hi, i'm having a little problem on my 69 f100 with the 302. a few weeks ago i cracked my passenger manifold. after waiting for my new headers i finally got them on. i have tightened then up and the drivers side is great, but the passenger side leaks bad at #4. and now the #3 plug is getting alot of blow by, it's really noticable. the header isn't too bad till i step on it, might be able to tighten it a little more.(it's a cheap ebay one and they didn't flatten enough of the tube so you can barely get to the bolt, 4 of the eight bolts are like this so tightening is 1 flat at a time all the way down.). before the manifold cracked i never noticed any blowby at all.
any ideas on what to check? i pulled the plug today and didn't see any sign of cross threading? could it be moisture got in and stuck the exaust valve?
when i had the headers off i cleaned the block and didn't notice any cracks or anything.
thanks
mark

 

Suggest check clearances and make sure the headers are not resting against the frame or crossmember, etc., a very tight fit may be sufficient while engine is at rest but cause problem with engine torque.

Installed Hooker Longtube Headers on 65 F100 with T18 and 302. Once in place, appeared clearance on passengerside, where the header flange meets the crossmember, was minimal, if any, while at rest but could cauge binding on passengerside header with engine torque. Used angle grinder with 4" grinding wheel and remove small amount of material. If should find the insufficient clearance between headers and frame, etc., might want to consider spacers.

For info purposes: had some extra insulation leftover from resealing wood burning stove, looks like a rope but has the same inside dia. as the starter cable. Threaded it thru the insulation and taped both ends. Would not think headers vs exhaust manifold would present and additional heat issue with the starter cable, one of those situations where light bulb came on in considering what to do with leftover insulation. Anyhow, rambled on enough, had an urge to chime in and add my $.02.

 

that's why i went witht the shorties. there's lots of room around the headers. i think it's the starter selenoid, it clunks but doesn't pass enough volts till it gets warm. the main problem is the spark plug blow by and the leak on #4. i keep tightening it but it still leaks. i might have to pull the header and try a liquid gasket type solution. why does this always happen with this truck, my last one was never any trouble.lol and only in winter.

 

You could try getting a good set of new header gaskets, and a set of stock exhaust manifold gaskets, put the stock ones next to the head and the others next to the headers, I have done this before and it actually works very well, seals good and did not seem to need much retorquing afterwards, or you could take the offending piece to a machine shop that has a table like a big belt sander and even them up that way also...