In the race car I have a nice 3200 or 3400 RPM convertor that I am planning to reuse. The problem is that it has the small 5/16" bolts in the mounting lugs. Does anyone see a problem with enlarging them to 3/8"? They have come loose several times before, and while they were not loose loose when I pulled the motor Monday they were loose enough that there are marks on the flex plate where it had moved around under the bolts. That is with clean holes, locktight and ARP bolts. I think half the problem is that the holes in the flex plates are larger than they need to be.

There is enough meat in the lugs to go one size larger and there is about 1/4" under the lug.(Between the lug and the torque convertor) So it looks like there is enough room to run a tap through it. I just don't want to ruin a good convertor.

Thanks,
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've done it on a 440 equipped street/strip car I had before with no problems.

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There's no such thing as too much cam....only not enough engine!

Thanks Scott! I plan to have 440 power!

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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 have done that as well.. be sure to not drill into the converter.. and use a bottoming tap fir the threads.

Greg

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I once stripped one of those holes out, so I drilled it out larger and tapped it for a Time-sert (like a heli-coil but solid, supposed to be stronger). I used the shortest Time-sert I could get, which I believe was 1/2" long, and it just barely fit. with the ~1/4" clearance underneath. I'm pretty sure I didn't use a special bottom tap (for blind holes), so you shouldn't need to worry about that. Hell, if the threads at the bottom aren't quite finished it'll just be a tighter fit!

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
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Thanks guys, I feel better now. Off to run the drill through a convertor!

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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 once owned a converter that had been tapped out for 5/16" bolts and I could never get them to stay put. I think the problem is a 5/16 fine tap requires a #1 drill which is .228". That means that some of the 1/4" threads will remain and weaken the new ones. I like the idea of using some kind of insert or I think CNC dude knows about some special kind of tap that might work. This question came up before regarding drilling out the block for 1/2" head studs.

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

Seymour, i'm trying to think back to when I did this in the 440 car I had. If I remember correctly, the previous owner stripped one of the converter bolts out and just left it out without trying to fix it. The next size up thread-wise was the same as the GM small/big block bolt size. So I just drilled the wallowed out hole for that tap drill size and re-tapped(under the car with the tranny still in). GM had the ideal converter bolt length-wise for flexplates and never had any trouble out of it. Just keep the drill straight as you drill it and the tap also as you re-tap and you should be fine. Put some lok-tite on the threads as a precaution which i'm sure you already do Rick.

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There's no such thing as too much cam....only not enough engine!

If drilling them out, I think I'd just go for 7/16" bolts.

Lou

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Home of Slant6-powered fun machines since 1988

Currently they are 5/16 fine and there is room in the lug to go to 3/8" but not 7/16".

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

Rick, I mistakenly thought we were going from 1/4 to 5/16. A 3/8 tap requires a Q drill .332" in diameter which should clean out the 5/16 threads. Mine had been taken apart many times and maybe the real problem was cross threading. Anyway it looks like an upgrade from 5/16 to 3/8 is possible. Easier to do out of the car than in it.

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

Tap drill size for a 3/8"-16 tap is 5/16", so you should be able to remove all the old threads. If you want to use 3/8"-24, the tap drill is even larger (size Q).

I said earlier that I didn't use a bottom tap for the time-sert, but looking at it again I don't know how there'd be enough clearance underneath to fully tap the lug with a standard tap. (I just pulled the engine out of this exact car, good timing!)

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Sorry I never gave an update on this, but it worked out fine! Drilled slowly and tapped them slowly too. Just used a regular tap and it worked Ok. I did order new grade 8 bolts with the heads drilled for safety wire so they should stay put.

Rick

_________________
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