Author Topic: Lapping small pistons  (Read 7396 times)

Offline Danny M2Z

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Lapping small pistons
« on: September 07, 2013, 09:11:52 AM »
G'day. What a lovely site. I am a refugee from the Yahoo groups (Got 'Neo'd  >:( ).

http://yahoo.uservoice.com/forums/209451 says it all!

I was looking through the posts about lapping pistons etc. and thought I might share a technique for an adjustable lap that may be of use to somebody. I have not managed to read all the posts so I apologise if this is 'old hat'

Measure twice - cut once. You can't put it back!

Offline steamer

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Re: Lapping small pistons
« Reply #1 on: September 07, 2013, 10:31:34 AM »
Nope!   Haven't seen a die stock used as a lap holder....cool idea!....Why Not!

Thanks for that!

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline petertha

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Re: Lapping small pistons
« Reply #2 on: September 09, 2013, 06:55:53 AM »
Thats neat. I'm interested in the technique for some future work. Can you elaborate a bit on materials & lapping compound? Is it kind of a progressibe coarse>fine grit process? I always assumed the 'lapper' part had to me softer than the material you were cutting so as to embed the lapping compound. Looks like you have aluminum on aluminum? Or maybe they both wear, but who cares as long as you get down to the desired OD & finish? How do you then flush off lapping grit off the piston? How do you go about gauging the piston size to its desired final dimension - snug fit into known cylinder bore or soemhting?

Offline Danny M2Z

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Re: Lapping small pistons
« Reply #3 on: September 09, 2013, 07:34:35 AM »
Thats neat. I'm interested in the technique for some future work. Can you elaborate a bit on materials & lapping compound?

The piston is cast iron. The lap is aluminium. The diestock allows the lap to be adjusted like a thread die. The lapping compound is progressive 600 grit, 800 grit and then 1200 grit.

If I need a finer finish jewellers rouge is handy mixed with oil. Lol, I have even used toothpaste and a friend uses cigarette tobacco ash to polish glass lenses

I wash the lapping fluid off with petrol and a toothbrush. That is why I do not use diamond, very hard to clear from the piston without an ultrasonic device.

To snug the piston to the liner I use 'tallow'. It is very old idea. Rendered pig's fat but it gives a lovely feel to the fit. With a small diesel engine the fit must be precise. Yep, I snug the liner to the piston without removing the piston from the chuck. That's when to also turn the contra-piston.

Cu Later * Danny M *
« Last Edit: September 09, 2013, 07:45:25 AM by Danny M2Z »
Measure twice - cut once. You can't put it back!

Offline petertha

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Re: Lapping small pistons
« Reply #4 on: September 13, 2013, 07:54:17 PM »
Excellent thanks.
I also noticed the slot phasing of your aluminum lap shown in the pic with the die holder - an open through-cut so it can contract; 2 external slots, phased equally apart. And the pinch grub screws occur on either side of the through-slot. Is this the general layout principle? I guess to facilitate uniform ID reduction as its adjusted through lapping?

Can you elaborate on the clearance dimensions. For example, if the piston was 0.500" target finish diameter, then the lap ID would be bored to ???.

About how many times would you make a tightening tweak?

Offline Brian Rupnow

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Re: Lapping small pistons
« Reply #5 on: September 13, 2013, 08:32:31 PM »
DannyM2Z--I simply have to ask. What are you using for rings on the piston in that engine. If you finish up with a lapping compound of 1200 grit, there is no way that cast iron rings are going to seat properly. Of course, I am thinking i.c. engine. I suppose if it is a steam engine, then smoother is better.--Brian

Offline Jasonb

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Re: Lapping small pistons
« Reply #6 on: September 13, 2013, 08:46:51 PM »
Brian, its common not to use rings on glow (nitro) and compression diesel engines of below 1" bore and the cylinder and piston are lapped to each other to get a good close fit.

J

Offline Brian Rupnow

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Re: Lapping small pistons
« Reply #7 on: September 13, 2013, 08:51:00 PM »
Okay--Thank you JasonB

Offline dieselpilot

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Re: Lapping small pistons
« Reply #8 on: September 13, 2013, 08:59:37 PM »
Just curious, how well does the fit work out when you're sizing the piston upside down like that? Is the cylinder or piston tapered any?

Offline steamer

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Re: Lapping small pistons
« Reply #9 on: September 13, 2013, 11:05:09 PM »
Yes the cylinder is tapered and tighter at TDC..

An excellent treatise on this subject can be found here.

http://www.modelenginemaker.com/index.php/topic,1908.0.html

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline Danny M2Z

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Re: Lapping small pistons
« Reply #10 on: September 15, 2013, 01:07:31 AM »
Excellent thanks.
I also noticed the slot phasing of your aluminum lap shown in the pic with the die holder - an open through-cut so it can contract; 2 external slots, phased equally apart. And the pinch grub screws occur on either side of the through-slot. Is this the general layout principle? I guess to facilitate uniform ID reduction as its adjusted through lapping?

Can you elaborate on the clearance dimensions. For example, if the piston was 0.500" target finish diameter, then the lap ID would be bored to ???.

About how many times would you make a tightening tweak?

G'day
To answer your questions (Sorry about the delay).

I made the lap the same external dimensions as the threading dies that are used with that holder, including the two dimples for the adjusting screws as these also prevent the lap from rotating in the holder. The cuts were made by hand with a junior hacksaw.

The cylinder bore is 0.510" so I bored the lap about 0.512". This hole will probably slowly grow as the lap wears.

I lap until for a minute and then clean the piston, measure the diameter and test if the liner will start. The liner has a very small taper. I lap until the piston will just fit to the exhaust ports and then use a finer lapping paste with only very brief bursts of gentle lapping as it is very easy to overdo it at this stage. 

I have a very old book 'Model Aero Engine Encyclopaedia'. To quote "Tallow is very useful in preventing the parts from sticking when close fits are being obtained. The correct fit is such that, with tallow the parts will pass fairly freely. They will then feel a little stiff when lubricated with fuel."

I make a tweak to tighten if the lap stops cutting, maybe once or twice. It all depends on how close I made the initial piston diameter. With my small diesel engines I probably (try to) turn them about 0.003" oversize.

With a small diesel the piston/liner fit is a little more critical than with a small glow engine because very good compression is required for starting (in my opinion).

When I shoot a wild pig, my neighbour makes sausage from the meat. Part of the deal is that apart from some sausage I get a jar of tallow. Tallow is pig's fat that has been rendered. It also gives a nice finish when turning aluminium (used it when out of kerosine) but then it smells a bit.

Regards from the land of the kangaroo

* Danny M *
 
Measure twice - cut once. You can't put it back!

Offline dieselpilot

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Re: Lapping small pistons
« Reply #11 on: September 15, 2013, 02:40:42 AM »
Dave, Ramon's thread is excellent. I've just never seen a piston made this way (still on the barstock) and fitted to the liner upside down. I'm just curious how well this works.

Greg

Offline steamer

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Re: Lapping small pistons
« Reply #12 on: September 15, 2013, 03:05:59 AM »
Hey Dan,

There's a thousand ways to skin the cat....this is one

As I understand it, Ramon has used his engines successfully in competition for many years...so....he's doing something right!

But

It's not the only way...and I'm sure Danny is successful doing it his way.

Please continue! :cheers:

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline dieselpilot

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Re: Lapping small pistons
« Reply #13 on: September 15, 2013, 02:15:57 PM »
I read Ramon's thread when he posted it, and again now. He uses the typical method. This method being presented here is a bit different, or am I not seeing the photos correctly?

 

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