piston fit

[Jim Allen]

Interesting stuff!

I have been making pistons for engines from .21 to 1 cube for a few years now.

I have been using RSA 431 T6 with great results. The T6 heat treatment for this alloy has only been around the last 3 or 4 years, (could be longer)

no need to heat treat them.

As for machining them PCD inserts are the only way to go. Also TiB2 coated endmills are great in this material.

If machined correctly there is no need to lap them in with any abrasives.

Brad & ken,

This is good information to have on hand because sometimes the 444 alloy is not available. I usually keep more than 200 finish machined pistons on hand from .8035" to .8045" in .0001" steps, for fitting to honed cylinders. I have the same number of heads which are carefully fitted also to prevent any detonation in the space where the head fits inside the cylinder. The only thing left to do to the pistons is to de-burr & remove any sharp inside corners on the bottom inside skirt radiuses. So far this has prevented any piston cracking or breaking for the past two years. I finish hone the wrist pin hole to size & we are presently using a pressed in wrist pin set up.

Jim Allen

 

[Terry Keeley]

Thanks for the replies gents, what a wealth of information these forums are. Nothing can replace "real world" experience!

Looks like the RSA-444 has the lowest expansion rate and is the strongest of RSA's model engine alloys:

http://www.rsp-technology.com/RSP%20Technology%20-%20Micro%20Engins.pdf

444 & 431 are close tho.

I'm starting to think about re-chroming and grinding some CMB 67 sleeves for my SAW program. I'd like to try some lower taper with the high Si piston material, think there might be some gains there. Still have my NSK Falcon grinder and am going to fit it to my new Emco Super 11 with the Pratt Bernerd collet system. Here's a video of the grinder if interested:

 

[Jim Allen]

Terry,

Did you mean 431 instead of 441? The chart you show clearly shows why 444 would be a better choice. A look at the thermal expansion, stiffness, specific stiffness, thermal conductivity, elongation, hardness, ultimate tensile strengths at various elevated temperatures & fatigue strengths says a great deal! Is there a difference in the price of the two alloys?

Jim Allen

My pictures: https://www.intlwaters.com/gallery/index.php?cat=10141

 

[Marty Davis]

Not having a machining or materials background I can't comment on alloys, etc.

I can comment on the finished product.

It is a VERY fine line between too hard (brittle) and not hard enough for a piston.

I have had pistons that were so brittle that they failed quickly with either a breaking of the skirt off or actual powdering of the piston itself.

I have also bought engines that were great for 1 or 2 runs and then the fit went away because the piston was to soft.

I can tell you without reservation that the piston material on one of the major brands of engines is perfect for logivity while the material for another of the major brands is much to soft and will not last.

I know also that the roundness and taper of the liner inner bore is one of the super important components of fit and longivity. I have used a helical lap to make sure the piston is round and a Sunnen Hone on the liner bore to make sure it is also round. Using Cork Bond for a liner finish reduces the amount of time to get the "shinny band" at the top of the seal and also makes the liner finish all over as perfect as I have seen.

I don't have the capability to make pistons so most of you experts comments about alloys and machining technique is WAY over my head and capabilities.

Great info, but not something that I have the ability to utilize.

 

[Terry Keeley]

Terry,

Did you mean 431 instead of 441? The chart you show clearly shows why 444 would be a better choice. A look at the thermal expansion, stiffness, specific stiffness, thermal conductivity, elongation, hardness, ultimate tensile strengths at various elevated temperatures & fatigue strengths says a great deal! Is there a difference in the price of the two alloys?

Jim Allen

My pictures: https://www.intlwaters.com/gallery/index.php?cat=10141

Yup! Corrected!

Will try to get my hands on some 444...

 

[Jim Allen]

Marty,

With more than 50+ years of building from scratch various size ABC & AAC engines I completely agree with two very important things you stated. "I know also that the roundness and taper of the liner inner bore is one of the super important components of fit and longevity. I have used a helical lap to make sure the piston is round and a Sunnen Hone on the liner bore to make sure it is also round. Using Cork Bond for a liner finish reduces the amount of time to get the "shinny band" at the top of the seal and also makes the liner finish all over as perfect as I have seen."

Liners no longer have a cross hatch pattern to hold the lubricant & help to seat the piston. The very smooth, round, hard chromed surface is there before the engine is ever run. Hard chromed cylinders are first honed with a 61 grit stone for size & the exact taper amount on the Sunnen honing machine. They are finish honed with a 83 grit stone to a mirror finish. There is no cross hatch pattern visible in these cylinders! Other engine builders such as John Erik Faulk, John Ellis & Profi have found the same things to be necessary for maximum HP & maximum engine life.

Jim Allen

 

[Marty Davis]

Marty,

With more than 50+ years of building from scratch various size ABC & AAC engines I completely agree with two very important things you stated. "I know also that the roundness and taper of the liner inner bore is one of the super important components of fit and longevity. I have used a helical lap to make sure the piston is round and a Sunnen Hone on the liner bore to make sure it is also round. Using Cork Bond for a liner finish reduces the amount of time to get the "shinny band" at the top of the seal and also makes the liner finish all over as perfect as I have seen."

