Hello All:

I have some ST G 20/.15D engines that I recently bought that have stuck contra pistons. I cleaned the engines well and got them apart but the contrapistons weer/ are very tight. I heated the cylinders and drove the cp out to enable me to clean the cylinder and cp well. I can put the cp back into the sleve but it will not go past the top if the cylinder ports. No, there are no burrs on the ports. I am unable to run these engines due to not being to adjust the compression.


Is there a way that I can get the cp to fit into the sleeves and work properly. How about lapping the two together with a compound? I do not have the tools to make a proper lap. Oh yes, I did ruin a cp by attempting to "sand" the cp to move in the sleeve! Lol

Any suggestions?

Is there somebody in the U.S. who works on Diesel engines?

Tia,

Franchi

I wonder how much experience you have with diesel engines.
Because contra pistons are supposed to be "stuck" and only move under running conditions.
Did you try to RUN them?
I'm afraid you ask for suggestions now that the damage has been done.

ruining a cp by attempting to "sand" it is no laughing matter.
I go: AAAAAAAAAAAAAARRRRRRRRGGGGHHHHHH!
Kind Regards, Hans.

A method I use is a pedestal drill used as press.
With the barrel removed and placed on a suitable block, use a short piece of rod in the chuck and push the contra to the top of the bore from below, then reverse and push it down again.
A dozen or so cycles often does the trick.

... and keep adding oil.

Hello Gazimoto:

When I say stuck, I mean unmovable!!! I doubt that my drill press would provide enough push to be able to move the cp. I will try to do it with a drill press later.

Once the cp gets past the ports, it will not go any farther. One could beat on it with a wooden dowel and a large hammer but what is the point? The cp has to be free enough to be moved by the compression adjusting screw or the combustion pressure. There is no way that these cp will move in the cylinders without being lapped.

In the past, I have heard of people lapping the cp to the cylinder via a lapping paste. I may try this as my mate has some 1600 grit diamond material that may work well. I do know tht one can got undersize very quickly on a cp! Lol

When I received these parts, the cp were installed in some of the cylinders. I heated them to 300F and then drove them apart with the addition of a good oil. I was afraid the the cp would break,but none of them did. Even this primitive method took many blows with a large hammer and a wooden dowel.to free the parts

Stay well my friend,

Franchi

Hi Franchi; from your problem description, it seems that you are driving the contrapiston down

through the cylinder. The contra piston has no business traveling past any port. As has already

been described, you are supposed to drive the contrapiston UP through the top of the liner.


Tony

Years ago I was discussing small diesel engines on a phone call with George Aldrich and mentioned I had picked up a VA Mini-Elfin engine and the contra-piston was very tight. George said he had the same engine and used a drill press and a wooden dowel to work the fit with jewlers' rouge. Like Tony said, only fit it at the very top of the cylinder, don't go too low.

George Alfrich loved small diesels and I picked up several AH minis for him.

I still have the engine and wanted to put it on an .020 size Playboy or similar so I may dig it out and get to work.

Here's a picture of the engine. This is the .41cc version, I think they made a .020ci version too.

Hi All:

No, I am pushing the cp from the bottom towards the top.

A friend of mine may have some rouge or very abrasive that I may try. I do know that one can remove too much metal very quickly! Lol

Thanks for the tips.

Franchi

George cautioned me to go very easy because you might not need to polish it too much.

That's your problem right there. If the CP is now out of the cylinder it should be entered back from the top of the cylinder.
The CP should never be pushed through the regions of the cylinder where the regular piston passes.

The reason why (as told to me by a very good Aussie engine builder) is that a used engine will collect carbon crystals in the combustion chambers rim between the contra and the cylinder wall.

These are extremely hard and sharp and will easily score the liner.

The very good engine builder I know will tell you that the contra must be pressed in from the BOTTOM, carbon crystals notwithstanding. The carbon threat is dealt with by careful cleaning.

Yes, of course it must be pressed in from the bottom!

I've been wondering how many of the respondents above recommending the contrary have ever assembled a conventional diesel.

The taper lapped into the the cylinder dictates this necessity.. The bore is wider at the ports than at the top. Inserting from the top and tapping it into place will ruin it's fit.

Tapping a tight contra from just above the ports to just below the cylinder top and back again a few times to loosen it slightly is another matter. It's acceptable practice.

I'll confess to that, if we are talking about removing the contra completely from the cylinder....

I never remove the CP from the cylinder, just moving it up and then back down is usually enough. I assumed cylinders could be made with a double taper for entering the contra from above.

And just to confuse things even more this very good Aussie engine builder has indeed devised his own design contra pistons that install from the top of the liner!
http://www.modelenginenews.org/~mode...tml#DCO_CONTRA
But they do seem to be unique in that ability and unlike a ST G 20/.15D.

Just to clarify here,and most Australian posters know to whom I refer, we had this exact talk years ago about an old well used PAW diesel that had its contra wound right down by my own stupidity and was colliding with the piston crown. And it was frozen in place.

The fix was to drive the contra right down and out the bottom of the liner (with the warning of 'this is not really preferable due to the possibilities of carbon crystals existing") and the bore and contra checked before reassembling from the bottom will much oil used.

This procedure worked, no damage was done and the engine runs like a dream.

