously reaches less and less high up at the TDC.
There is no slopping or slamming of these parts as the piston is always under a compression load at regular rpms (below about 22-24000rpm).

When you reset the piston properly, you only alter the position of the rod under tension. But it should never experience tension during running, so in effect you have not altered anything to the running engine and the performance is the same.

It is only engines that are being pushed into high revs that will need a reset of the piston, as there might be tension in the rod at some parts of the engine cycle.

My tach race engine was in dire need of a reset and I doubt that engine has ever done 22K in it's entire life. It had an audible click when run through by hand.

All I know is that once reset there is a flat spot on the top of the rod that could be felt. And I doubt it was machined that way from the factory.

On a car engine when a rod bearing is bad you hear a knock and I have seen flattened rod journals in excessively worn engines. A cox engine is no different in load/tension etc. Except in a smaller scale.

Much like a rod journal the ball socket is a bearing surface that must have correct gap or play.

Sounds like you are comparing a four-stroke with a two-stroke engine. When you turn a cox engine over TDC and it has compression you will not hear any click as the rod is constantly under compression.

Here is a picture of the wear the rod can make in the piston (there is no flat spot, just a wear on the piston);

[/quote]

That's not quite how I see it...

The rod wears away at the piston during running and the piston continuously reaches less and less high up at the TDC.
There is no slopping or slamming of these parts as the piston is always under a compression load at regular rpms (below about 22-24000rpm)

It is only engines that are being pushed into high revs that will need a reset of the piston, as there might be tension in the rod at some parts of the engine cycle.[/quote]

Can't agree Kris.
The weight of the piston aided by centrifugal force will overcome the compression every time and in effect the conrod becomes
longer, the compresion ratio alters along with ignition timing.
I used to fit billet forged rods to Cosworth engines because the standard ones would 'grow' (prior to shearing) altering
the compression ratio amongst other unwanted nasties which in a Turbo charged engine is a very expensive mistake.
I forget the exact physics but with forced induction it was desirable (tho not possible) to lower the C/R at high revs/boost.
J

John Goddard wrote:
The weight of the piston aided by centrifugal force will overcome the compression every time and in effect the conrod becomes
longer, the compresion ratio alters along with ignition timing.

I have not seen that on a cox engine running below 20000rpm. There is a balance between rpm and piston weight, and there would be a transition rpm where the conrod starts to experience strain. Has anyone seen any wear in the piston of a cox engine suggesting that there would be strain forces involved at regular rpms?

A four stroke engine is very different as you do have strain forces for every second evolution.