Expanding worn pistons

[WindyParticipant @windy30762]

I am not sure if it was this forum but there was a thread explaining about how to increase worn cast iron piston dimensions with heat.

Could some one advise me about using this method please?

Paul

 

Edited By Windy on 16/04/2013 16:57:05

[WindyParticipant @windy30762]

Expanding worn pistons

[john kennedy 1Participant @johnkennedy1]

Windy, I remember reading about that process on here. **LINK**

Try doing a search. John

[Francois MeunierParticipant @francoismeunier96697]

upon heating to red and quenching in oil, your worn piston do hardly enter in the cylinder, amazing, it gives some meat to lap again, the increase in dia was about 0.02 mm for a 25 mm piston, but not not regular, more pronunced on the lower, thinner part, somewhat oval too…, may be not quenched properly.

Zephyrin

Edited By gedeon spilett on 16/04/2013 21:28:25

[Sub MandrelParticipant @submandrel]

Hi Windy,

Its been mentioned in ME. As I recall you soak it at red heat for a good while as if you were normalising it and add a thou or two per inch, then re-lap it.

Obviously you can only do this once.

Neil

I found this – makles it nice and simple, eh?

"

Irreversible Carbon Diffusion in Growth of Cast Iron

1984 MRS Fall Meeting.

K. Nagaoka^a1 and M. Souma^a1

^a1 Faculty of Eng., Hokkaido University, Kita 13 Nishi 8, Kitaku, Sapporo, 060, Japan

ABSTRACT

In gray cast iron carbon diffuses irreversibly and the result is a growth remaining castoff voids and graphite redistribution. The origin of primary growth is in the mechanism of carbon diffusion which is controlled by a ratchet at graphite where carbon is carried by vacancy flow. Growth of flake cast iron in oxygen reduced air at low oxygen content is accounted for by self carburizing due to a reaction of penetrated oxygen and graphite flakes. For the irreversible carbon migration by gaseous carrier another ratchet is suggested. Cracking theory for primary growth is replaced by castoff void theory, and oxidation theory is modified."

[Paul LousickParticipant @paullousick59116]

Try this link for making piston rings.

http://users.tpg.com.au/agnet/make%20piston%20rings.html

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Neil ,

As I'm sure you know already – there are almost as many Great Experts in the academic world as there are in model engineering – a knowledge based filter is nescessary when reading some of their astonishing works !

What is in that paper is not wrong but it can be explained much more simply and has in any case been known about by engineers and metallurgists for many years – possibly centuries .

Several metal alloys grow a little with time when heated above a critical temperature but beware they are also degrading – heat grey cast iron for long enough and you'll get a crumbly sponge .

There are several ways to increase the size of a cast iron component by a very small amount by more controlled and less damaging processes .

(1) Using variations of case hardening without the final hardening bit – for instance heating in a bath of finely ground cast iron with an added activator and leaving to cool naturally .

(2) Using deliberately initiated creep – contrive a moderately heat resistant jig which acts so as to force the sections of the casting in the desired direction . For a piston this would be some variant of circular wedges in/on a mandrel fitted inside piston and moved progressively by a screw or pressing arrangemnt as heating progresses .

Methods of adding larger amounts of metal involve weld deposition and Chrome plating .

Very large pistons have sometimes been given emergency repairs by sleeving .

NB: Method (2) above works well on some aerospace materials including Titanium and was once used quite extensively for recovering the proper shape of distorted aeroengine components and putting deliberate distortions into things for experimental purposes .

Personally for a tiny piston on a model engine I'd just make a new one – advanced repair methods are only of use on higher value components or where a new one cannot easily be obtained .

Regards ,

Michael Williams .

[Ramon WilsonParticipant @ramonwilson3]

Hi Windy,

I confess I very rarely visit this site these days – totally frustrated by the spoiling of so many threads due to the adverts covering the posts. Fortunately this is not one of them.

Just like to confirm that over the years I have successfully reclaimed several worn engines using this method which I first found described by my engine 'hero' George Aldrich in either Model Aircraft News or Strictly IC. It's possibly one of my posts you are reffering to.

Just heating the cast iron to a nice bright red and letting it soak in the flame a bit before letting it cool slowly will increase the diameter slightly however quenching it – in oil – will give a bit more growth – about 1-2 thou on a 3/4" or so diameter. It can vary depending on the cast iron and as Neil says you can only do it the once. It's surprising sometimes just how much more material you have to play with.

Lapping the bore before lapping and refitting the piston is better than just relapping the piston but I'm sure you'd be aware of that.

Hope that helps a bit more

Regards – Ramon

[WindyParticipant @windy30762]

Thank you for all the explanations it was Ramons posts that I was trying to remember.

Memory at time needs a shake up.

Paul

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