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My '71 Olds Rocket 350 seems to either have a blown head gasket, cracked head or worse a cracked block. By reading many past posts I understand that a compression test is not the best method for testing these problems. My motor starts and runs good but as it warms up it starts missing getting worse the more it warms up. Until it wont idle at all. But if you give it some gas it will keep running. The other night after keeping it running for about 10 or 15 minutes until it starting missing worse than ever, I decided to do a compression check. Keep in mind this was the worst its ever missed . Any way on the #4 and #6 no compression at all. So I know somethings wrong big time. But the funny thing is that after letting engine cool down the compression goes back to normal, its even a little higher than any other cylinders for #4. Well I know I'v got something wrong in this area, but wonder if I should try some K&W block seal before pulling the head off. Will this help me to determine which problem, Blown gasket, cracked head or cracked block. I just want to get to the root of the problem this time, I pulled the head off before and could see no visible problems. Now its become much worse. Sense it gets worse as engine warms leads me to believe its a crack. But Im pretty much a newbie at all this and may be overlooking some other approach one of you may have. Oh and I get white smoke out of exhaust on start up but goes away after a few minutes. Lots of bubbling while engine is running in radiator.

Today started up and let run until warm. This time it didnt miss as bad as last time but was still bad miss. Did another compression check and all ok this time only # 4 higher than the rest.
I plan to pull head and take to machine shop to have check over But I am wondering should I do block sealer before this?

sick Olds Rocked 350
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