Opposed twin means two cylinders opposed flat design. Briggs, Onan, Kohler brands were the majority back in the day. Briggs or Kohler IMO would be better choice than onan, but that just my opinion due to the cost and availablity of parts. Before the ovH valve V revelation Flat head twins was the standard engine used in larger LTs and garden tractors. Although an older design if you can find good one and that model Craftsman LT will take Opposed twin then yes it is good engine and many are still in service and running good all over the country. The briggs was good engine if you kept the cooling fins cleaned out, decarbonized the combustion chamber about ever 5 years, and kept oil in it. Probably the most available now days?

Hey RC,

Is there any way to know when decarbonizing is needed? I hate to take the head of just to "take a look". I am talking K301 here, not a twin.

I remember reading this in an owners manual somewhere it was routine timely maintenance. IMO Depending on how old the engine is or how the comb. chamber designed. IMO flat head require it usually after few years (5 to 10 depending on how the oil was changed) of hard use while OHV designs rarely require it. Your 301K I think an older flat head design. That engine will run along time. So. depends on how it runs, if it hard starting, slow or unusual acceleration, and you have done the normal, like cleaning plug, good spark, carb. and air filter, then the next thing I would look at would be combustion chamber carbon build up. These engines usually take thick multiply head gasket. I have used these over and over several time and rarely had to replace one after I cleaned it up especially on tiller/pushmower type flat engines.

"......is this a good type of engine to install on a 42" Craftsman mower?"

Your best way is probably to use the same "style" engine that came with the tractor.
You may have clearance issues etc. if trying to use a different "style".

IF you post the Sears 917.xxxxxx number, we might be able to look up the "correct" engine.

Other issues are things like-
What are the PTO dimensions?
Which charging system?

Bill is right. You are probably best off replacing like with like. The full 917 number is needed to tell what type of engine was original.

Most 42" AYP/Electrolux (917 code) came with single cylinder Briggs, flat head through the late 1990's and the OHV after that.

The opposed twin was a good engine, but no longer made (emissions) and had rather poor fuel economy. It is very wide which will cause fitment issues, plus you have two exhausts/mufflers to contend with.

If mower came with opposed twin, it is usually easy to replace with later model single or v-twin, but not other way around.

Even converting a flat-head single to OHV single is a challenge, because of different exhaust port and muffler location. You may find the savings of a transplant eaten by a $90 muffler.

Briggs produces a CD which is an IPL (Illustrated Parts List) and it contains information on external clearance dimensions for those who are thinking about an engine swap.

I am a fan of the Briggs Opposed Twin engines. My Wards Signature 2000 had an 18HP, which ran strong but sadly had been abused by the former owner(s). It blew so much oil smoke I was afraid the neighbors were gonna call the fire department on me. I nursed it along for a while, and it smoked like crazy, until one day it blew a rod through the block, rather quickly converting itself from a working engine into a parts donor. I don't blame the engine for that, but rather neglect from the former owner.

But I had prepared, and bought a spare engine -- an IC (Industrial Commercial, 20HP version of that same engine. This one had received much better care from its former owner. I swapped in the new engine one weekend 5 years ago and have been happily running it since.

The Briggs Twins have a most unique sound, as compared to the single cylinders. And they tend to run smooooothly. Ggoyeneche mentioned poor fuel enonomy, and yes, my Twin does gulp the gas. I can get Maybe an hour and quarter out of the little gas tank on the Wards Signature mower, compared to almost 2 hours on my backup mower which has a single cylinder 14HP.

Yes you can get the same technical horsepower from a single cylinder, but the torque spread across two cylinders makes the power and performance of the Briggs Twins a thing of beauty. If you want to go out there and take down 12" field grass all day long, this is your motor. They love to Breathe, so keep the air filter clean, keep the cooling fins cleaned, and keep up on the oil. I use Bradd Pen 30W oil formulated for offroad air cooled engines, and I change it at around 10 to 15 hours.

"Briggs produces a CD which is an IPL (Illustrated Parts List) and it contains information on external clearance dimensions for those who are thinking about an engine swap"

The B&S IPL is no longer available. I tried to buy a new one last year as mine is "worn out".

As for the B&S opposed twin being smooth, yes but the Kohler Magnum opposed twin was the smoothest engine I have ever had.

Walt Conner