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Author Topic: Lapping pistons  (Read 750 times)
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Fourfingers
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« on: March 03, 2018, 03:00:09 PM »

Sorry to be a forum hog, but help needed...
Having grown a piston (painful) I am advised it shoild be lapped for a good fit. Much talk of lapping here and there and encouraging noises re. the benefits, but .... no word I can see of how to do it!
Any advice most welcome.  No, I dont have a lathe or any useful gear other than a drill, Brasso and a wrist riddled with RSI / carpal tunnel thingy. (Dont ask).
Thanks
Ff
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gossie
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« Reply #1 on: March 03, 2018, 04:30:18 PM »

Brasso, tooth past, elbow grease.
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Konrad
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« Reply #2 on: March 03, 2018, 05:23:53 PM »

I like to run it rich and wait. But if that isn't practical I'd go to a lens polishing house and try their finer or finest abrasives*. Mix up a slurry to the consistency of toothpaste, maybe even wetter.

First you need to find where the piston binds in the bore. Clean everything very well. Allow the piston to drop into the cylinder note where it stops repeat several times. Note: make sure the piston isn't acting like a piston. That is the cylinder should not be sealing against the table top. Put the slurry in the cylinder just above the ports.

Now with an old set of gudgeon pin and conrod. Mount the piston on these. Fill the gudgeon bore with wax to keep the abrasive out of these bores. Now with the piston on these old parts slide the piston into the bore. Moving the piston with a 1/4 turn twist try to make a 30° twist (ramps, scores) as you push the piston up the bore. Rotate the same way as you pull out the piston (this makes a 1/2 turn of the cylinder for each up and down motion).  After about a dozen of these strokes clean and test as you did earlier. The piston should drop a bit further into the cylinder. Note at first the piston will actually bind earlier as the abrasive will actually knurl the piston at first.

If the engine now runs you don't want the piston to drop much more that the linear equivalent of 10 Degrees of crank rotation. Clean and reassemble run the engine run another break in cycle.

If using toothpaste look for one that is used for Sensitive teeth as it seems to cut better.

How did you come to the conclusion that the piston has grown? And what type of materials is your P&L made from. I assume it is a mehanite (fine grained) cast iron piston in a leaded cylinder.

All the best,
Konrad
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faif2d
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« Reply #3 on: March 03, 2018, 09:24:31 PM »

It also helps if you insert the piston into the cylinder upside down when lapping.  That way you will wear the piston in a taper with the crown slightly larger than the skirt. The cylinder will be tapered no matter which way you insert the piston.  Be very slow in the lapping as you can not add material after you have gone too far.
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Fourfingers
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« Reply #4 on: March 04, 2018, 05:10:22 AM »

Many thanks for helpful replies so far.  To answer Konrad, I 'grew'the meehanite piston by cherry bombing .... dull red hot then quench in oil.  Others say a slow cooling incl the famous George Aldrich.  Who am I to argue?
Its from an old Taifun Rasant completely devoid of compression, but otherwise internally OK.
I assume the liner is hardened steel.
Next problem: compression still very poor.  Anyrhing else to do save a re-bore?
Have seen mentioned careful tapping inside piston crown to flare it slightly.  Not happy about that - piston now brittle I guess, have already chipped it!
Or the tube cutter trick?  No less than Rustler mentioned this: score a groove(s) around piston (where?, assume near top) this raises two tiny ridges, carefully lap (!) to fit ..... no, I didnt think it a workable plan either, but you never know!
Or, how about a cheap and cheerful home-made chrome plate of liner/piston?
 Dammit, a rebore is easier......
Did see a very useful tutorial just now by Ramon on Modelenginemaker.com all about lapping ...a PhD opus almost!
john
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Konrad
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« Reply #5 on: March 04, 2018, 12:32:20 PM »

If you know about Modelenginemaker.com you are golden.
 
Be aware that manufacturing, repair and running are three different processes.
Having heat treated the piston is putting this project into the repair and manufacturing area.

You will want a dummy cylinder (honing sleeve) to true up the now distorted piston.

Also adding hardness  to a material will actually make the material denser (smaller for the same mass). To grow a material you would want to soften allowing for larger gain sizes. Hence the key, slow cool down process in an oven.

Knurling (pressure cutting a groove) does work to expand materials, as apposed to cutting a grove by removing material as with a single point cutter.

