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Kraft engine department 76 to 80 Empty

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Kraft engine department 76 to 80 Empty Kraft engine department 76 to 80

Post  FlipStart Sat Aug 14, 2021 4:19 pm

Hi all! I had started a talk about my years working for Kraft Systems in their model engine department in another thread but will continue it here. Kraft made a .61 displacement R/C engine back then and I just grabbed a couple of crank shaft's out of my storage unit, down the street a ways. If I can remember how to post pictures, or figure it out again, I will show them, one is kind of interesting. By for now.Kraft engine department 76 to 80 Dsc_1710


Last edited by FlipStart on Sat Aug 14, 2021 11:18 pm; edited 1 time in total (Reason for editing : To add an image)
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Post  Coxfledgling Sun Aug 15, 2021 2:44 am

I will try and find your other thread which sounds interesting.

No left hand thread but porting "opposite", clock and anticlockwise crank rotation ?

Of course I could be completely wrong...
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Post  Oldenginerod Sun Aug 15, 2021 5:23 am

@Coxfledgling wrote:
...... porting "opposite", clock and anticlockwise crank rotation ?

That'd be my guess!
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Post  FlipStart Sun Aug 15, 2021 2:26 pm

You guys are absolutely correct! The crank on the left was custom made at Kraft along with about 5 others that are for running the engine in the opposite direction of rotation. There was a request by enough people that Rodger decided to make a few cranks. He cut them to shape and had me drive them down to National City to a shop that did heat treatment work and had them hardened to some Rockwell number. Then with the cranks hardened he ground the final surfaces to size for the bearings and rod.

I wasn't originally suppose to get one as I didn't really have a need or request for one but there was an extra or 2 so I asked if I could have one and it was OK.

For many years my plan has been to build a P-39 lightning with 2 Kraft .61's for power. That was my dads favorite war plane and mine also. It could still come true.

This should be the link to the start of the Kraft engine department story. 2 posts I believe. 1 and another.
https://www.coxengineforum.com/t15397-wasn-t-the-70-s-a-great-time
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Post  FlipStart Sun Aug 15, 2021 2:34 pm

@Coxfledgling wrote:I will try and find your other thread which sounds interesting.

No left hand thread but porting "opposite", clock and anticlockwise crank rotation ?

Interesting point about the threads!.. It would have been better to have cut reverse threads as well. I should probably use Loctite when I get her going.
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Post  Coxfledgling Mon Aug 16, 2021 1:40 am

Since these cranks ride on ball race main bearings ?, It looks as if the prop driver would have a "taper" and the crank have a split collet with a taper, so the collet locks onto the crank ?

Cranks are/seem simetrical not "desaxe" ( that spelling may be incorrect ) ?
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Post  davidll1984 Mon Aug 16, 2021 12:17 pm

Cox engine cw crankchaft Does not use difrent tread I have never had a problem with the propeller becoming loose Hand Shake Thumbs Up It's different crank must be hard to find
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Post  FlipStart Tue Aug 17, 2021 1:01 pm

@Coxfledgling wrote:Since these cranks ride on ball race main bearings ?, It looks as if the prop driver would have a "taper" and the crank have a split collet with a taper, so the collet locks onto the crank ?

Cranks are/seem simetrical not "desaxe" ( that spelling may be incorrect ) ?

Yes Right again. The prop driver is tapered and rides on a split collet. The collet sits up against the inner race of the bearing and the prop driver seats to the collet before toughing any other part of the engine. We had no problems at all with that system. It was very simple and reliable. The collets and drivers were outsourced I believe. I don't think we had to do anything to them at all before installing on an engine. Once in a while I would get one that was really on there and had to use heat and the puller to get it to break loose.Kraft engine department 76 to 80 Dsc_1712


Last edited by FlipStart on Tue Aug 17, 2021 5:46 pm; edited 1 time in total
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Post  FlipStart Tue Aug 17, 2021 1:10 pm

@davidll1984 wrote:Cox engine cw crankchaft Does not use difrent tread I have never had a problem with the propeller becoming loose Hand Shake  Thumbs Up It's different crank must be hard to find

Good to know, thanks. I won't worry too much about the Kraft engine loosing a prop in that case.