But anyway, I have never heard of a normal full bore solid contra piston ever being inserted from the top of the liner during assembly.

Aren't they lapped as a complete set to fit a tapered bore as well as the piston and as such surely driving a normal contra out of the top of the bore is as probable as being able to do the same with piston?

Thanks for the link, I might have read that once or twice before but it doesn't hurt to read through it again...

Interestingly, the counter piston is made conical such that the upper part is too large, this will then compress and seal towards the cylinder.
This CP could possibly be entered in to cylinder from the bottom, if the cylinder is tapered enough, but there is certainly a high risk of catching a the ports...

Sorry gents, but I think you're misunderstanding David's process for making and fitting contrapistons. The contra is NOT installed from the top of the liner, as you assert, Chris. What the article is referring to is that the tapered, thin-walled contra is checked for fit by being inserted (not pressed) into the top of the liner. If it will enter the bore for a certain distance without jamming, then it's the right size. It can then be installed in the usual manner from the bottom of the liner. It's a very clever method that seems to pretty much guarantee a good result.

The article possibly gives the wrong impression by saying, "It can now be thoroughly cleaned and pressed into the cylinder top". Yes, it is being pressed into the cylinder top (because that's where it resides) - but it's being pressed in from below.

Hope that clears things up.

Steve

If There's a high risk of a CP catching on a port, then surely the same risk exists for that happening with a Piston. At the ports the cylinder taper should be such that both of them are "not tight" rather than "loose". After-all Compression starts just after the ports are closed by the piston on the way up.

Also this use of the word "crystal". It describes a substance that exists in a particular structural 3-D "Lattice like" arrangement of its Atoms and Molecules. Materials with such a structure are not necessarily hard. The Carbon based residue left in a model engine running standard diesel fuel blends is not going to be in the form of diamond. It's going to be more like that of soft Graphite.

How do I know this? I studied Metallurgy and Materials Science for three years in my first degree. That was almost fifty years ago, but I don't think things have changed that much.

In passing, I don't think that our friends Fanchi's stuck Contra is due to solidified carbon residue. The problem is more likely to be due to corrosion. Good old rust.

Thanks Steve, I must have read that wrongly and stand corrected.

If There's a high risk of a CP catching on a port, then the same risk exists for that happening with a Piston. At the ports the cylinder taper should be such that both of them are "not tight" rather than "loose". After-all Compression starts just after the ports are closed by the piston on the way up.

Also this use of the word "crystal". It describes a substance that exists in a particular structural arrangements of its Atoms and Molecules. Materials with such a structure are not necessarily hard. The Carbon based residue left in a model engine running standard diesel fuel blends is not going to be in the form of diamond. It's going to be more like that of soft Graphite.

How do I know this? I studied Metallurgy and Materials Science for three years in my first degree. That was almost fifty years ago, but I don't think things have changed that much.

Just having reread all of this I realized that there is an uncanny parallel between this new contra piston design and tapered cup expansion plugs or "Welch Plugs" that are used in full sized engines.

Couple that with the twice mentioned top of the cylinder "the contra piston end will just enter the top of the cylinder bore" and "and pressed into the cylinder top" and its not hard to believe that the contra was 'meant' to be installed that way.

If you do any "sanding," do it only on the contra, and not the cylinder, unless you know for sure the cylinder geometry is wrong. It's a lot easier to make a new contra than a new cylinder if your sanding goes awry. When I make new contras (for the new engines I make), I polish them to proper fit in the lathe using 800-grit wet-dry paper or finer. With 800-grit, the cutting is very slow, so there is less risk of taking off excess material. You can use a drill press or drill motor in a pinch. I make new contras extra long and then face them to length after polishing to fit. This removes any taper at the ends caused by the polishing. You won't have that luxury when sanding an old contra, so be careful to avoid rounding the ends.

Yes, a conical contra would work well even in a parallel bore, but it would have to be entered from the top...

Once in the bore it would also be easier to move down, as compared to moving it up. Hmm, I think I have few engines that will act just like that when warm...

Hello all:

WOW! I am really impressed by the amount of information that I have read re. cp fits.

I think that my cp are round and not tapered, my cylinders are tapered, my pistons and cylinders are clean, smooth and free from any form of rust. Perhaps these parts were made to fit only one combination of cp and cylinder.

These engines, mostly ST G20/.15D were modified with custom heads that look like what Davis now uses for his head design. I would like to get these engines running again but the tight fitting cp will not permit this. There is no wally that I can get the cp to enter the cylinder from the top! I guess that I could heat the cylinder and cool the cp to insert the cp but what would happen at room temperature? Perhaps I would then have a shrink fit. That would be fun!!!!!!!!!!!!

Thanks for the suggestions.

Stay well my friends,

Franchi

I have dug through some old magazine to try and check if my memory has failed me regarding the CP and its removal and refitting.
In the few written instructions I have from the 70-ties there are some note on the webra record engine, from 1977. The instructions clearly state that one should push the CP upwards though the top for disassembly and push it in from the top (in a vice) upon assembly. In the event of a too sloppy fit on the CP one can try to increase the tapering of the CP by fitting a suitable steel ball from the top and gently tap it on a hard surface. This will expand the top part a little, but if done too much there is a risk of cracking the CP and hence to ruin it completely....