Re-machined piston bosses

[Jim Allen]

A simple two part fixture allows the re-machining of existing stock piston bosses so that the custom connecting rods used can be guided in the upper end where the rotational speeds are much lower than the bottom end. The total clearance in the upper end between the piston bosses is now maintained at .005" to .006". I use two hardened steel washers on both sides of the bushed connecting rod. The clearance in the bottom end between the crank haves is .015" to .017". Allowing the connecting rod to float back & forth when the engine is running at WOT can not be a good thing???

Jim Allen

 

[brad nichols]

Great work as usual Jim!!

Brad

 

[Jim Allen]

Great work as usual Jim!!

Brad

Thanks. The question is, what is the reason for doing this?

 

[David Bryant]

got a general (sort of inter-related) question for you Jim:

How critical is wrist-pin fit? Should it be a very snug/tight fit into it's bore? Or is slightly lose okay as well? (gas or nitro engines)

 

[Jim Allen]

Good question David,

It would appear from building many high RPM (25,000 to 35,000), 1.2 cu in gas & .40 cu in to 1.1 cu in nitro engines, that any looseness in the piston's wrist pin holes or the upper end of the connecting rod can cause piston & connecting rod failures. It is also necessary in the majority of these high speed engines to guide the connecting rod between the piston bosses where the rotational speeds are much lower compared to the bottom end. I purposely posted a photo of the fixture & small hand press used to do this in all Nelson racing engines. This is a small but important feature that is not done in my manufactured engines. The easiest way to do this is by honing both the connecting rod & the wrist pin holes.

Jim Allen

 

[David Bryant]

thank you. It appears that a "press fit" into the honed boss is optimal?

 

[Jim Allen]

Doing this solved the piston, upper end connecting rod failure problems found in the FAI MB 40 engine (32,000 to 35,000 RPM). These pistons were honed at Aero Precision Machine. It is also possible to bore the wrist pin holes in the piston to the correct press fit with the fixture I showed. Only polycrystalline coated diamond, solid carbide boring bars will work for cutting the silicon aluminum. THE HOLES SHOULD NEVER BE DRILLED & THEN REAMED TO SIZE!

JA

 

[David Bryant]

Doing this solved the piston, upper end connecting rod failure problems found in the FAI MB 40 engine (32,000 to 35,000 RPM). These pistons were honed at Aero Precision Machine. It is also possible to bore the wrist pin holes in the piston to the correct press fit with the fixture I showed. Only polycrystalline coated diamond, solid carbide boring bars will work for cutting the silicon aluminum. THE HOLES SHOULD NEVER BE DRILLED & THEN REAMED TO SIZE!

JA

View attachment 100233

used in a mill?

 

[Jim Allen]

YES! Holding the piston in a "V" block type fixture. Use gage pins, not an inside micrometer or a bore gage, to get the hole to the exact correct size.

 

[David Bryant]

YES! Holding the piston in a "V" block type fixture. Use gage pins, not an inside micrometer or a bore gage, to get the hole to the exact correct size.

View attachment 100235

View attachment 100236

View attachment 100237

I see, nice

 

[Steve Seebold]

Nice work Jim, are you doing it on a CNC or conventional machine?

 

[Jim Allen]

Everything I do in my home shop is done with manual machines. I have hundreds of thousands of dollars invested in measuring tools, drills, end mills, materials, equipment, machines & anything that would be found in a very well equipped tool room. It would take some time to photograph what can be found in this tool room. There are very few jobs that I would not be able to do.

Jim Allen

 

[Steve Seebold]

You’re right Jim. A good toolmaker/machinist can do anything a CNC machine will/can do, the CNC will just do multiple parts faster.

Someone told me once “A CNC MACHINE TAKES A GOOD MACHINIST AND MAKES HIM AN EXCELLENT PRODUCER”.

I have a Tormach PCNC 1100 in my shop. I paid right around $14,500.00 for it, brand new. It’s not as fast as a Haas but I can hold +/-.001 with it all day long.

I used to own a real CNC shop where I had a FADAL 3016 box way mill, a Haas TM-1 Toolroom mill and a heart attack.

Now I call myself Mid Day Machining because I seldom get to the shop before 11:00 AM and I’m gone by 5:00 PM.

I’m doing it now mostly for something to do as I get older and to keep my mind sharp. I turned 75 the end of October.

 

[brad nichols]

You guy have us envois of your shops and capabilities!! You can always have the tools but you have to have the skills to use them!! You can be a journeyman but you need to strive to be a craftsman!

Brad

 

[Jim Allen]

Brad,

I think that without the 4 or 5 year training programs similar to what European tool makers went through, learning what is necessary to qualify as a toolmaker becomes very difficult, if not impossible. I hired many of these tool makers, from Italy, Germany, Poland, Switzerland & Yugoslavia who went to academic school for 1/2 of the day & were trained for 1/2 of the day in the basics of tool making. Several of these people that I hired had numbers on their forearms from being in German concentration camps.

Learning to become a toolmaker is a dying art! How many present day machinists have read & understand what is written in the Machinery's Handbook or The New American Machinist's Handbook???

Jim Allen, Tool Maker, M.E.

 

[BobHorowitz]

Jim,
How many can do the math?
Bob

 

[Jim Allen]

Bob,

They all could do the necessary Trigonometry & Algebra math that is required for a tool maker. They had a deep understanding in the use of a sine bar, a planer gage, gage blocks, height gage, surface plate, dividing head & heat treating oven use. They were also extremely competent with all the basic machining operations such as single pointing threads, parting off various materials, OD & ID grinding, surface grinding, milling machine & lathe operations, tooling, etc., etc., etc. They not only understood exactly what they were doing, but they understood the tools that were being used.One of the Yugoslavian Tool Makers was a math professor before he escaped to become a tool maker.

I spent 27 years working with these people. They never stopped learning new techniques & ideas as the technology as well as the tools progressed through the years.

Jim Allen

 

[BobHorowitz]

I was referring to the people trying to come into the profession today.

 

[Jim Allen]

I'm not sure Bob, because there are no schools or programs available. In my 6 years experience at Aero Precision Machine building the Nelson .45 pylon racing engines I saw many machinist that could program CNC machines without any knowledge of the things I mentioned before. They certainly qualified as machinist, however they would not be qualified as Tool Makers.

 

[Brad Christy]

I was referring to the people trying to come into the profession today.

Bob,

What's the joke....? "Two weeks ago, I couldn't even SPELL 'engineer'.... Now I are one."

Thanks. Brad.
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