Howdy all. <O</O I recently rebuilt my 302 SBF, and on its first start up it ran for about five minutes before blowing out one of the frost plugs and dumping its coolant all over my feet (my bride was inside the car on the loud pedal, and I was setting timing and watching for leaks). My first thought was that there had been an air pocket in the water jacket, causing an over-pressure as it warmed up (though the 16# rad cap valve never vented any pressure). <O</O In an attempt to avoid any air pockets for attempt #2, I removed the thermostat, sat the car about 20 degrees nose-high, filled the coolant directly into the thermostat housing neck until full, then attached the upper rad hose and topped off the rad to fill the hose. As for the frost plug, I re-installed it as best I could without pulling the motor back out. </O Now, in hindsight, it was probably dumb of me to put the same plug back in the same hole and not expect the same results, because that's exactly what happened. It ran for two minutes and blew out the same plug, dumping coolant all over my buddy's feet (this time I was behind the wheel where it was safe and dry). I'm still not sure what caused the original failure of the plug. It may not have been installed right, or may just have been a bad plug from the start. The frost plugs are one of the few things on this motor I didn't do myself, so I'm pretty pissed about it. The engine shop that did the machine work for me installed them as a "favour" when they were driving in the cam bearings. Thanks for the big time saver, *****s. Anyway - I figure my options right now are to pull the motor and install a NEW frost plug in there correctly (there's no way I can properly drive it in with the motor in the car, there's just no room to work), or I could throw a dummy block heater in that hole and tighten it down so I know it's physically held in place. I didn't see any evidence of oil in the coolant, or water in the oil, so I don't think I've got a head gasket breach, but who knows? I'll start it without the rad cap on for attempt #3 and see if it starts pushing coolant out immediately. If so, there may be cylinder pressure getting into the water jackets. But I've got to get it to hold water before we can test that idea. Am I a fool to just seal it up with a block heater and try again? I'm all in favour of doing a job right, but is this "ingenuity" or "stupidity"? I think I've spent more on coolant than I have on speed parts!
Well if it really is a 16ib cap and it did not vent I would have to think wrong plug(s) or someone bent them up pretty bad on the install.
hey just a suggestion before you pull the motor..go get a rubber freeze plug they got em at napa and just about any auto parts store..try this first and see if ti holds or blows out another one..I would be suspect that the one those guys did as a "favor got set in crooked or wasnt seated correctly or was the wrong one all togeather ..try the rubber plug and get it up and see what happens from there. Just my 2 cents
oh BTW those rubber plugs do have a bolt in them and a nut you screw in til it gets real tight they work ive used them in a pinch before and if its hard to get at than no one will see it eather!!
i had a 429 that was doing the same thing, i put in a block heater and it would push the block heater out just enough to leak at high rpms but when the rpms came back down it would seal again, i ended up taking off the heads and found a hairline crack in the head gasket.
I'd think faulty plugs, buddys Firebird did sort of the same thing when he used some cheapo freeze plugs changed out the cheapos, for some br*** and they've held great.
Study the hole carefully...try to get in there with a wire brush. There may be rust pits in there from its previous life.
Run a compression check before you fire it up again. That'll rule out several things if the compression checks ok.
How did you put it back in? Compression test and take a caliper and check the plug hole for roundness.
sounds like a cheap or bent soft plug,if there was enough compression leaking to blow the plug out from a head gasket blown or a cracked head you would have been showered by antifreeze coming out of the filler neck and the 16 pound cap would never hold that much pressure,take a wire brush,or sand paper,or emory cloth,clean the hole real good,either drive a new freeze plug back in or use a rubber one with a bolt
Thanks for the advice, everyone. I was not able to get a close look at the bore condition due to the extremely close quarters up in there, but I was able to pop in a rubber expansion plug and get it tightened down. The compression test checked out fine; all 8 are pushing strong and even. I fired it up again about two hours ago and it ran like a champ for a good half hour. Life is good. I should be ready for the first drive very soon. Thanks again guys.
Just in case, put another of the expansion plugs in your tool kit...like carrying an umbrella to prevent rain!
I used to have a 427 Ford that blew expansion plugs on a regular basis. Back in the 60's you could get expansion plugs that had a bolt in the center with a T-bar (like a molly bolt) on it that you turned sideways and put in the block hole, then straightened up and tightened up the center bolt. The only way those came out was with a piece of the block attached (I know).
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