franchi

Hi:

I got out my Cub .074 Diesels and put new "O" rings in them but that did not change the running characteristics of either.

Both engines seem to have good P&C fits and one will start via an electric starter. The other one did not start at all. The one that ran showed 7000 RPM on an old white plastic 8-4 Top Flite.

The compression screws appear to be screwed down as far as they can go but the new "O" rings on the contra was very tight to squeeze into the cylinder. I had to resort to using a vise to squeeze the contra into the cylinder. Is this too tight? I have no idea if the contra is mooving by flipping the prop. It appears to be the same no wether where the comp scred is positioned. Can I screw the contra down unstil it hits the piston in these engines? The fuel was a new can of DDD.

I would like to get these running!

Any suggestions?


Stay well,

Franchi

AMB

Franchi the vise was not the best idea I hope you did not distort or burr contra, It should have been greased and a dowel about the same diameter used to push in with alight tap with a hammer Liner of course on flat surface (wood) remove the contra by putting a small rod down the hole where the compression screw goes
and remove and check contra for any burrs, it can be rubbed with a little polishing compound to get a little looser fit ( not much) O ring will still seal if too tight you will bend the rod
by forcing it up with piston
With the polish trick yo do not need much off the diameter you will still get a good seal with the O ring good luck martin
maybe others in the group may have a better suggestion
if you can polish off about .00025 inchs or a wee more you should be ok to have piston push up contra

franchi

Hi AMB:

Thanks for the tips!

I pressed the contra with the "O" ring into the cylinder after deburring the bottom edge of the cylinder. When the "o" ring hit the cylinder is when I had to use the vise to get back the well greased o ring. The vise jaws were lined with thick and soft aluminum.

I will return to the basement to try your suggestions .

Could I "sand" some material off the o ring easier that the cylinder wall to get a better fit of the O ring?

Thanks again,

Franchi

AMB

Franchi Without the O ring do you have free movement of the contra in the cylinder??, if you do yes, you can thin the o ring martin

grease it of course for the trial fit

franchi

Hi AMB:

I just worked on the Cubs. I made sure that there was no burr on the contra or the cylinder. I did reassemble the engines and I did notice that a small amount of o ring had been shaved from the o ring.

Both contras can now be pressed into the cylinders by hand even though it is difficult. The piston still can not push the contra back up in the cylinder by compression. The one engine that ran, would only run with the compression screw bottomed out on the 8-4 prop. I guess that that is the only size that I can use on this engine! Lol

I may try to run them both soon but the one does not have much compression. Where may I find a new P&C for these engines?

I did have an address for a chap who hopped up these engines for FF. Do you have any idea as to who it would be?


Thanks and stay well,

Franchi

qazimoto

My Feedback: (1)

Fanchi,

have the OK Cubs got the "cross" device between the comp screw and the contra-piston?

AMB

Franchi clue less on parts, maybe flea bay or some of the guys on the site. If piston iron you may be able to grow (expand it) with the oven trick
Maybe you can mark the compression screw or slip a piece of tubing over it so it just clears the piston as long as a no strike,on lowering, on running it will bang up against the screw quite a normal thing for contras in cylinder design, rule 1-2-3- NO piston strike on contra

On my PAWs on Sharmas I have lowered contra till it just touched the piston the slowly hand flipped to clear, you only need to do this once if at all little fuel or oil in the exhaust will provide a liquid cushion when piston BTC (method that I used) make screw backed out enough to allow this to happen martin

2 objects cannot occupy the same space at the same time

qazimoto

My Feedback: (1)

Will the engine fire on a prime with the fuel supply disconnected? Is the fuel supply working having checked for a blockage?

Frankly I'd say that as soon as you resort to using an electric starter on small diesels such as the OK engines then you're lost the plot!

If they won't handstart with decent fuel on a lightly pitched 7" prop knowing that they fire on a prime under the conditions noted above then there's something seriously wrong and it's quite likely a consequence of heavy handiness with the starter.

The rods may be bent or the rod little ends may be excessively worn out or any of a dozen other causes besides the suspected contra piston "problem".

