I've got a Lee Custom K&B .61, which I've owned nearly 45 years. Great running engine. First used it to power a model called a Taurus, which I initially flew with a second-hand Fox .59 (two glow plugs in that engine). Subsequently, I installed it in a Phoenix 5 (another pattern plane, but this one with a fiberglass fuselage, the first of many, and my first experience with retracts - Rhom Air).
With the advent of more powerful Schnuerle-ports, old fashioned loop-scavanged engines like the Fox and K&B lost popularity - except for Clarence Lee's really good running K&B .61 which is such an easily starting smooth runner that it maintained favor with nearly everybody other than the hardcore competitors into tuned pipes and all that. Anyway, I later used my Lee Custom .61 to power my Andrews Aeromaster. Lost that model to crash which damaged the crank due to switch failure. Interestingly, said switch failure directly led to ProModeler because in my frustration at a switch bringing my model down, I resolved to back it up with a second switch (I was always quick at math and I understood to my core how astronomical the odds of both failing on the same flight really were). So I soldered a second lead to my battery pack and ran two switches.
Then a mentor with a Royal Cessna 337 (push-pull), which years ago I had built for him (architect, now deceased but with more money than God), asked me to do the same for him. he wanted ten because he had 10 models. Low volume serial production exposure at a tender age taught me a important lesson as before that, money earned was based strictly on what i was doing at the time - mowing, painting, etc.
So fast forward to today and we still offer dual-lead batteries plus really good quality switches ($10). Speaking of switches, now they're heavy duty (at 20A they're rated for four times the current and are more reliable). Moreover, in addition to redundancy, these days two switches means the receiver can draw twice as much current (10A total) as through a single lead (rated at 5A) so this is another important - added reason - for using two switches.
Anyway, before long folks from a club across town caught wind of what I was doing and just like that I had a small sideline business. One, which like today, grew based solely on word of mouth! Note, this first product was a dual-lead 500mAH 4-cell NiCd battery pack and today (fast approaching 40 years in the hobby business), I continue offering dual lead packs (as do plenty of others). Note; while I make the claim of being first (although being a youngster, it never occurred to me to try and patent it, added to which, today I also know it would have been impossible as a practical matter to defend said patent even had I thought of trying to obtain one) finding other suppliers isn't difficult, or rare.
Meanwhile, present day packs in our ProModeler line-up range in capacity from 1000-5000mAh as are dual leads for the purpose of two switches. Math is the root of my single best suggestion for protecting your model that two switches trump one because switches remain the most common point of failure compared to receivers and servos. This despite the common use of surface mount PCBs, which are uber reliable compared to back-in-the-day practice, when discrete components hand-soldered were how things were done). Switches are still the most common hardware failure. Doubling them up is cheap insurance.
But back to Mr. Lee, I returned the crash damaged engine (in the Aeromaster I crashed due to the switch failure). he returned it running like new with a nice note. This engine powered a Great Planes Sportster before ultimately finding a home on a Pica Waco (a scale .61 size biplane) where it lives to this day! Lots of good memories.
Why do I mention all of this? Principally to wish Clarence Lee a happy birthday - 4th of September, 1923 - his bio if you're curious: https://www.modelaircraft.org/...
. . . and to mention; you can still buy a Clarence Lee Custom .61 for less than 200 bucks brand spanking new!
Mr. Lee is in great shape and I especially bring this to your attention if you're into flying old school pattern planes. If you're not, believe me when I say it doesn't get much better - not in terms of good flying model airplanes that are simple, fun, and not a lot of money.
Anyway, if I may be allowed to continue to meander to my final point (reminiscing about batteries with twin leads and switches was a feint versus my real purpose of this missive), which is this; look up SPA (Senior Pattern Association). Facebook page and everything! It's largely composed of old farts reliving their younger days. Along with competitive juices flowing as usual (pipes and retracts) the simplicity and fun of a fixed gear model like a Kaos, KwikFli, or Taurus powered by a loop engine (like Mr. Lee's sweet running .61 Custom) remains a proven source of inexpensive fun. here's what I recommend as a good recipe;
Get a Kaos, add a Lee Custom K&B .61 w/muffler with an 11x7 prop, and guide it with your choice of receiver plus five DS105CLHV servos and a 2S1000mAh pack. Go fly. Perform three loops as close to directly in front of you as you can, or nail three perfect spins. Maybe look up some fellow doing the same maneuvers and I'll bet you a milkshake after a day of this it'll be tough to wipe the grin off your face!