I would imagine another Kraft .61 CW crank would be very hard to find as to the best of my knowledge only about 6 were made, at least by Kraft technicians. I suppose it's possible other company's might have sold specialty parts for the Kraft.

I don't know much at all about RJL but they're engine looks like a Kraft .61 to a large degree I'm thinking.
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Post  FlipStart Tue Aug 17, 2021 1:17 pm

OK, I think I left off on pistons. I think pistons for the .61 went through the most changes of any other part, except for a completely different carburetor of course. I don't really know what the first engine serial number was but I'm think it had to be A00001 as the earliest serial number engine I have seen is A00282 and it has all the original parts the first engines came with, which is a fair bit different than what it ended up with.
Several changes were made over the early years as some parts were not being reliable in actual in field service. They could run the engine here, test fly it and think they were putting it through every possible stress it would encounter in the field, but nothing can duplicate what the public can dish out. Time is a factor here also.
Anyway back to pistons. In going to head over to my storage unit today and se if I can get to the cheese box drawer of pistons. I'm thinking I have some examples in there. I wasn't at Kraft till most of the early changes had taken place with the engine parts. Mike filled me in on the early engine parts and what the problems were. I also worked on customer engines while doing the repairs. A lot of times I would just upgrade a customers engine besides fixing the problem it was sent in for. Sorry I keep drifting away from pistons.
The earliest pistons (as far as I know) were made there at Kraft out of bar stock aluminum and all the cutting happened there. A really interesting feature of the first pistons was that they put a Teflon coating on the crown and I'm not sure why. The obvious thought would be to help prevent carbon buildup, it was white and I'm wondering if it had anything to do with heat dissipation or something along those lines; I never did find out for sure. I think maybe Mike told me but I forgot. At any rate the Teflon coating was not sticking to the piston well under use and was abandoned at some point fairly early on.
By the time I got to Kraft they were using a cast aluminum piston that of course needed far less machining and I think was lighter and was quite strong, although some of the castings would have cracks or areas of missing metal at the very bottom of the skirt. We had to boar and hone the wrist pin hole, cut the inside skirt area a little, cut the crown and cut the ring groove and wrist pin keeper slots and that was about it.
The .61 used a modified Schnuerle-port design which had a big window in the back side of the piston and a boost port in the cylinder wall opposite the exhaust.
I'll post some piston pictures if I can get to the pistons. My storage unit is cram packed.

There were a couple more changes to the piston assembly but not the piston itself. (as far as I know)Kraft engine department 76 to 80 Dsc_1713
I was able to get the box of pistons and cylinders out and I'll go through it and hopefully find a few different pistons.
The one pictured here is a very last run assembly as it has the stainless steel wrist pin clip with one end cut off. This was my idea actually; not using stainless steel but cutting off one end before installing the clip.
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Post  FlipStart Mon Aug 23, 2021 10:44 pm

I suppose I should continue with the Kraft engine department story before too long. I was looking through my cheese box drawers where most of the engine parts are and knew I had this way cool prototype head but didn't see it and others besides. I was getting worried I had somehow gotten rid of it but didn't think I would have done THAT.

I did weed out some of the production stuff some years ago as I just had quite a bit. And I'm not quite use to looking in the drawers yet after the lower number of things. I did find the heads. I had then at the bottom of a drawer with separator paper towels over them and a layer of Prop drivers and split collars on that, then another paper towel separator over that and connecting rods on top. so the heads were burred a distance down.
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Post  FlipStart Sat Aug 28, 2021 3:49 pm

I'm thinking this would be my most valuable/collectable engine as it's never been run and is a very early model with all parts that the engine had when first released for sale. Many things were changed in later runs.

