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K&B 61 c 1960s

Started by FlyinBrian, August 31, 2018, 10:45:29 am

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I picked up a K&B 61 from the Bootie fair at the NATS, it appears to be in remarkably good nick although a bit gummed up. It is actually so clean that I am wondering if it is a MECOA engine however i have not seen anything like it on the MECOA site.

The prop shaft screw ahead of the prop driver is only 1/4" dia probably UNF  but at the mo I can't check the thread. It is also very short at about 1" this makes fitting props awkward and unsafe. I think that originally there may have been some kind of hollow bolt used, or is perhaps the prop shaft/screw actually an insert and can be changed for a longer one.

Any help appreciated
Basic Research is what I do - when I don't know what I'm doing!.


Not 100% certain, but...

Most of the larger K&Bs had a screw-in prop-stud rather than a threaded front-end on the crankshaft, so it may be just that your engine has a very short stud in it. The studs were usually just lengths of 1/4UNF studding, so you can easily make your own from studding, steel bar and a die or by cutting the threaded bit from a 1/4UNF bolt.

If you use studding try to find HT studding rather than mild steel - most of the mild-steel studding from non-specialist suppliers is actually made from case-hardened cheese (at best it's cheddar, but a lot of it is over-ripe brie) with mechanical properties to match. Also don't be tempted to use Stainless Steel studding, as this tends to be rolled from softer grades of stainless and bends quickly. If you don't have a lathe then the best bet is probably to find a suitable HT bolt and cut the plain bit off, chamfer the cut end with a dremmel and then clean up the thread with a die. If you DO have access to a lathe I generally find it's simpler just to make the stud from scratch. Start off with something like an M10 HT bolt (that's the easiest way to get known-good material), turn it down to size and then screwcut the basic thread, cleaing up the threadform with a decent die. This would allow you to cut the chamfers at both ends before the final thread cut (much neater IMHO). Of course if you are just fitting a prop and standard plastic spinner you could always just replace the stud with a short bolt and have done with it...

The really BIG advantage of these prop-stud configurations is that you can fit the old Irvine metal spinners without any adaptors. Just make the stud the right length to go all the way into the brass spinner-retaining nut at the front...

If the stud isn't finger-loose then try screwing a new nylock nut on then off again - the stud will usually come straight out. If that fails then use two nuts locked together.

€0.0007 supplied,

There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...


September 02, 2018, 20:16:20 pm #2 Last Edit: September 02, 2018, 20:17:54 pm by FlyinBrian
Thanks Peter - more or less what I had come up with - honest!

I made a similar query to Mr B Winch esq and he replied thus, little info on the prop stud though :'(.

Start email
Yo Pom Brian (from Oz Brian. HoHo)

What you have there, in my considered opinion, is one of many hundred K&B .60 engines that were made on contract sometime in the 70's for the USA army to power target drones. According to the info I obtained at the time, there was a bit of a problem and the army changed their mind or something else and the engines were at the K&B factory in great numbers so...sell 'em to modellers worldwide which they did. We could pick them up here for about $60 which was quite a good price considering they had the Perry pump and carbie. They went like shite off a hot shovel, as the saying goes but they did have a problem. The bearings tended to wear reasonably rapidly and quite a few ended up on my bench for service but....the liners could not be removed. The liners were steel and of very thin section so, when hot, the expansion on the port areas swelled the liners a small amount and this prevented them from being withdrawn from the cylinders. As they were designed as a disposable item, I would say certain short cuts were made during manufacture and they weren't designed for later service. The engine will run quite well but don't let it get overheated if you plan to service it in the future. As to the fuel, they did run quite well on straight fuel so I would try this first or use your 4s fuel as 15% modern day synthetics will be more than adequate.

The plugs had a good element in them but needed a good and powerful 1.5 Volt supply and, remember, modern day batteries, NiMH, are rated 1.2V.                     11/7 - 12/6 propellers are in the correct range and they start very easily.

Let me know how you get on.


Brian Winch
End email

Nice of him to go into such detail eh.

Basic Research is what I do - when I don't know what I'm doing!.


OK, I got a couple of High Tensile bolts but the thread is no longer than the existing stud. If I get a suitable Die? whats the chances of extending the thread or will the bolt be too hard?

Alternatively I can cut off a small (about 7mm) off the threaded end and use the bolt as is with the prop sitting mainly on the non threaded portion. Means if I use thicker or thinner props I'll need corresponding slightly longer or shorter bolts to keep in the field box.

I love this hobby
Basic Research is what I do - when I don't know what I'm doing!.


A decent die should cut the bolt without any problems - the steel is stronger but not that much harder. Assuming you don't have access to a lathe, for this sort of thing the usual approach is to lightly hold the diestock in a vice and then screw the bolt through it to cut the new thread with a little tapping oil, tallow or (if you don't have either) some  high-pressure grease like a lithium wheel-bearing grease or similar. Remember that it's a whole turn forwards and then 1/3 turn backwards to crack-off the swarf. If you have a SPLIT die then it would be worth fully opening the die for the first pass and then doing a second pass with plenty of lube and the die closed down. This will give the most accurate threadform.

When you've cut enough thread you can trim the bolt head off with a hacksaw, and then gently put a 45deg chamfer on the cut end with a disk in a dremmel to clean up the cut thread.

There is also no problem with using a bolt directly into the crankshaft if you wish. 

There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

The Saint. (Owen)

You could consider using a sleeved nut.
Electrickery is the work of the devil.
Proper aeroplanes are powered by engines.


Owen, tanks for that

Basic Research is what I do - when I don't know what I'm doing!.


I have a brand new ( no box...) K&B 65 complete with silencer and radial mount for sale if anyone is interested....See the For sale section...
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