Mower Engine-Blown valve?

Mowing lawn this morning, hit a dense patch of grass and the engine sounded like it would stall so I raised the mower to relieve mulch. A loud pop and flash, similar to an M-80 (maybe not quite as loud) and a blue flash out the right side of the mower. (Muffler is on the left, so I knew it wasn't a backfire.)
A friend suggested the "finger over the spark plug hole" compression test which it appears to have failed: no vacuum or compression in the chamber.
The question...How much would a valve repair be if I take it back to Sears? ...To anyone else? (This is a 5 year old Craftsman 22" 6.75hp).
Thanks, 1s&0s
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It could be a piston or rod, the question being the metal debris will ruin everything and since its 5 yrs old is it worth fixing.
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<< since its 5 yrs old is it worth fixing. >><p>I doubt it's a rod or piston...there would be grinding and other messy noises, which there are none of.
And I may replace it. It just depends on how much it might cost to fix. Anyone know how much a blown valve might cost to repair on a 6.75hp engine?
1s&0s
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Ive blown a hole in a piston, cracked rings, and only had smoke, no grinding, no noise . No one can say how much to repair as even you dont know the problem or extent of damage. Or extent of debris in the motor. This type of damage comes from wear, wear from not maintaining the mower with good oil or overworking it. Is it a temsumseh, they are the bottom of the barrel, rarely worth rebuilding. Valves dont take the brunt of wear on Temsumeh or in a load up. They have crap bearing and oiling design. Sears has honda powered mowers now, cheap. go get a 20 yr honda , use better oil, change your oil more often and dont beat yor machine and expect it to last.
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Well I never heard of a "blown valve" doing as you described and I think you can forget that. I 'might' be a blown head gasket which wouldn't be much. If it has thrown a rod resulting in the described action, it should be obvious by looking at the side of the engine. If it threw a rod and didn't bust the side of the engine, you would not have seen anything out that side when it occurred, if there is a hole in the block, throw the engine away. There would not necessarily be any other noise associated with throwing a rod.
You should be able to stick a dowel or long rod in the spark plug hole and tell if the piston is going up and down when you turn the engine by hand. This will tell you if it has thrown a rod absent a hole in side of block.
Walt Conner

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This same thing happened to the mower of a friend of mine. No compression - valve not moving. We opened the case and noticed that the plastic cam gear had broken in half. Got the $20 replacement gear from sears, put it back together, and it ran like a champ.

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