10.75 or 11.5 torque converter bolt pattern? - Pontiac GTO Forum
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post #1 of 2 (permalink) Old 09-04-2012, 11:06 PM Thread Starter
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10.75 or 11.5 torque converter bolt pattern?

I'm in the market for a new contintental torque converter. Is there anyway to tell if I have a 10.75 or 11.5 bolt pattern on my flex plate without pulling the transmission? Contintental told me there are @ 3 to 4 weeks to ship so I really don't want to take the car off the road for a month or pull the transmission just to messure the flex plate and then reinstall it. The transmission and engine is a 400 & turbo 400 out of a '68 B-body car.
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post #2 of 2 (permalink) Old 09-05-2012, 04:01 PM
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Drop the dust cover off the bottom of the trans/flex plate. Measure the distance between two adjacent (next to each other) converter bolts.

Rubbing a little math on it, the length of a chord (straight line that cuts through a circle) is equal to the diameter of the circle (what you're looking for) times the sine of half the "subtended angle". Well, there are 6 equally spaced bolts and a circle is 360 degrees, so we know each bolt hole breaks the circle up into a 60 degree segment (360 divided by 6). If you have a 10.75 inch bolt pattern, then the distance between two bolts is going to be 10.75 times the sine of 60/2. The sine of 30 is .5, so 10.75 times .5 is 5.375 - 5 3/8". If you have a 11.5 inch bolt pattern, the distance between two bolts is going to be 11.5 times .5 = 5.75 - 5 3/4". Soooooo.... if the bolts are 5 3/8" apart, you've got a 10.75 inch bolt pattern. If they're 5 3/4" apart, you've got a 11.5 inch bolt pattern.


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