Author Topic: Kratmo Castings  (Read 3213 times)

Offline Wolfgang

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Kratmo Castings
« on: June 15, 2017, 11:36:29 AM »
Looking for something completely different I stumbled across an ebay auction for a set of castings for a Kratmo F 30, an aero engine designed an built around 1935. Mostly out of curiosity I put in a bid, and to my surprise the auction took only 4 bids. Top one was mine, and now I am the proud owner of a set of Kratmo castings, acquired for a whopping 60 €.

Sooo: What do I have here? Is this a collectors item? UNESCO world heritage? Much to precious for a beginner like me, who has so far built two engines from barstock?

Or is it just another set of castings, and I should have a go at it and see what comes out?

After a couple days research I found a source for plans for antique model airplanes, and they even had a plan (including some build instructions) for the Kratmo.

One of the previous owners has made a start on the castings, but obviously did not get very far. As far as I can see he must have known what he was doing, and the castings are not ruined in any way. That would be up to me, then! :)

My greatest concern, of course, is to make a mistake at one of the castings and to locate a part partly outside the casting. That would most probably the end of the journey, or does anybody know a source for replacements for these castings?

Advice welcome...

Wolfgang

Offline Jo

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Re: Kratmo Castings
« Reply #1 on: June 15, 2017, 01:10:33 PM »
 :embarassed: its the big one, watch your fingers.

They normally go for about E100 new so E60 second hand is ok. They are very desirable as we suspect they are no longer available (especially outside Germany).

Jo

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Offline Wolfgang

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Re: Kratmo Castings
« Reply #2 on: June 16, 2017, 08:20:43 AM »
Ok, so they are not quite as rare as I thought, and I will give it a try. I will post a build log here, but it may take some time before I get around to it. Too many projects, to little time...

Offline Jo

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Re: Kratmo Castings
« Reply #3 on: June 16, 2017, 04:06:40 PM »
Snap, but they are not mine  :embarassed:

Jo
« Last Edit: June 16, 2017, 04:10:46 PM by Jo »
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Offline Wolfgang

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Re: Kratmo Castings
« Reply #4 on: June 17, 2017, 12:10:02 PM »
Interesting, many thanks for the photos. So the carburettor body and what seems to be the cover for the scavenging ports (?) are missing from my set. Unfortunately, they are only sketched on the plan I have, no dimensions. I should be able to come up with something functional, but duplicating  the original "as cast" look will be difficult.

By any chance, do you know of a good set of photos or a build log somewhere on the net?

Wolfgang

Offline Jo

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Re: Kratmo Castings
« Reply #5 on: June 17, 2017, 04:35:05 PM »
Yes you are missing the transfer port cover and the carb.. nothing difficult to make from stock materials. You can achieve the as cast look using a vibrating hand engraver (if you have the patience).

Sorry I don't know of a build log for this specific engine. There might be one on a German forum as these casting sets were available about 10 years ago there.

Jo
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Offline Wolfgang

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Re: Kratmo Castings
« Reply #6 on: June 21, 2017, 06:14:56 PM »
I looked around on the net a bit and at least found a couple of photos, seems they are from an old ebay auction. This one looks particularly discouraging, where on earth shall I find a 65 cm left-handed airscrew?

Wolfgang
« Last Edit: August 07, 2017, 09:14:21 AM by Wolfgang »

Offline Wolfgang

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Re: Kratmo Castings
« Reply #7 on: September 20, 2017, 07:12:38 PM »
Another set of Kratmo castings was just offered on the german ebay site:

http://www.ebay.de/itm/222651374597?ul_noapp=true

I am still pondering over my own set of castings, maybe someone else is interested in this one.

Just wanted to let you know...

Wolfgang



Offline Wolfgang

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Re: Kratmo Castings
« Reply #8 on: December 27, 2019, 11:28:47 AM »
I finally made a start on the Kratmo Castings. First step was a check of the operations already performed by the previous owners, and I immediately found a problem: the rear crankcase half was opened up to 54 mm instead of the 52 mm required by the drawing.



The outer diameter is 55 mm, so the wall thickness is only a half millimeter, not considering any centering errors. Not a lot for an engine this size!

I am at a loss how to fix this. The difference in crankcase volume is probably insignificant (and could easily be fixed by pressing in an aluminum ring), but I am afraid the crankcase is not strong enough for practical use, or even for a test run. It may even collapse at one of the next steps, e.g. while pressing in the crank bearings.

With no experience nor equipment for pouring aluminum, making a new casting is out of my reach. I could possibly make a new crankcase from bar stock, but that would definitely ruin the look of the engine.

Ideas, anyone?

Offline Ramon

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Re: Kratmo Castings
« Reply #9 on: December 27, 2019, 11:59:49 AM »
Hello Wolfgang - comiserations on finding the case over bored but I wouldn't consider this too bad a set back.

Personally I would make an aluminium ring a loose fit on the 54mm diameter- about 0.05 - 0.1 down, the inner dimension say 50mm.