Liners no longer have a cross hatch pattern to hold the lubricant & help to seat the piston. The very smooth, round, hard chromed surface is there before the engine is ever run. Hard chromed cylinders are first honed with a 61 grit stone for size & the exact taper amount on the Sunnen honing machine. They are finish honed with a 83 grit stone to a mirror finish. There is no cross hatch pattern visible in these cylinders! Other engine builders such as John Erik Faulk, John Ellis & Profi have found the same things to be necessary for maximum HP & maximum engine life.

Jim Allen

REALLY beautiful work. Just way above my capabilities.

I am enjoying the thread, but don't have much to add to the discussion. Keep it up....We can learn from this information.

Isn't it interesting the areas that people get "into" in our hobby. Some like the racing part only and don't want to do much themselves, some do a little of both, etc etc. Personally, I enjoy the hull building part, optimizing everything and for sure the racing part.

The engines available today are a great starting point for those that love to optimize. For those that don't, they are really competitive right out of the box.

 

[Chris Wood]

i personally hate engine work lol. i have done some and it drives me batty.. that is why i have dad do that.. i build the boats and set them up..

some people have a nack for certain things like marty mentioned. me i am not a machinist.. never have. never will. so most of this is above my head. one thing i do find interesting in these motors is the lack of consistency. some come out of the box real close.. others look like a beaver worked on it and needs all kinds of fixing.. lol

with today's modern machines i can't figure out how diamentions are so far off from engine to engine.. I understand when a tool dulls and it is change out time but other than that it makes little sense to me

 

[Jim Allen]

What you're saying is very true Chris. At Aero Precision we are using late model Mori Seki & Makino CNC machines to make parts that require critical sizes, finishes & close roundness tolerances. (typically .000050") Being able to control these parameters allows for production engines to easily be built to the same performance standard. We previously bench tested every engine sold to a customer until we got the engine building process down to a numerical science. Now everything is built according to the numbers we found through extensive testing to be the best. Each & every part is inspected while the machines are running. Believe me when I say, if you cannot repeat whatever is the best number found, engine performance will vary all over the place.

Jim Allen

 

[dwilfong]

It is easy to tell when time is up or never got started with a eng.

I can tell if the eng will make good power from how the piston feels in the eng.

What I am explaining is what I have experience with piston fit.

went threw 3 batches of pistons in my 1.01. 4 or 5 in each batch.

The piston tapper was the hardest part to get right as in what I felt was good.

With the plane old 4032 in stock sleeves thy needed almost .00015 relief at the top to seat right.

The wear band was just above the pin when lapped in and thy stopped abruptly. No mushy feeling just stopped right before TDC.

After a few tanks of fuel 2 or 3 thy would have almost no pinch at the top.

But with the plug in you could get a seal with no hissing right after thy passed the ex port. I could hold it at TDC for as long as I wanted and it would pop back and go back to TDC and bounce back again.

If I just flip the eng over with my hand it would bounce back and forth a FEW times from the seal.

No drag at all just bounce back and forth.

That is when I knew the eng is right.

Did not take long to get to this state with the 4032 but you hade to be on it good to not run it lean as this would go away.

As long as I ran the eng at the same mixture and I keep my eye on the ex smoke it was the bomb.

But just one day of leaning out to see where it would go and it was down hill from there.

I have a eng right now that has the same piston in it for 2 years. Gallons of 60% fuel threw it.

Hard to keep a plug in it any more as the fit is gone. But Dang it runs good if you are carful with the needle.

Most are stuck with stock liners as thy cant make there own. The 4032 is a very good choice for those that can't make there own custom liners as it is easy to lap them in a used liner.

I use a flex hone of Levigated Alumina to polish the liners in conjunction with the lapping.

Easy to get and cost affective for me.

Just sharing what I have done and found that works for me.

 

[Brad Christy]

Terry,

I will refer you back to my post #174 and repeat. RSP Technologies (the company that makes the stuff) suggested RSA431 over RSA444 for our application.

Chris,

I completely agree. With the machines available today, even the cheap ones, these engines should all be within .001" of each other. I can't think of a single reason they can't.

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

 

[dave roach]

I have been running one of Brad pistons for 8 years with no problem.The sleeve is going south.

 

[NeilLickfold]

Terry,

I will refer you back to my post #174 and repeat. RSP Technologies (the company that makes the stuff) suggested RSA431 over RSA444 for our application.

Chris,

I completely agree. With the machines available today, even the cheap ones, these engines should all be within .001" of each other. I can't think of a single reason they can't.

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

The main reason some companies liners are different sizes, is because of the chroming process. So it becomes easier to hone a liner to a minimum clean up and then fit a piston and head as compared to the extra time in honing out a sleeve to a specific size. When honing a sleeve, the hone only takes out the average of the average of the area. So around the ports, the honing process will take more off the side with the least area. When grinding, it will just take off material through the centreline of the grinder, either straight line path or the programed path if it is cnc. Liner batches are often turned with about 0.0002 inch size variation, sometimes 0.0003 inches, depending on weather conditions and tool wear and material stress.

Neil

 

[Terry Keeley]

Looking forward to trying this again, had pretty good success 20 years ago and now with the internet and some help from very knowledgeable people I'm sure it will be even better.

Here's the grinder we built using a Compact 5 & the Falcon unit:

Think it'll be a little better fit on my new Super 11!

 

[Jim Allen]

Terry,

You will love what this machine can do. I have mine set up to do all the engine building stuff including cylinder grinding with the Themac tool post grinder.

Jim