1960’s wankel engine rebuild – Fichtel & Sachs KM48

(LINKS FOR SACHS WANKEL SERVICE MANUALS ARE AT BOTTOM OF PAGE)

My first true passion and hobby was rebuilding engines. Oddly enough, the first engine that peaked my curiosity was the OS 49PI type 2, a 4.97cc single rotor wankel engine intended for use in model aircraft. My father purchased a used one for me on eBay around the age of 13, and I had a blast tinkering with it. The OS wankel is a screaming (17,000 rpm) methanol fueled engine which can crank out a whopping 1.1 metric horsepower while weighing less than a pound, and fitting in the palm of your hand. This unique little power plant paved the way for me to discover larger and equally peculiar engines.

os_wankel_1
Isn’t it cute!

The next engine I was able to get my hands on was 303cc single rotor engine made by Fichtel & Sachs for Arctic Cat Panther snowmobiles back in the late 60’s/early 70’s. It is essentially a renamed Sachs KM914B sold to Arctic Cat by Curtis-Wright. The 303cc single rotor engine was probably the most common Sachs engine, as a number of them were not only in snowmobiles but also equipment like water pumps, generators, and other types of power equipment.

I do currently have a 303 that I pulled from a barn find Panther snowmobile; but that project is for another day.

In total, Sachs made several sizes of air cooled rotary engines for different applications in the 1960s to 1970s. These included:

KMS-4 – 58cc – handheld chainsaw engine

kms4_chainsaw
KMS-4 Chainsaw

KM3 – 110cc – lawnmower engine

km48_lawnmower
KM3 Lawnmower

KM37 – 106cc – various types of power equipment (similar in appearance to KM48)

KM48 – 160cc – various types of power equipment, marine and ultralight aircraft engines

RC1-18.5 – 303cc – (rebadged KM914B) – snowmobile engine

KM914A/B – 303cc – various types of power equipment

KM24 – 295cc – snowmobile engine and motorcycle engine (same engine core used in Hercules W-2000).

It is clear that Sachs had really targeted the small engine market with their various sizes of single rotor air cooled engines. Despite their overall simplicity (very few moving parts) and general durability when maintained correctly, pitfalls common to the rotary engine design (poor fuel economy, higher operating temperatures, noisier than usual) eventually saw the rotary phased out by the late 70’s in favor of more economical and familiar piston engines.

While the Sachs brand of air cooled rotary engines did not dominate the small engine market as intended, they remain as good examples of thinking outside the box. You can still find these little power plants running strong in the hands of a caring collector. Even examples which were thrashed with no air filters and low quality 2 stroke oil (if any) will still pull like a mule at high engine speeds. That is where the wankel design really shines. In a reciprocating piston engine, the piston must quite literally stop and change the direction of its travel possibly hundreds of times a second.

piston_animation
Image courtesy of pl.wikipedia.org

In a wankel engine, the rotating assembly experiences no such start/stop motion like a piston does at top dead center (TDC). Additionally, by enhancing the port geometry on a rotary engine, airflow massively increases at higher engine speeds. This usually results in less torque a lower engine speeds, but much higher power at high engine speeds where piston engines usually lose torque.

wankel_animation
Image courtesy of wikipedia.org

Another interesting point of observation is the existence of three (3) separate chambers per rotor. If you take a look at the animation above, you can observe that the apexes of the rotor create barriers between these chambers. Many arguments have been made as to whether the wankel is more analogous to a two stroke or four stroke single cylinder engine. I like to equate a single wankel rotor to a three cylinder two stroke engine, where the swept volume of one rotor face is equal to the swept volume of one cylinder. To complete three combustion cycles on a single rotor, the eccentric shaft must spin three times resulting in six “strokes”, three up and three down in the rotor housing. In my opinion, this really makes the wankel rotary a six-stroke engine.

Now to the subject of this article; my latest rotary engine project! This Sachs KM48 engine project is the culmination of about 4 years of collecting NOS (new old stock) parts and an older running engine. I began by buying a NOS KM48 engine core which had never been run. Interestingly, the package had some markings indicating that it was built under contract for the English Ministry of Defence (MOD). I did some digging and found the following information:

“Norton made 500 of the KM48 core engine under contract to Thorn-EMI. That company had a military contract to manufacture a mobile radar system called Cymbelline. I don’t think they made many complete systems and, last I heard, Thorn-EMI still had several hundred unused engines in their original packaging from Norton.” – Richard Negus on rotaryownersclub.co.uk/ (possibly the same Richard Negus as in the ex-Managing Director of Norton Motors Ltd?).

So it would appear that one of the engine cores I picked up was a part of some wankel engine generator for the Ministry of Defence back in the day. Interesting bit of history and all the more reason to turn this core into a proper running engine!

It took several more years of collecting various new parts on eBay as they popped up (new fanwheel, magneto and points assembly, ignition coil, spark plugs). I was very close to having a complete engine, but I still needed the power take-off flywheel, carburetor, and engine cowlings that I knew I would never find online as parts. I finally got the break I needed when a local craiglist ad popped up for a complete running (but worn out) engine. I snatched it up immediately. Yes! Time to finally build a complete engine to like-new condition. First, I began by stripping down the old engine carefully:

Major disassembly:

Of course, some tools needed modification to keep disassembly moving:

Getting closer:

Finally the wankel inside is revealed!

20180516_203955

Irons are in pretty good shape:

Now it was time to combine the NOS core engine with as many new parts as possible to construct a like-new engine!

20180518_145858
new rotor on the left, old on the right

The final result, with that beautiful rotary “brap” sound:

I am very happy with the way this engine turned out! I use Maxima Castor 927 at a 30:1 premix and have had excellent results. Castor oil, especially the revered maxima 927 formula, is notable for withstanding extreme temperatures where other oils simply break down on a molecular level. I find this especially suitable for wankel engines as these powerplants do get hotter than similarly sized piston engines, especially near the exhaust side of things. The bing carburetor can be a bit picky with fueling at times, but with a little tweaking the engine purrs like a kitten. I may just keep the engine on a stand and enjoy the sweet exhaust note on a sunny day!

Manuals:

RC18.5/KM914 and KM24 (303cc & 295cc) manual

KM37/48 (160cc) manual

One thought on “1960’s wankel engine rebuild – Fichtel & Sachs KM48”

  1. Hi Kevin. I have several major parts for a KM24, and I’m trying to put together the remainder of what I need to get an engine running. I’m also considering casting new parts using 3d-printed lost-PLA, with a view toward experimentation. I really appreciate your generosity in putting this page together. If you have any more tips for me to get my own engine running (like where I can find seals, bearings, etc.), boy, I’d really appreciate the help! My parts came from some broken up Malibu Grand Prix hand-me-downs. Thanks again! – Dave Klingler, Albuquerque, New Mexico

    Like

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