wheelhorse66 10 #1 Posted February 11, 2014 Im thinking about boring my 8 HP Kohler,but how much more HP can I get with the .010,.020,and .030? how much more HP do each of these produce with the 8hp? Share this post Link to post Share on other sites
sorekiwi 761 #2 Posted February 11, 2014 You'll get very little gain in horsepower by reboring those standard sizes. You'll gain a fraction of cubic capacity , but you'll lose a fraction of compression ratio. Where you will notice a horsepower increase is regaining a good seal in the bore with good rings and tight clearance on the piston. This actually isn't gaining horsepower, its reclaiming horsepower that went away as the engine wore out. 3 Share this post Link to post Share on other sites
wheelhorse66 10 #3 Posted February 12, 2014 Right now it has a standerd piston in it with a little slop if I bore it .010 over and put a .010 over piston in it will this solve the problem?and how much bigger is the .010 piston than the standard? Share this post Link to post Share on other sites
Tom. 70 #4 Posted February 12, 2014 Hi, Re-boring the engine will reduce the slop in the piston. I am currently in a similar position, I have a K181 which smokes and has slop in the piston. I took it to my local engineering shop that measured it for me, they did it for free I would presume most shops would, and it showed that the cylinder had very little wear and it wasn't necessary to re-bore it. They said the rings and just worn so they recommend I get a new STD piston and STD ring set and fit them after honing it. I would definitely recommend taking it to somewhere who can measure it up for you and see if the cylinder has worn much or not. Keep us updated! Tom Share this post Link to post Share on other sites
shorts 182 #5 Posted February 12, 2014 Take it to a reputable machine shop and have it professionally measured, then they can help you make an informed decision instead of everyone on the net shooting in the dark and making uninformed comments that could cost you hard earned money, yours not theirs. If you don't have the factory manual for your engine either find it online and read it or better yet get a paper version so you can use it for reference now and in the future Share this post Link to post Share on other sites
SousaKerry 500 #6 Posted February 12, 2014 and how much bigger is the .010 piston than the standard? Ummm yeah that would be .010" over.... 1 Share this post Link to post Share on other sites
sorekiwi 761 #7 Posted February 12, 2014 You need to get the bore measured properly before deciding what oversize you need to go to. .010" over is the first rebore size, but your bore may not clean up at that. To compound things, a Chinese .010" over piston will probably be a little different from a Kohler .010" over piston. Even if you have the tools to measure the bore properly, there are some traps. Most of the wear in a bore is on the thrust side of the cylinder. So if (for example) you can measure that you have .008" wear, a .010" oversize piston may still not work. Of the .008" wear, you might have .006" on the thrust side and .002" on the other. Boring the cylinder .010" over is only .005" per side so the bore wont clean up on the most worn side. A good machinist can fudge it a little to compensate, but ideally the center of the bore should stay in the center of the block. So, as the others have said, take it to a machine shop, have it measured to see if a rebore is necessary (and if it is, what size piston to order), order your rebuild kit, bring the piston to the machine shop and have them bore the block to the piston you have in your hand. 1 Share this post Link to post Share on other sites
richmondred01 2,205 #8 Posted February 22, 2014 While it torn down check the crank. Most likely will have to be ground. Grinding should cost about 60 bucks. I'm assuming you will be replacing the rod anyway and an unsized rod should be the the same price as the standard rod. Do yourself a favor and don't cut corners. Share this post Link to post Share on other sites
