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Old Dec 14, 2012, 04:40 AM
It must have a machinegun
Scaledown's Avatar
Perth, Western Australia
Joined Sep 2004
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Since you fine people have helped me with three engines so far, I'm going for a fourth now.

I have this really nice looking FOK 1.5. Its problem is the piston is way too tight in the cylinder at TDC. It really jams hard. I tried starting it but when it fires it sticks on the next revolution. I spent an hour muttering What the FOK. Its obviously a dud. So my question is - can an ordinary guy armed with not much more than a spanner, a hammer and a saw free up this engine? What is this lapping business that I read about?
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Old Dec 14, 2012, 08:56 AM
Sticks, Tissue & old Diesels
brokenenglish's Avatar
France, Centre, Amboise
Joined Nov 2011
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Hi,
Lapping means individually fitting the piston to the cylinder by moving the piston inside the cylinder, usually with some kind of mildly abrasive substance (metal cleaning product, usually sold to make chrome or brass objects shine, or "jewellers' rouge".
It's a bit of an art and takes a bit of practice, but my own first attempt (decades ago!) was under the supervision of a real expert, and it came out right first time.
The important thing to remember is that it only takes a few seconds!
You need to remove the cylinder and the piston/con-rod assembly from the engine.
Then use a rod (thick piano wire or a screwdriver stem), through the hole in the big-end of the con-rod, to move the piston longitudinally, up and down the bore, while also rotating the piston, as it is moving up and down (a sort of "figure of eight" motion). As mentioned, this is done with a slightly abrasive cleaning product and it only takes a few seconds!
Keep in your mind that it's obviously better to stop too early (you can repeat the process) than to carry on for a few seconds too long (you'll need a new piston).
Finally, when the piston can just get over the tight spot, with a gentle push, STOP!!!
Then, thoroughly clean everything (petrol or similar)... you obviously have to get rid of all traces of abrasive, and reassemble with a little 3 in 1.
Warning! Don't use this process for a contra-piston!
The fact that this is no longer done for the current cheapo "replica" diesels (whereas it was standard practice for all quality diesels in the forties to sixties) is the main reason why recent vague look-alikes are so b-awful in relation to the originals.
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Old Dec 14, 2012, 09:02 AM
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Canada, ON, Hamilton
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Scaledown, I do not believe that you have a “dud”.
If the piston is “sticking” at TDC (like a newer ABC engine) I would guess that it has not been broken in. At least it is not “worn out”!
Heat the top of the cylinder area with a hot air gun, keep it hot, and then try flipping the prop. Is it sticking a little less?
Back off the compression (to reduce the load/strain on the rod) and continue cranking the engine.
As the cylinder gets hot it may expand and loosen up the piston.
If the “sticking at TDC” becomes less, add compression, prime with fuel and try starting.
Running the engine will loosen it up.
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Old Dec 14, 2012, 09:05 AM
Sticks, Tissue & old Diesels
brokenenglish's Avatar
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John's post above is also true and is a good probable solution.
I just answered your question about lapping but, as John says, you may not have to go that far!
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Old Dec 14, 2012, 09:09 AM
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Scaledown, I do not believe that you have a “dud”.
Looks like I posted just after brokenenglish!
His suggestion will work better than mine, as your FOK is not an ABC diesel.
Have fun, nice looking diesel.
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Old Dec 15, 2012, 08:15 AM
It must have a machinegun
Scaledown's Avatar
Perth, Western Australia
Joined Sep 2004
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Maybe I should have explained how bad it is. It sticks about 3mm below TDC. You can turn it over with a prop on it, but you can feel the flex in the con rod as it goes over the top. I'm afraid if I succeeded in getting it started the big end would wear out before the engine ran in.

I had some polishing paste so I had a go at lapping it using the method brokenenglish suggested, stopping, cleaning it and checking it a few times. Now it sticks about 1.5mm below TDC so I've made some improvement and I'll stop there and see if it runs now.

fiery suggested I use a 9x4 prop, which I've got.
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Old Dec 15, 2012, 09:32 AM
Sticks, Tissue & old Diesels
brokenenglish's Avatar
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OK so far. Another point that I should have mentioned is that your piston fit is too tight at the top of the bore, but OK lower down. This would seem to indicate excessive bore taper. If that is the case then, even when you've fixed your present problem, you may not have enough crankcase pressure to pump the mixture charge up the transfer ports.
This is a very common fault with the cheapo modern look alikes (which your engine isn't). The tight fit at the top gives the impression of good compression (possibly true), but the piston/cylinder fit is so loose at the bottom of the bore that they are impossible to hand start. You need an electric starter, so that the mixture is being pumped faster than the crankcase compression is leaking!
If that should turn out to be the case, you'll have a choice between using an electric starter (horrible on a diesel), scrapping the engine (if you don't care too much for it), or calling on the services of David Owen or Jon Fletcher (if you'd like a nice engine and a super runner)...
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Old Dec 16, 2012, 05:38 PM
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Hervey Bay, Queensland, Australia
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My Russian Made Elfin 149's exhibit just that problem.