Bond this ring in with standard JB Weld if possible or another similar metal bonding product. Abrade both surfaces with a scriber or similar so the JB has something to key into. Allow to cure over a good 24 hours, the longer the better, before setting up to remachine to the 52 dimension. By doing this you will not be stressing the case and the finished job will be virtually as strong as the original casting.

With regards to the prop you could have a try at making your own - I described doing similar on herehttps://www.modelenginemaker.com/index.php/topic,3233.210.html post 222.

Hope that helps  and that you don't find any other mishaps - always possible when something has been prior machined. Good luck with the rest of the build.

Tug

PS  I would keep the interference fit on the bearing to the minimum possible to retain it and heat the case before fitting - if the bearing is mounted on the crankshaft first it should just drop in the housing - the only stress being as the case cools.
« Last Edit: December 27, 2019, 12:02:51 PM by Ramon »
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Offline Alyn Foundry

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Re: Kratmo Castings
« Reply #10 on: December 27, 2019, 01:37:33 PM »
Hi Guys.

This might sound like a daft question but, as it's a two stroke won't it run in either direction?

Regarding the over machined casting....

If you made a Plaster of Paris/Dental plaster mould taken from the outside of your casting. Then produced a " Resin positive " I'd be quite happy to cast a replacement for you. With this method you minimise the losses due to shrinkage other than the final loss from the cast.

Cheers Graham.

Offline Ramon

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Re: Kratmo Castings
« Reply #11 on: December 27, 2019, 01:54:04 PM »
This might sound like a daft question but, as it's a two stroke won't it run in either direction?

Only if it is a 'side port' inducted engine Graham which, perusing the drawing above, this appears to be  ;) Whilst rear and front induction two stroke engines will break into a backwards run on occasion it is never consistent nor at potential RPM so not recommended.

As you say though it could be run counter clockwise if a conventional right hand pitch prop is fitted but if the original was designed to use a left hander and run clockwise then why not keep to it. Whatever way the airflow needs to be over the engine of course  :D

Regards - Tug
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Offline Admiral_dk

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Re: Kratmo Castings
« Reply #12 on: December 27, 2019, 06:55:59 PM »
If I remember correctly .... The Kratmo was published as a set of drawings and a build article in a German magazine around 1935 for those readers that would try casting and the rest of the process involved in making an useful engine for models (boat, car & airplane). It was later followed up with drawings for a few different sizes.

This means that there never was an official casting set .... as far as I know ....

When I started looking at eBay.de after motorcycle parts, CNC routers and model engines, some 17 years ago, I often saw Kratmo engines or castings and I never saw two that looked alike -> prices where low. A number of years later, they suddenly started to show up looking professionally made, not possible to tell apart and the prices were much higher - I really suspect that somebody made them to a reasonable high quality and sold them as originals in "original boxes", with manuals. The first batch all paper and boxes looked very old, but from the next batch they didn't even bother to make them look old - the same story with Feldgiebel engines and a number of rare very old German collectors aero engines. From time to time I also saw the castings for these engines for sale on eBay.de too about the same time ....

I have absolutely no problem with somebody selling modern quality replicas of the old stuff - but they should be honest and tell that the are replicas (a few did admit and I can't tell them apart from the other nice ones) ....  :old:

Offline Jo

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Re: Kratmo Castings
« Reply #13 on: December 28, 2019, 07:19:46 AM »
You are right Per. I have a recent set of castings of the higher quality which were done as a batch and they were not cheap  :paranoia:

The drawings were published in one of the German magazines and I believe were available as a set of plans from that publication see attached. I do not speak German so cannot translate the initial historical description of the engine from the article. Maybe someone who does could?  :)

Jo
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Offline Jasonb

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Re: Kratmo Castings
« Reply #14 on: December 28, 2019, 07:34:56 AM »
That drawing is still available, vth do them as well as the Felgiebel. But not the same engine that Wolfgang has, his is bigger.

https://shop.vth.de/bauplan-kratmo-10-3200868
« Last Edit: December 28, 2019, 08:09:31 AM by Jasonb »

Offline Jo

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Re: Kratmo Castings
« Reply #15 on: December 28, 2019, 09:13:51 AM »
But not the same engine that Wolfgang has, his is bigger.

Indeed, but it is the only description of the Kratmo engines that I have and I do not speak German.


Surus has let me show you that the big engine has a lot more castings in its set than the little one in his collection  ::)  :pinkelephant:

I still need to put together drawing sets for both engines  :thinking: I need some time.

Jo
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Offline Admiral_dk

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Re: Kratmo Castings
« Reply #16 on: December 28, 2019, 10:06:22 AM »
Your article basicly says that the telephone didn't stand still for a moment after they published the article for the Satyr Old Timer airplane in 1983 and the majority where asking for a sutibile old engine for the airplane. Since there almost only where the Kratmo 10 konstructed by Walter Kratzsch - Krat(zsch)-Mo(toren) - back in the nineteen thirties in Germany (and for much off the World back then) - the FMT magazine had Roland Schwartz create a modern set of drawings from the old article and do a control build to ensure the quality before publishing them.

