Has anyone drilled and tapped their torque convertor bolt holes where the flex plate bolts up to a bigger size. Mine has the 5/16 fine bolts in it and they have come loose a couple times. But the real reason I was considering going to 3/8" is that the flex plate holes are almost that large already. And the 5/16 bolts have plenty of room to flop around. It looks to me that a slightly larger bolt would be a better fit and may aid in them not loosening. Is there any reason not to do this.
Rick

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

use grade 8 fine thread..... don't see why not

I had an altered stock type converter once that had this done. The threads eventually failed, I believe it was because a 5/16" hole is too big to cut proper 3/8" threads. I found a table that shows that a 3/8-24 thread takes a Q drill, unfortunately I don't know what diameter Q is. You run into a similar problem if you try to re tap 7/16" head bolt holes for 1/2" studs. If there is enough metal in the lugs it might be possible to put in 3/8" fine helicoils.

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If you didn't drive it there, it's not a street car.

Thanks Seymour that is good info.

Rick

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

I went through this years ago but in reverse, trying to find a flexplate with smaller holes. I think at one point I had a bushing made even but never used them. Came to realize bolts were loosening because whomever had assembled the trans/motor back together didn't think those locating dowels were necessary.

Moral of the story, if they loosen somethings amiss, and its not the size of the holes and/or bolts in the flexplate. If the bolt head's clamping on the plate properly the flexplate won't move. Are you using generic hardware store bolts? I believe their head is a smidge smaller

If you insist on going to bigger bolts, I wouldn't use a helicoil just drill to the required size and tap. The last time I remember handling a converter, the lugs didn't have much extra meat around and a helicoil would only require even more being removed, risking cracking the lug. You'll also need a bottoming tap as there's not much room between the lug and converter.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Performance converters are tapped for 7/16" fine thread. You should be able to redrill and tap your 5/16" holes for that. You will probably also have to tweak the holes in the flexplate. I'd use a good step drill bit.

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ARP 12pt flexplate bolts seem to have bigger heads on them.

Safety wire? Those heads are awfully thin to drill though.