gcb

ANY engine should at least have the backplate taken off and inspected for residue, particles, etc. Flush it out with something like kerosene. Check if you can see something binding on the piston or cylinder. Also determine if it is the piston/cylinder or the backplate/rotor that is causing the binding. I got a MARZ one time that needed a thicker backplate gasket to solve a binding issue.

My vote (guess) goes for holding the compression screw. BTW, the dot on mine is black...if that means anything.

George

Recycled Flyer

What about the possibly it is simply (or was) a threaded hole that could bear down onto the top rim of the liner?

Keep in mind that these engines are blind bored and the liner may need a stop in order to locate the vertically correct position.

Recycled Flyer

duplicate

123Cat

The Russians know what they are doing

first man in space ,

first dog in space ,

liquid metal cooled submarine reactors

ffkiwi

That sounds quite feasible-equally it might be a pin to locate the liner circumferentially-or even a combination of the two......which is feasible if the liner has a slot somewhere on its top edge circumference.

ChrisM
'ffkiwi'

DaveyMo

Some really good and enlightening discussion going on here - it's much appreciated! The following close up photo shows the that there are two apparently drilled holes near the red dot. The usual piece of piano wire for compression screw friction is missing.

Warren B

One of the holes is for a wire "clicker" which promptly fatigued through in the expected spot on mine - a stupid idea!

A spring under the screw head works OK on my grey KMD, the black one holds setting without any lock.

Recycled Flyer

Copy paste from ACLN issue 81

( My memory ain't that bad after all!)

"What makes the KMD unusual is the lack of a regular
cylinder head. Instead, the cylinder is a one-piece
aluminium die casting with a hardened steel liner. The liner
is essentially a tube with 1.7mm wall thickness, seating on
its lower edge and prevented from vertical movement by a
single grub-screw bearing down on a V-notch in the liner’s
top edge. This is a simple and surprisingly effective
arrangement. The cylinder assembly extends to below the
exhaust port, where it is mounted to the crankcase with
four M3 machine screws. In typical first-generation
schnuerle ported diesel engine style, the porting is a little
small and constricted, but has quite reasonable port
durations: 1400
exhaust, 1340
transfers and 1280
boost."

ffkiwi

Aha-now we know-and as I speculated a few posts earlier-that same grub screw would also prevent any possibility of the liner rotating within the cylinder casting-and getting slightly out of alignment with the transfer passages and exhaust opening. Clever engineering!

ChrisM
'ffkiwi'

DaveyMo

Chris:

It appears you were very close in your speculation! In the process of ferreting out the answer to the red dot , I learned the definition and purpose of a gland packing. I'd heard of the term before (and had seen them in water valves), but never knew what it meant. Thanks.

DaveyMo

Recycled Flyer: Great detective work on your part! You are doing something right in keeping the memory in shape. Many years ago here in the states, there was a saying along the lines of "You win the kewpie doll" when someone solved a problem. I guess you could say that it applies here. Many thanks!
Dave...

DaveyMo

George and Warren:

You were both very perceptive in picking up on a couple of questions only implied in my post on this mill. The compression screw turns with little friction, so I'll put your suggestion for a spring under the screw head to good use, Warren.

The back plate will be the first part to come off after a little more soaking, George. A question: Do you know good sources of gasket material of varying thicknesses here in the states?

Thanks to both of you!

Dave...

gcb

The material I used on the MARZ was some ancient "Perfect" gasket material I had. Guess what...it's still available:
http://www.perfectpartscompany.com/P...eMainFrame.htm


George

DaveyMo

Greetings All:

The saga continues. I did a tear-down and internal cleaning and found very nice innards with great fit! I've heard that the contra piston needs a tight fit, but this one might have too much of a good thing. The adjustment screw pushes it in all right, but getting it back out with anything like reasonable force is impossible. It takes my big bench vise (padded), an appropriately-sized hardwood dowel centered on the contra, and a rather heavy hand on the vise bar to push it back home. I can't imagine engine compression or combustion moving it when the adjustment screw is backed out.

My thoughts for your review. Perhaps try exercising the contra piston in and out using penetrating oil or alcohol to loosen things up. Or, remove it and reduce the diameter ever so carefully using the technique discussed by Maris Dislar in his ACLN issue 81 article.

Gents, your thoughts?

