I had to destroy mine to get 'em out, 5/16 or 3/8 I'm certain. And much longer than you'd think.
http://www.ebay.com/itm/NOS-1978-19...ash=item2cb76179d4:g:xRsAAOSw44BYSpiS&vxp=mtr
EDIT with ebay link: This guy incorrectly measured them as M10s. That'd mean they're 3/8 if he was even close on the thread-gauge. And his length is 2 1/2 " converted to US units.
http://www.ebay.com/itm/NOS-1978-19...ash=item2cb76179d4:g:xRsAAOSw44BYSpiS&vxp=mtr
EDIT with ebay link: This guy incorrectly measured them as M10s. That'd mean they're 3/8 if he was even close on the thread-gauge. And his length is 2 1/2 " converted to US units.