Does the fuel have 25% castor oil?

Sorry but it's small diesel 101.

[:@]

fiery

My Feedback: (2)

There is a need for a "Small diesels - operation, care and feeding - 101" tutorial sticky topic on this forum.

gcb

franchi,

Why are you pushing the contra up from the bottom? When I replaced the "O" ring on the .075 I had, I pushed it down from the top.

Turn the contra while inserting it from the top to see if it fits better in one position than others (contra/cylinder out of round).

Lose the starter! Flip it hard by hand with that 8x4 prop so you can take advantage of the flywheel affect. Once you can run off a prime you can switch to a smaller prop.

AFAIK the Cub .075 uses the same piston and cylinder as the Cub .074 glow. There is no size difference, the .074 and .075 designations were to differentiate glow from diesel. I think, however, that the diesel used a different conrod.

Good luck with it.

George

Diesel Die-hard

There's an article about the OK diesels at:

http://modelenginenews.org/ad/ok_diesel.html

It includes some operating tips which seem to work.  The comment about using an oily fuel is particularly appropriate since this raises the effective compression ratio for a given setting and also improves piston seal. It thus improves both starting qualities and the range of available compression settings..

I've run into a few of these with bent wrist pins as a result of electric starter use - a death sentence for one of these engines.  The pins are rather poorly supported at the ends since the piston bosses are really short. As George rightly says, lose the starter!  These engines start just fine by hand on an 8x4 prop with an oily fuel and a reasonable helping of ingition improver. .

George is also correct in recalling that the diesel versions of the OK models used a different rod.  The diesel rod is of steel as opposed to the aluminium alloy used in the glow models. Small end wear is a fairly common problem with these engines and does create difficulties because it reduces the compression ratio for a given contra piston setting.

You also won't get anywhere if the three-armed spacer on top of the contra piston is missing.  Se the above-mentioned article for a discusion about that component.

A correcty-fitted O-ring creates no difficulties at all in sliding the contra-piston into the cylinder - a finger will do it..  Sounds as if you used too large an O-ring. The lip on the contra -piston is there to prevent owners from raising the compression too high, thus minimizing the possibility of a hydraulic lock. These lightly-built engines don't take a hydralic lock at all well!  Some experienced diesel users used to turn the lip off the contra piston, thus increasing the high end of the available compression range.  However, this should only be done by an owner who really knows his diesels and has a sensitive flicking finger that weill keep him out of hudraulic locks. It's also unnecessary with an engine in good condition with good fuel.

Hope this helps!!  My examples start and run very well - yours can too, I'm sure!

franchi

Hi All:

Than you for all of the tips!

What I will do next is to:

Add Castor oil to my Davis Fuel. Davis suggested that I add 6 oz. per gallon if I were to use the engines for Control line use.

The O rings must be too large as they take a large amount of thumb pressure to get them into the cylinder. I may attempt to sand down some O rings for a more frindly fit. The rings that I am using are .375"-.500". Perhaps they are too thick.

Yes, I am using the three legged compression shock absorber in the engine.

When using an electric starter, I am very careful to hand flip the engine before using the starter. There is no way that I can had start these engins with the fuel that I am now using. I have other diesels that I can hand start very easily.

I do not think that there is any mechanical damage to the engines as they look very nice inside. The one has no evidence of ever been run.

Right now I other fish to fry. The electric generator always wants to run very lean thus it misses a bit and only produces 106 volts! The stator on the riding mower needs replaced. I can not avoid the Honey Dew list of painting the trim on the hous repairing a roof, washing the hous etc. etc. Lol

B.C. Canada. Where do you live in B.C.? I used to live in Terrace. What a beautiful area in which to live!

Stay well my friends,

Franchi

Diesel Die-hard

There must still be something wrong wih your engine, franchi - all of mine start very easily by hand.  How's the base compression?  Are the lower cylinder and backplate seals good? 

Terrace - know it well, since my brother used to live in Prince Rupert and I was often through Terrace to visit him - used to stop there quite often for a meal. I live in Coquitlam, just to the east of Vancouver.