Increasing Single Cylinder Engine HP

Jack1940H · Dec 29, 2017

I have an older Kohler 12HP engine on my Gravely 812 tractor that has serviced me well over the past 30 years. It runs fine but I know it needs valves and was thinking about doing a complete rebuild once it is apart. While apart I would have the block rebored for a new piston and thought maybe it could be bored large enough to accept a piston used in 14HP engine. My question is can the 12HP Kohler block be upgraded to produced 1 to 2 more HP and if so what are the modifications needed?

tomplum · Dec 29, 2017

You know, many of these K series were kissing cousins. I would think if one looked at it from a cost standpoint, you wouldn't do much unless you had some donors. Parts are around if you are dedicated to finding them and putting the $$ machine shop costs into them. From a restoration point of view, I could see it though. Fun reading: http://www.ihcubcadet.com/KillerKohler/killer kohler part i.pdf

buickanddeere · Dec 30, 2017

Jack1940H said:
I have an older Kohler 12HP engine on my Gravely 812 tractor that has serviced me well over the past 30 years. It runs fine but I know it needs valves and was thinking about doing a complete rebuild once it is apart. While apart I would have the block rebored for a new piston and thought maybe it could be bored large enough to accept a piston used in 14HP engine. My question is can the 12HP Kohler block be upgraded to produced 1 to 2 more HP and if so what are the modifications needed?

Check on the garden tractor puller sites . My kid’s tractor has a K341 with an insert bearing road, re-ground cam, high compression forged piston, over size intake valve and a billet head . No idea of the HP but at 4000rpm it spins out the custom pulling tires in high gear with the variator handle all the way ahead . Starts and isles like stock. Still use the factory muffler . .

Egon · Dec 30, 2017

The rebuild will probably show a few horse power gain.

Going for a full fledged upgrade just depends on the amount you want to spend. It could involve a rebuild on which the crank pulley will be the only original part!

Buying higher horsepower new might be a whole lot easier.

Rich B1 · Dec 31, 2017

Do a rebuild, that alone will bump horsepower back closer to new. While it is apart, do some research on Harley KR and Ford flathead V8. There is a lot you can do at the valve pockets in a flathead to get better airflow.

buickanddeere · Dec 31, 2017

375-CH-4K Stock Camshaft | Vogel Manufacturing

VM .

under Rod Bearing | Vogel Manufacturing

Stainless Steel Valve - 1.38" (Kohler Single Cylinder) | Vogel Manufacturing

crazyal · Dec 31, 2017

Can you just buy a new surplus engine cheaper? If possible I bet one of the V-twin Honda clones from Harbor Freight would give you all the power you would ever want. Not sure if it would be possible to mount but boy would it be fun trying.