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Jaymac

18hp kohler to 20hp rebuild ????

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Jaymac
Hi all from Australia, I recently purchased a Sunstar 20hp Tractor with a 48 inch mowing deck and a blown 20hp kohler magnum. It appears that the rod has punched a hole in the alloy casing. I have managed to locate an electric start 18hp magnum for a good price however the owner said the motor ran quietly but it was a bit smoky. Ive managed to get both for around $650 AUS (~$500 US) which leaves me with a bit to spend and still a good buy. Can anyone tell me if rebuilding the 18hp case with the 20hp internals and heads (providing no damage and a new rod or two) is a feasible option or am i better off simply rebuilding the 18hp. I would prefer to keep original 20hp if i can wing it. Any help would be greatly appreciated, fantastic site, has had me reading about my new Sunstar for hours. Kind regards Jaymac

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D-17_Dave
I know the case is the same so I would think the parts should all interchange. I would look closely at the bottom of the cylinder that lost the rod to make sure there isn't a chunk of the cylinder wall missing. Also there may be some internal damage such as cam, lifters, governor, etc. from the rod banging around. BTW, welcome to the club house.

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a-cman
The 18hp and 20hp Magnum engines are actually quite different, very few parts that will interchange. You can find the parts and service manuals on the Kohler site under the classic engines area. I have both Magnum engines, in 2 different tractors, myself, and have found them to be very different when parts are needed.

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Jaymac
Thanks for the rapid response. I have found the service manual for the M18 & M20. The differences appear to be in the stroke as the bore is identical. In the dimensions there are some differences but the graphics in the pdf file are a bit fuzzy. They appear to indicate a larger diameter crankshaft and possibly wider motor (due to longer pots??). Everything in specs is identical except what is listed below, however this does not include all dimensions just those associated with a rebuild. Specified differences are: 18HP 20HP Horsepower @ 3600RPM 18 20 Displacement (cu. in.) 42.18 46.98 Stroke 2.75 3.06 Compression 6.0:1 6.6:1 Crankpin O.D. New- 1.3733/1.3738 --- 1.4993/1.4998 Crankpin O.D. Max wear limit 1.3728 --- 1.4988 Piston and Piston rings Thrustface O.D. @ D1 - New 3.1203/3.1210 --- 3.1208/3.1215 Thrustface O.D. Max wear limit 3.1181 --- 3.1186 TFace to Bore Clearance- New 0.0035/0.0052 --- 0.0030/0.0047 Piston Pin O.D. New 0.6247/0.6249 --- 0.7499/0.7501 They have different valve guide depth guages in special tools as well Well I guess ill just have to open her up and have a peek inside. I have to rebuild the 18 anyway, i guess i will find out more mid operation sm01 Thanks again for any help. Kind regards Jaymac

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