Dave....

brokenenglish

Dave, are you aware that the contra-piston should never be removed through the top of the cylinder, but should always be tapped down, through the bore, and removed from the bottom end. This is due to the taper on the bore, obviously.
If you're having trouble getting the c/p out through the top (I'm not surprised!), that's normal.
If you were already aware of all this, please accept my humble apologies!

ffkiwi

I would be very very cautious in doing anything to ease the fit of the contra-unless you are an experienced model engineer or do precision engineering as a job. The optimum fit is +/- about 1/50,000" between a fit that is too tight, just right and too loose-or about 5 times more critical than the piston liner clearance! Unless you are very experienced with lapping or precision grinding it will be hard to work to this degree of accuracy-and that presumes the liner is decently round, and the contra ditto. [FWIW NZ's top engine man, Harvey Westland told me when I once interviewed him for a magazine article-that as little as 2 microns out of round in a liner has a detectable effect on performance.......admittedly he was talking in terms of top level pylon engines!]
I do not think you can get an accurate feel for how the contra will respond in use with a prop on the engine, by testing the part in the disassembled liner as you describe above.

ChrisM
'ffkiwi'

Recycled Flyer

I would try and take the sharp edges off the contra piston well before reducing the overall diameter and see how that goes first.

qazimoto

My Feedback: (1)

Good quality gasket material for model engines is available from Ken Enya at Enya Direct. He has 0.2 and 0.5mm thicknesses, it's cheap and it works well..

See: https://cs206.xbit.jp/~w041133/store...index&cPath=31

It is quite usual for a racing diesel to have a "stiff" contra. If it moves down alright, then it can usually be brought back by the following method. Please try it before you attack the fit of the contra.

#1 Disconnect the fuel supply.
#2 Wind the comp screw to where you want it to be.
#3 Prime the engine against a closed piston and turn it over a few times.
#4 Wack the prop hard.
#5 Repeat #3 and #4 until the engine fires for a decent burst.

This should move the contra. It is not a good idea to start the engine on a high compression setting and then back it off as it warms up.
Try and find a setting where it will start undercompressed and rapidly warm to smooth running after about 10 seconds.
This way you don't wear out the contra fit by constantly moving it.

gcb

Good point! I try to leave mine at the running setting and adjust my starting procedure. That way you should only need to adjust compression if you change prop, fuel or if the weather changes a lot. In those cases, I let the engine get up to operating temperature then lower compression until RPM drops off, then increase to maximum. Since this is for a diesel newbie, when you get to maximum RPM there should be a "flat spot" where as you increase compression RPM does not increase. Further increase will bring RPM drop indicating that it is over-compressed. I like to back off compression to just where the RPM starts to decrease. Of course you must needle the fuel as you reach these optimum compression settings. Figure that final adjustments are obtained by adjusting to actual flight conditions. A little rich or a little under-compressed will usually fly OK due to combustion characteristics of a diesel. Over lean and it will quit. Over compressed may induce excessive wear.
Of course these are generalizations and there are other ways of running your diesel. Good luck with it.

George

DaveyMo

Brokenenglish: Yes, perhaps I'd heard of that need to remove the c/p from the bottom. Thanks for the reminder!

DaveyMo

Chris M: I really appreciate your reference to tight tolerances for the contra piston. The micron measurements referred to in the Dislar article gave me pause, and now you've just confirmed that I really don't want to put my meat hooks onto that snug fitting c/p!

Recycled Flyer: If worse comes to worst, I'll give your "minimalist" approach a try.

Qazimoto: Thanks for the five-step rundown on how to reset to a lower compression point. Also a good tip on avoiding excessive wear. The link for gasket material is appreciated; I'll add it to the link that George thoughtfully supplied earlier.

So, what's the consensus (if one can be had)? Can anything be gained by removing it (for cleaning only) and then reinstalling with light lubrication? Or, should I just leave it as is and work with what I've got?

Thanks.

Dave....

AMB

DavyMo I think the KISS principal applies here, back off the compression screw a half turn from contact with the contra and try and start, there is a good chance as the engine heats the contra will "bang up" against the screw
Had a tight PAW and it worked for me, you may save yourself a lot of work regards martin

fiery

My Feedback: (2)

KMD Review in ACLN No. 81.

http://www.dkd.net/clmodels/acln/acln81.pdf

steve111

Leave it as is, I reckon. A lot more harm than good comes from pulling these things apart and messing with them. You may well find that while the contra won't move with what you think is 'reasonable force', it will shift quite happily with the comp screw backed off, a prime to the side of the piston, and a good hard flick. I've also had a couple of engines where I didn't touch the comp screw for so long that the previously-adjustable contrapiston became stuck and wouldn't even shift with the aforementioned technique. In this case, winding it in a quarter of turn unstuck it and it would then back out normally.

Recycled Flyer

Simply try heating the outside of the engine with a heat gun before starting as it expands the liner long before the the contra piston - that should help matters along a bit.