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So I have this old beast with 90 460 in it.. Kinda a bita truck, bita this bita that. Engine is bone stock to the best of my knowledge. EFI . My problem is broken exhaust manifold bolts. 3 to be exact. Got one out ok. Well a royal pain but enough threads left to be functional. #2 not so good. Drilled it out to helicoil the bolt, but the tap doesn't seem to want to start. Doing this by hand, stuffed under the fender. The tap will just catch, then chip off the piece and keep spinning. Its actually just hogging the hole out. Are these heads high nickel content or something weird I am not aware of. I've hand tapped more than a few holes and this just doesn't feel right. Put a lot of heat there to attempt to take the temper out of the hardened bolt. That shouldn't harden the cast, right? I'm really frustrated. Don't want to mess up the hole and REALLY don't want to pull the head. Any info is appreciated . Thanks.
 

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What kind of tap are you using? I would start with a tapered tap then use a bottoming tap to finish.
 

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It might be too late for you at this point but you can rethread with a 10m tapered tap and finish with a 10m bottoming tap.10m bolts have the same pitch threads as a 3/8 course thread bolt and being slightly larger,they are a great fix for stripped or damaged 3/8 threads.Trust me on this. Have done it many times in situations like yours.
 

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So I have this old beast with 90 460 in it.. Kinda a bita truck, bita this bita that. Engine is bone stock to the best of my knowledge. EFI . My problem is broken exhaust manifold bolts. 3 to be exact. Got one out ok. Well a royal pain but enough threads left to be functional. #2 not so good. Drilled it out to helicoil the bolt, but the tap doesn't seem to want to start. Doing this by hand, stuffed under the fender. The tap will just catch, then chip off the piece and keep spinning. Its actually just hogging the hole out. Are these heads high nickel content or something weird I am not aware of. I've hand tapped more than a few holes and this just doesn't feel right. Put a lot of heat there to attempt to take the temper out of the hardened bolt. That shouldn't harden the cast, right? I'm really frustrated. Don't want to mess up the hole and REALLY don't want to pull the head. Any info is appreciated . Thanks.
if you are using a 3/8 helicoil tap the hole needs to be 25/64 dia
 

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Discussion Starter #6
Yes 25/64 drill for the helicoil tap. The tap that came in the helicoil package. It's a tapered tap. I'm getting into the meat of the tap and hoping it will finally get a bite. but so far it has just been hogging out the hole. By hand , using my variable speed Milwaukee. Just doesn't want to start.
 

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Yes 25/64 drill for the helicoil tap. The tap that came in the helicoil package. It's a tapered tap. I'm getting into the meat of the tap and hoping it will finally get a bite. but so far it has just been hogging out the hole. By hand , using my variable speed Milwaukee. Just doesn't want to start.
maybe you could do a test piece on apiece of scrap first and see whats going on . maybe a piece of bolt still in there you could use an x or y drill to go a little bigger on the hole but sometimes taps are marked wrong because people are being pushed to make numbers!
 

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I had this same problem, although the head was not on the engine so I had more room to work. Here's what I did. Take a 1/4 inch thick piece of steel and drill 2 holes in it. One hole to align with the hole you're trying to tap, and one to align with the nearest adjacent hole with good threads. Bolt the steel piece snugly to this hole. When you're satisfied that the other holes are perfectly aligned, start tapping. The tap should thread relatively easily through the softer steel. This helps to hold it straight as you start into the cast iron. Take your time and use some cutting fluid to help lubricate the tap. Good luck!

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