Worn out P/S?

[BobBonahoom]

Guys,

What are the symptoms of a worn our piston/sleeve? I have a 21 Nova DD with absolutely no pinch left at the top. It has trouble holding a needle on the stand, goes lean (high RPM) then calms down, then repeats. Very erratic. Is that what you would expect?

I have the contact info for the guy in Ohio who squeezes them, but wondered if the strange things I am seeing are symptoms of a loose fit?

 

[TimothyJones]

Sounds like a bearing,hole in fuel system sucking air maybe...

I have worn out motors and personal experience haven't had that issue.

 

[Ray Sametz]

Guys,

What are the symptoms of a worn our piston/sleeve? I have a 21 Nova DD with absolutely no pinch left at the top. It has trouble holding a needle on the stand, goes lean (high RPM) then calms down, then repeats. Very erratic. Is that what you would expect?

I have the contact info for the guy in Ohio who squeezes them, but wondered if the strange things I am seeing are symptoms of a loose fit?

Bob, I had a motor with absolutely no pinch, but when run and brought up to temp and shut down, I turned it over by hand and it was tighter then a bulls azz in fly town. The piston was expanding from heat just right. Think your problem is somewhere else. OMHO

Give it a try.

 

[dave roach]

Bob

Like Tim said i would be looking at a fuel problem.I have a engine with no pinch at the top and runs great.I would replace all the fuel line if it was me.It could have a broken tube in the tank.Had a friend that was having problems get his boat to to go for 4 weeks we work on it and what to a race and the boat was sitting and the stand with a full tank of fuel and i what down to run my boat and come back there was fuel all over the table from his boat and it had a small hole in the fuel line.replace the fuel line and his problem was fix after 4 weeks.

Dave

 

[BobBonahoom]

That's helpful. I was sure it was an air leak in a fuel line, tank or something like that. Changed all the fuel and pressure lines, pressure tested the needle valve, fuel tank, made sure the filter was clean and not leaking and I even changed carbs. I checked everything and came down to either bearings or the loose fit. I will try the bearings next as I have a spare set.

 

[dave roach]

Bob

The guy her in Ohio does great work on sleeve it done on a CNC machine. If you get one pinched form him go with the lite pinch.

Dave

 

[John Beardslee]

John is currently recovering from back surgery and out commission for a few months and isnt taking on any work. His piston and sleeve pinch work is excellent. Worth waiting for!

Thanks John

 

[Terry Keeley]

I'd also look for the motor itself sucking air somewhere. Crank seal, head, exhaust, carb, sleeve to case...

 

[tom galdys]

Check and clean the pressure fitting in your tuned pipe. Especially if it is pointing down.

 

[BobBonahoom]

Good call Tom. I always point them up and I put a filter on the pressure line, but I never thought to check the fitting and the filter!

 

[Jim Allen]

"What are the symptoms of a worn out piston/sleeve? I have a 21 Nova DD with absolutely no pinch left at the top."

Bob,

A loose fitting ABC or AAC piston & sleeve, in a tuned piped engine, doesn't necessarily mean the engine in question is worn out! ABC or AAC tuned pipe engines can still run strong even when the piston has lost most of its "pinch" at the top of the stroke. It is a proven fact that a looser fitting ABC or AAC tuned pipe engine will run faster, but will not produce high torque amounts at low & mid-range RPM's. The lower torque amounts can easily be seen when attempting to pipe up a looser fitting engine that is under load. A much more important factor for ABC or AAC tuned piped engines is how round the piston, sleeve (within .0001" works best) combination are, as well as how round they remain at WOT temperatures & RPM's. Sound mechanical design of the piston, sleeve combination make this possible!

Jim Allen

 

[BobBonahoom]

I found my problem. It is an air leak near the carb. Fixed it and everything works well now.

Good info from Jim Allen on piston fit, etc. I seem to recall that years ago the first ABC engines used a cast, very high silicone aluminum alloy because it's thermal coefficient of expansion was a better match to the brass sleeve. With a better match the engine did not require as tight a cold fit because fit was maintained across all temperatures. I could be wrong about that, but that is what I seem to recall. Does anyone out there know if this is right? Jim perhaps?...

 

[Jim Allen]

Bob,

In the very early days of ABC & AAC engine building, a tight fit between the piston & sleeve, plus a course crosshatch honing pattern was used to compensate for the "out of round" (+ or - .0002") condition of the piston & sleeve. A roundness (+ or - .00005") of both parts is absolutely essential before anything works properly! We eventually discovered that the tight fit method does not correct "out of round" problems & it does not extend high RPM engine life! Some engine builders thought that higher amounts of silicon in aluminum alloys used for pistons would prevent piston scuffing or engine seizing. This is also untrue, as the answer to this problem involves the cylinder taper amount & the piston top taper amount. You are correct about what cold fit can & should be used with the many possible piston, cylinder alloy combinations that can be used. We have tested & used in production miniature racing engines the following piston alloys; A-390, Dispal 250, Mahle-138, RSA-431-T6 & RSA-444-T6. We have tested & used in production racing engines the following cylinder alloys; C 36000 brass, C 54400 phosphor bronze & 4032-T6 aluminum. All of these alloys require different cold fits! However, Nothing works properly until the involved parts, the piston & the sleeve, are properly machined & finished! The correct mechanical design of these parts is also absolutely necessary!

Jim Allen