Offline Jasonb

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Re: Kratmo Castings
« Reply #17 on: December 28, 2019, 10:13:13 AM »
There is a bit about them in this old (and now politically incorrect) book towards the end

https://docplayer.org/56564840-Jahres-inhaltsverzeiehnis-band-8-lieft-seite.html

Those castings don't look too difficult to carve from solid, the most work would be in the cylinder casting but it may be possible to do that in steel and silver solder on some of the bosses which would make it quite easy. Also quite easy to make patterns if anyone wanted to cast them.

Online steamer

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Re: Kratmo Castings
« Reply #18 on: December 28, 2019, 12:41:52 PM »
There is a bit about them in this old (and now politically incorrect) book towards the end

https://docplayer.org/56564840-Jahres-inhaltsverzeiehnis-band-8-lieft-seite.html

Those castings don't look too difficult to carve from solid, the most work would be in the cylinder casting but it may be possible to do that in steel and silver solder on some of the bosses which would make it quite easy. Also quite easy to make patterns if anyone wanted to cast them.

The flapping wing model on page 128 is interesting as well.......
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Offline Wolfgang

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Re: Kratmo Castings
« Reply #19 on: December 29, 2019, 12:42:28 PM »
Many thanks to all who have responded, and special thanks to Graham for his generous offer to cast a replacement part! I am not sure yet whether I will have to go down that road, but in any case I will take a mold from the crankcase part before I start cutting metal.

My fist attempt to fix the over-bored part will be the JB weld method proposed by Tug, although I am not 100% sure about the outcome. Did some CAD modeling of the engine:



 and I think the highest stress on the crankcase walls will occur at the places of the little red circles:



While there is room for a reinforcement at the crankcase side walls, there is unfortunately very little room between the crank web and the back wall:



So the reinforcement can not go "around the corner", and I suspect the crankcase might fail at exactly this point. But we shall see...

That being said, my confidence in the Kratmo as a useful engine has dwindled over time. Biggest concern is the long and thin crankshaft, together with the large overhang from the front bearing. Any engine put on an RC model will have ground contact with the prop sooner or later, and even the slightest touch to that large prop will cause a bent crankshaft. To make the inevitable repair even worse the connecting rod is permanently attached to the crankshaft, a feature I have not seen on any other engine before.

So my current plan is to build the engine and see if it runs, and then it will probably end as a dust collector on the shelf.

My castings are almost certainly from the one of the replica batches, they just look to good for 1935. The drawing, however, seems to be a reprint of Walter Kratzschs original drawing. It is definitely hand-drawn, not produced by CAD. Print quality is rather poor, partly. It came with four pages of build instructions written by Walter Kratzsch himself. They are in German with a print quality far to bad for optical character recognition, so probably useless for most. But there is nothing special in there, and for an experienced engine builder the drawing alone should be sufficient. Got it from here:
https://manualzz.com/doc/4430645/antik-plan-liste---modellbau-and-kopierservice-gerold-kirchert
They also have plans for the smaller Kratmo engines, and a few others.

Wolfgang



Offline Ramon

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Re: Kratmo Castings
« Reply #20 on: December 29, 2019, 04:29:04 PM »
Hello again Wolfgang,

Well, I would agree with your synopsis the wall section does look extremely thin. That said the standard JBW is a very strong product and given that you don't intend to use the engine in an airframe on a continuous basis I still believe it would be worth a try - I certainly would have enough faith in it to try myself. If it's possible it might be better to bond in a solid blank and remachine the complete housing for a slightly smaller diameter, possibly thinner bearing to give a bit of extra strength?

Whatever - good luck with your JB repair  :ThumbsUp: - I strongly recommend the 'standard' over the 'fast' version - the former having much stronger properties and a higher tempreature resistance - the latter, though possibly not of great importance in this case, another factor in it's benefit.


Regards - Tug
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Offline Wolfgang

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Re: Kratmo Castings
« Reply #21 on: January 08, 2020, 04:31:12 PM »
Made a start on the crankcase repair. I turned a substantial ring from aluminum, making it a light sliding fit in the crankcase, degreased everything with acetone, roughed the glueing surfaces up with sand paper, and glued the ring in with JB Weld:



Let it dry for two days, centered it in the four jaw, and turned the overbored section to size:



The JBW withstood the cutting forces without problems, and while the repair is not invisible, everything is concentric and up to size now. The remaining clean-up of the opening towards the cylinder will have to wait until both crankcase halves are bolted together and bored out. I will probably support the ring with a close-fitting plug inside the crankcase during that operation.

Many thanks, again, to Tug for the tip!

Wolfgang
« Last Edit: January 08, 2020, 04:53:10 PM by Wolfgang »