The iron and steel one I have is nice in all other respects, so it may get a rebore and new piston to make it serviceable.

It does run, but needs an electric starter, which displeases me.
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Old Dec 16, 2012, 11:22 PM
It must have a machinegun
Scaledown's Avatar
Perth, Western Australia
Joined Sep 2004
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Electric starters seem to be not very practical for old motors because you need a spinner, and so often the prop shaft is so short its hard just to get a modern prop and a nut on it, without the thickness of the spinner backplate as well.

Where does one get a new piston for an old motor?
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Old Dec 17, 2012, 02:53 AM
Sticks, Tissue & old Diesels
brokenenglish's Avatar
France, Centre, Amboise
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Quote:
Originally Posted by Scaledown View Post
Where does one get a new piston for an old motor?
New pistons don't come as spares!!! They have to be made and properly fitted to the cylinder!
In any case, if part (most) of the problem is excessive bore taper (which is likely), then obviously, just installing another piston won't cure the problem.
You'll see at the end of my earlier post, I mentioned the work of David Owen and Jon Fletcher, both these gentlemen are in Oz, they do superb work and your engine could no doubt be made into a very nice runner... if you think the engine is worth it... and the answer to that question can only be in your own mind...
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Old Dec 17, 2012, 06:26 AM
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The windy west coast of Sweden
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This may be a long shot, but it did however happen to me once:

An Indian Mills 1.3, purchased second-hand, showed just the symptoms mentioned above, i.e.the piston fit was very tight (in a harsh and "squeeky" way), just before the TDC and up. Also the the contra piston movement was very limited.
When examining, I found a very hard-to-remove carbon deposit of some kind, in the upper part of the sleeve....
God knows what fuel or oil the previous owner had tried to use...

No solvent (known by me) could soften this carbon, but after (literally) cooking the parts in motor oil, and, while still hot, using a "lapping tool from hell...", consisting of a piece of green scotch-brite glued to a dowel, I finally managed to get the upper sleeve clean enough to allow things to move more freely....i.e. the engine was now ready for a careful run-in....
Although this worked, I wouldn't recommend the "scotch-brite-treatment" to anyone...after all there are more gentle abrasives, like Autosol etc....

Summary: Watch for carbon...!..."experimental" fuels could really mess things up..
I may bark up the wrong tree, but I thought I'd mention this...


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Old Dec 21, 2012, 11:05 AM
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Canada, ON, Hamilton
Joined Oct 2005
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Parra Wasp 1.5 specs

Alberto sent me a copy of the Parra 1.5 Wasp review from Aeromodeller. (See previous posts with pictures at #3679, #3685)
This 1.5 cc ( 0.09) is putting out the power of a sport 2.5 cc ( 0.15) and is actually doing better than the Cox Tee Dee 0.09! Rather amazing little engine.
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Old Dec 22, 2012, 12:39 AM
Greggles47
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Sydney OZ
Joined May 2007
397 Posts
John,

It shouldn't come as any great surprise that it performs better than a TD. It's a bit like comparing a Spitfire to an F16. The TD is a loop scavenged Iron/steel motor from a bygone era. Where the Parra is either ABC or AAC Schneurle ported modern motor. The TD was a great motor and can still perform well given the age, while the Parra is from this century.

This is not meant of any criticism of the TD just a note about the relative ages.

Regards

Greg
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Old Dec 22, 2012, 02:26 AM
Sticks, Tissue & old Diesels
brokenenglish's Avatar
France, Centre, Amboise
Joined Nov 2011
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Quote:
Originally Posted by greggles47 View Post
John,

It shouldn't come as any great surprise that it performs better than a TD. It's a bit like comparing a Spitfire to an F16. The TD is a loop scavenged Iron/steel motor from a bygone era. Where the Parra is either ABC or AAC Schneurle ported modern motor. The TD was a great motor and can still perform well given the age, while the Parra is from this century.

This is not meant of any criticism of the TD just a note about the relative ages.

Regards

Greg
Well said Sir!
The TDs are among the (many) historically great model aero engines. What the Parra will achieve remains to be seen. However, based on what I read here, it does seem to be better than most commercially available oriental or eastern european stuff...
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Old Dec 22, 2012, 04:19 AM
It must have a machinegun
Scaledown's Avatar
Perth, Western Australia
Joined Sep 2004
1,257 Posts
I've given up on that FOK. I can't even get a pop out of it now, even with an electric finger. I'm thinking it must be a case of excessive bore taper. With a good engine you can usually feel a bit of suction going on at the bottom of the stroke, but on this one I feel nothing.

To cheer myself up I got my Taifun Hurrikan running, which I posted about on page 288. I made an aluminium block into which I inserted an Enya needle assembly. I've figured out this engine is the RC version minus throttle. That extra ring on the back is a clever clamping mechanism that clamps down on the throttle base. That venturi that came with it must be from something else, because it doesn't go far enough into the clamp to hold properly. It kept falling off after 3 minutes of running. But not before it recorded the best rpm of all of my 1.5cc diesels!

I'm pretty sure an Enya 09 throttle will fit perfectly and there's one on ebay at the moment.
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