I was doing repairs and this one cane in with a letter that said the owner had bought it from a hobby shop and knew that the engine had gone through several upgrades since this one was made. He hadn't used it and simply wanted it brought up to current specs. It was the oldest serial number I had ever seen and thought it would be a shame to modify it. Besides which it was easier to just send him a new engine with all the latest parts. So that's what I did. Then I worked a deal with the company to purchase this engine. It probably wasn't anything particularly collectible back then but I bet it is now. It's serial #A00282 so it should be the 282nd engine sold. I also made sire I had all the original documentation it came with.

Kraft engine department 76 to 80 Dsc_1715
Kraft engine department 76 to 80 Dsc_1716
Kraft engine department 76 to 80 Dsc_1717
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Post  davidll1984 Sat Aug 28, 2021 5:53 pm

Huh... The prop driver is tapered and rides on a split collet. The collet sits up against the inner race of the bearing and the prop driver seats to the collet before toughing any other part Huh... With this configuration it would be a good candidate to install a flywheel with a clutch What is the max rpm of this engine ??
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Post  FlipStart Sun Aug 29, 2021 11:11 pm

With the 11 7.5 Zinger it averages about 12.5K depending on if a Kraft carb or a Perry is being used.

Kraft engine department 76 to 80 Dsc_1718
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Post  FlipStart Thu Sep 02, 2021 1:04 pm

Here is the last Kraft engine I was flying right up to the point where it made contact with the asphalt, and I don't mean wheel side down either. We were using this lone tree beside an abandoned asphalt something. I was doing my usual crazy stunts and was partially under the for shade. Suddenly I made a mistake and my plane (don't remember what it was, some low wing) went behing the tree too. I tried to get sight of it again but I heard it getting closer fast and didn't react quick enough. Wack, there it was on the asphalt out in front of the tree.

The engine basically survived pretty good as I remember. It might have broken a crankshaft but maybe not. It did break the muffler but that's a given in almost any crash, The carb has a ding but otherwise OK. I have the bearings out but I don't know if maybe it's because the crank broke or not.

This engine ran really good and strong. One of the machinists at Kraft (actually Phil's full size airplane mechanic) cut me a cool cast iron Dykes Pattern piston ring for it. The low drag of their design in the non power stroke is especially good for model engines as internal friction is a big power loss in such small engines especially. It's like twice the height of a stock .61 ring and I think I got a pretty good shot of it through the exhaust port with the help of outdoor lighting and fill flash. It's the grey area between the 2 typical looking carbon areas. You can see this engine has a much later serial # and stamped in a different spot than my collector unit.

I want to get this one going and back on a plane again. .60 size planes are really fun.

Kraft engine department 76 to 80 Dsc_1719
Kraft engine department 76 to 80 Dsc_1720
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Post  FlipStart Thu Sep 09, 2021 11:07 pm

I got this crank case that had the bottom half machined away in the Bridgeport. I don't really know why. Maybe someone was making another cutaway engine for display in the lobby but I doubt it.

Anyway kind a cool to se the transfer port channels and boost port. I kicked up the flash power to illuminate the insides more.

Kraft engine department 76 to 80 Dsc_1726
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Post  Oldenginerod Sat Sep 11, 2021 5:24 am

@FlipStart wrote:I got this crank case that had the bottom half machined away in the Bridgeport. I don't really know why. Maybe someone was making another cutaway engine for display in the lobby but I doubt it.

Anyway kind a cool to se the transfer port channels and boost port. I kicked up the flash power to illuminate the insides more.

Kraft engine department 76 to 80 Dsc_1726
Maybe the beginning of an attempt to build a twin??  Huh...
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Post  FlipStart Sat Sep 11, 2021 7:52 am

Ya know, I think you might be right. The guy that was Phil's airplane mechanic did build a twin and I can't think what configuration it was. He might have done some experimenting and this crank case might have been the opposed twin thinking.
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