If, after tearing it down, you do not SEE a crack......you might want to use a method I learned from an old motorcycle mechanic years ago. Clean the surfaces of the block just as clean as you can get them using a mineral spirits type solvent. After cleaning with the mineral spirits, spray the surfaces of the block with aerosol carb cleaner or aerosol brake parts cleaner (the idea is to remove the solvent from the surface of the block). Allow the block to dry completely. Next, spray the entire area where you suspect a crack might exist with a "talcum powder based" aerosol deodorant. You want to get a good coat of talcum powder stuck onto the surface of the block. Next, use a heat gun, hair dryer, or a heat lamp to warm the block. What will occur (if a crack exists) is the solvent and old oil will be expelled anong the crack as the block warms and the crack will be revealed as a dark line against the white backdrop of the talcum powder.

Several years ago, Briggs & Stratton had a problem with an oil leakage in their vertical shaft lawn tractor engines. The bolts holding the sump to the bottom of the engine block were not tightened securely at the factory, resulting in the oil leak you ask about! I have repaired a few of them. Here's what must be done: there are about 12 bolts holding the sump to the block. And, you cannot access them all without removing the engine from the tractor, or whatever it is bolted to! therefor, you must remove the engine from the tractor frame, drain out the oil, turn engine upside down, remove the sump and bolts, remove the old gasket and clean the surfaces. Have ready a new gasket! Using some automotive heavy grease, apply some grease to the block edges, and stick the gasket on it. Clean up the edges of the sump, and wipe it out. now, before installing the sump, make sure the timing marks on the cam gear match up with the timing mark on the crankshaft Also make sure the governor gear is in its proper position on top of the cam gear. A bit of that heavy grease will keep it in place. Now, if all is good, carefully slide the sump down over the end of the crank-shaft, and into position. (Note) Before putting on the sump, it is a good idea to replace the sump seal at the crankshaft opening. Install the bolts, and tighten well with a regular length 1/2-9/16 box wrench, in an alternating pattern. How tight?? TIGHT!! Just don't break them or strip them out! Reinstall the engine, install the recommended amount of 30W engine oil. And you should be good to go! With all of the parts at hand, and with the engine in the machine, from start to finish will take 4 hours, for the novice, less for the experienced mechanic!

Hello again, thanks for all the constructive comments, I say it looks like the cutter belt could have done this damage. I say this as I inherited the mower (24 hrs use) and when I first used it the cutter belt kept coming off. Well when I read the mannuel I noted the lever to tension it was intentioned however when I tensioned the lever it was rock hard, tight. Now when I look at the sump it is worn only on one side of the hole the one on the side as if the belt was really tight and and there is some scuffs on shaft itself however I don't think it very bad. I'm hoping to just replace the sump and re tension belt. This is the reason the first time I put the bottom seal in from underneath it didn't work. I have not much experience of lawn mowers but I have rebuild a good few car engines. Surely I am capable of this work?

I would venture to guess you have- Either A blown head gasket OR A bad crankcase breather You can usually spot a blown head gasket by removing the valve cover and starting the engine/ Have a rag handy. A blown gasket will typically blow out a "mist" on the push rod side.