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Rebuilding a K301 in an AC 712S


Michael

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My 712S AC garden tractorI bought new as a kid is very tired and needs rebuilt as it has a fair amount of blowby and is laboring to mow when it shouldn't.  It has already been rebuilt once, and it recently was run a very short time with low oil, and before I throw a rod and break something I'm thinking i'll just rebuild it.

My question is, what was different on the inside to give the 10, 12, 14 and 16 hp ratings.  I'm thinking I'd like to increase it to 16 hp, I just don't know what i need to do to get the 16 hp.

I hoping to buy all new internal parts including a crankshaft if I can find one and just have a brand new engine.

If anyone has a recommendation on the best place to get an engine kit I would appreciate that as well.

Any help will be appreciated.

Michael

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rbstuartjr

Well for starters the main differences between the horsepower engines stated above are the blocks and bore size. The max you can bore a k301 is .030. There are ways to make extra horse power from that engine but that would be boring it out to max size, changing cam profiles and even using a different crank. You can’t bore out a K301 to make it a k361, there’s not enough metal in the cylinder walls. I would just bore it to make the cylinder walls true and do a rebuild kit for that engine. There are websites like Brian  Miller’s  that explain everything you can do to make more HP out of these engines. I’ve used the engine rebuild kits that you find on eBay. I’m stuck on rebuilding my k321 because my local machine shop closed down and didn’t open back up. 

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Have done a fair amount of work on a flow bench with the iron K series blocks. Larger carb (K341) will get you more power, but you may sacrifice some part throttle "drivability" or acceleration response because of the reduced vacuum signal. THE SINGLE MOST EFFECTIVE modification for air flow and power is a good 3 angle performance valve job by someone who knows what that is and how to do it. Doesn't require any special parts, factory valves can be reground to the right profile and the seats can be reground to the right angles and diameter. Milling the head will increase compression slightly and improve torque. Just be careful to keep adequate clearance over the valves. Getting too carried away milling head can cause sealing issues.

Edited by RAC
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In addition to what RAC and the others have said, I have rebuilt the old Cub Cadet K series engines several times, (but not for the same tractor). There are usually people on Ebay or Amazon that sell Stens rebuild kits for this engine. There are usually a couple of options, but you'll need to pull the engine apart to see what you need. Good original cranks are hard to find, and I have been burned on Ebay by someone misrepresenting what they had. Hopefully the crank can be reground, but the standard undersize is .010, and  beyond that you get into custom machining for babbit bearings, and so on. it can get expensive pretty  quick if you need to grind it more than .010 under. The camshaft can have issues too, usually with the points lobe having a groove worn into it causing inconsistent timing. Usually the lobes for the valves are ok. 

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14 hours ago, bnolte said:

In addition to what RAC and the others have said, I have rebuilt the old Cub Cadet K series engines several times, (but not for the same tractor). There are usually people on Ebay or Amazon that sell Stens rebuild kits for this engine. There are usually a couple of options, but you'll need to pull the engine apart to see what you need. Good original cranks are hard to find, and I have been burned on Ebay by someone misrepresenting what they had. Hopefully the crank can be reground, but the standard undersize is .010, and  beyond that you get into custom machining for babbit bearings, and so on. it can get expensive pretty  quick if you need to grind it more than .010 under. The camshaft can have issues too, usually with the points lobe having a groove worn into it causing inconsistent timing. Usually the lobes for the valves are ok. 

There is another thing to watch for on K series engines from the K301 and up, and its a bit controversial. I used to hang around a lot on the Cub Cadet forum and there was a big controversy over whether the balance gears should be removed when overhauling the engine. The consensus seems to be that it doesn't hurt to leave them out, they don''t do that good a job of dampening vibration anyway. 

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The babbit bearing route will let you go to .030 under pretty easy. You can reuse the old rod and have it bored to accept the bearing as long as it's not been hot. Been a I since I've done one, I think maybe I used continental Y112 rod bearings, they're about $13 each.

 

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12 hours ago, BAP said:

This guy has a lot of engine parts for older Kohler and Briggs motors. He has rebuild kits and individual parts. 
https://isavetractors.com/engine-rebuild-kits/

You can actually call him...his name is Norman and his # is 207-298-9701....he is a great guy and answers his phone and will answer questions

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