Author Topic: Kratmo Castings  (Read 3570 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

Online 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...

Online 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

Online 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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Online 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
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

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:

Online 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 »