Again all these process need a spare cylinder (lapping/ honing) sleeve to true up the piston after the "growing process".

All the best,
Konrad

Then once you a few microns away from the finished size you can try to lap the piston to the cylinder.
  
Using the engine cylinder directly after what I think you have done will only destroy the cylinder as you try to get a round piston.
« Last Edit: March 04, 2018, 12:50:20 PM by Konrad » Logged

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Konrad
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« Reply #6 on: March 04, 2018, 12:58:18 PM »


Or, how about a cheap and cheerful home-made chrome plate of liner/piston?
 Dammit, a rebore is easier......

Please don't try to re-chrome. Chrome is a very toxic product as is the process! If the municipalities catch you plating without the proper controls you are looking at fines in tens of thousands of pounds!

I'm sorry I don't know your engine but the idea of running a soft piston in a hard liner doesn't make since to me. Both should be soft to allow the materials to form to each other. The key is that the cast iron will work harden during the break in (run in) process giving you the much sought after long wearing properties. Heat treating the iron to a high hardness is counter productive.
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Fourfingers
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« Reply #7 on: March 04, 2018, 02:22:43 PM »

Konrad
Not sure about microns ....I'm working in half inches!
Good point re chrome, my lungs are bad enough already.  You are right about distorting the piston.  I did exactly that and broke it being all agricultural.  Brittle too.
A little knowledge is a dangerous thing.  Off to the engine doctor next time!
john
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Konrad
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« Reply #8 on: March 04, 2018, 03:36:28 PM »

Bringing back the dead rarely is a good thing! So the horror movies would lead one to think.

The problem with chrome isn't the sulfuric acid fumes so much rather it is that the hexavalent chrome will break the DNA chain and form the base for a cancerous cell.

There's an engine doctor?
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Fourfingers
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« Reply #9 on: March 04, 2018, 03:55:26 PM »

Konrad
Engine Doctor?  Oh yes, we are lucky here in Blighty in having several geniuses (geniee?).
Well we have to have some compensation - right now I'll grab any!
It would be invidious to mention any by name, but previous posts will run a couple to ground.
Looking at some of the engine photo galleries you must have a few too.  Trouble is they are too busy (quite understandably) making their own creations - W16!!! - and so on to want to repair our sad bits of scrap!
john
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Konrad
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« Reply #10 on: March 04, 2018, 07:31:22 PM »

As a builder of high performance engines myself (FAI F3D), I can tell you it is a rewarding part of the hobby. I'd like to encourage you to find some flee market engines (low emotional value) and play around and learn how to improve them.

Now true my motto is "I improved it to the point it doesn't run". It was a hard earned motto!

All the best,
Konrad
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Fourfingers
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« Reply #11 on: March 05, 2018, 05:11:09 AM »

Hi Konrad
Thanks for your encouraging words.  Must admit to having 'won' a few shockers on evilBay!  I can now disassemble most motors, put them back the correct way and make them run as they did before....
Having no equipment at all, bar hammer and chisel, i have made and replaced gaskets (!) replaced crappy cross-head screws with cap bolts, re-fitted caged ball bearings, freed stuck contras and liners (for liners, loosen head screws evenly a tad and fire up on a prime ..... very satisfying).  And the height of my achievements - found new needle/roller bearings of the correct size and spec and fitted to Rivers Silver Streak!  Oh,and freed up a MARZ 2.5 with such a tight TDC pinch from new that it was impossible to turn over .... rod in grave danger.
As you see from previous posts, my skills as a blacksmith are sadly lacking.
So without a new shed, heated, plumbed in, wired up and having an en-suite ... to say nothing of gear ... your kind words of have a go joe fall on stony ground!
By the by, are your creations on the internet?
john
("How many engines do you need?"  Answer: 'about the same number as your shoes'.  Response:  Ouch!)
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Konrad
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« Reply #12 on: March 05, 2018, 09:37:01 AM »

No photos of my engines other than deep inside of some racer in an AMA magazine, this was around 1989. I'm told one still exists and I might have some parts at the old family ranch. By 1993 with a new wife I was almost exclusively running electrons.

We all work with the tools we have. To be honest you can do a lot with files and stones if you have the time. I know you have a neurological issue. But there is something therapeutic just getting something to move in your hands.

All the best,
Konrad
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