I just put a b&m 2500 stall converter in a 727 behind a 512 stroker. I used a beefy solid flexplate which bolted to the crank with six beefy ARP bolts.

Then the flexplate boltes to the torque converter lugs with four tiny little bolts!

What gives? I suspect leverage comes into play here but how does all that beef trickle down to four tiny bolts? I felt like I had to be extra careful to not shear them while torquing with blue loctite! I’m always blown away when I see ‘weakest link’ items- 1/2” front leaf spring bolts, the single pin that holds the swinging basket to the frame on a ferris wheel...

Obviously it works or they wouldn’t make them like that but does anybody smartener than me have an explanation?

It's 5/16 sae threads and should be cranked down, loctite helps, and should be checked periodically. The heavier 7/16 bolts need to be fairly tight as well. When I worked at chev, it was not rare at all for those 3 bolts to loosen on everyday drivers. Those were 3/8 or 10 mm. They started using locking compound too.

It's in a sheer situation, but not like dropping the clutch at 5000 rpm. A race converter uses the big bolts like the crank size. I'm sure there's some graph online to show the specs of bolt sheer.

Imagine the increase in strength when going from 3 bolts to 4. (Chevy/GM to Mopar)

Some of the 427 Chevys used six.....

They increased the size of the converter bolts for the hemis. The reason that they don't need to be as big as the crankshaft bolts, is because there is less force on the converter bolts than the crank bolts. Torque = force X moment arm length. The bolts further from the crank centerline (longer moment arm) have less force on them. It's lever theory, like you mentioned.

There is another factor involved as well and that is friction.

Friction between the flexplate and the converter lugs is what transfers the torque. The bolts are loaded in tension and should not see shear as we normally think of it.

R.

in theory but, being the fastidious type, I always try to leave all the bolts loose and turn the converter so that it is loaded against the side of the holes before final tightening.

I had three of the four loosen up on my 69 D200 a couple years ago. The guy who put the converter in 18 years ago apparently didn't torque them down. The was the truck was running and sounding I thought I had spun a bearing so I took the engine out leaving the trans in the truck. Needless to say I was pissed when I saw the torque converter flop after taking it out of the trans...

But the longer moment arm is still very much in play as to where the "friction" is applied, to be clear.

If you torque them to the proper specs you should be golden
The only ones I've seen or heard of loosening up was due to inattention to the tightening specs. or a out of balanced torque converter or motor
My current bracket car with a glide with a 9 inch torque converter that has 3 1/2 inch bolts in it due to the former owner running a blown B1 motor in it, most of the good race Hi Po torque converters for Glide use three 7/16 bolts with lock nuts on the other end, no threads in the converter lugs

Yeah when I first put a B.B. in my dart I thought how stupid are these 5/16 bolts. I later switched converter and the better converter had 7/16.
I still hate that threaded converter lug deal.
I had them back out and I swore it was a rod knock.