found 4 holes where the thread has disappeared in the 350 block (7/16 UNC), can you drill and tap a new thread to use recoils/helicoils to use the same head studs!, still has thread on the studs, unsure why the threads have gone, Ran the engine for not even 5 mins, can i use the steel head gaskets or just get some standard type ones!
saki,
Feb 20, 3:27pm
You can use recoils get a kit for the appropriate thread, uses an over size tap the start of the tap is the original thread size to guide the tap.
pieman33h,
Feb 20, 3:29pm
why not drill out to 3/8 and convert to studs, never pull the threads out again!
clark20,
Feb 20, 3:30pm
How many are "just" about to fail if 4 already bad! Maybe get them all done professionally.
mk3zephyr,
Feb 20, 3:42pm
Ive run a 7/16 tap through the good threads to clean them up, reason the 4 have failed by the look of it is because of a bad installation sometime previously, was actually thinking of tapping them out to 3/8 and putting 3/8 bolts in there are 2 on each side at the top of the head, not anywhere near the water jacket studs
jfp1,
Feb 20, 3:44pm
It's not uncommon to have to recoil the odd head bolt hole in Chevy blocks, especially big blocks for some reason. Because the holes are not blind, coolant seeps up the threads and can corrode them. GM made a special sealant for it's engines to stop the coolant getting at the threads and to stop coolant leaking into the oil wetted areas under the valve covers. 7/16" UNC is the size and various suppliers sell the re-coil kits but there aren't real cheap.
mk3zephyr,
Feb 20, 4:30pm
Just had a closer look and the offending holes have been "coiled" before and have ripped out the coil threads in the block included, also 7/16" is bigger than 3/8' so thats a no go. The bolts measure 3.5", Now if i was to tap the 7/16" hole a further 1/2" would i get away with using a 4" high tensile bolt and hardened washer in its place!, just a thought
mk3zephyr,
Feb 20, 4:42pm
see 2nd post above this one, post #7
mechnificent,
Feb 20, 4:43pm
Or you could get solid tube type inserts that cut their own thread as you install them. They take a bigger hole than the coil inserts do.
mechnificent,
Feb 20, 4:58pm
Go to an engineering supply shop and ask for rethread inserts. they are hardened and thread themselves straight into place nice and tight. You don't need a fancey drill or a tap, just a bolt and a nut or two to lock into the insert, then you turn the bolt till the insert is flush, unlock the nuts and take the bolt out ready for the next insert
They come in fine and course threads on the outside in whatever inside thread you want. The fine external threads are slightly smaller and are better for close places. in your case get the course thread on the outside and it will accomodate even a well oversized hole.
mk3zephyr,
May 7, 10:15pm
so will tapping the thread further with a longer bolt not work!, thought i could try that for starters, even a cap screw as they are pretty tough. its only the 1st 10mm or so that is stuffed, rest of the thread is there deeper down and plenty of it
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