steering rack bolt size

[mgglep]

Hi all
I have a mk 2 escort rack which had the usual spline shaft coupler attached to it I placed a 8mm bolt through it to clamp it up. Looking at it I thought this would be ok as the rack has a dent in the shaft so you psyically have to take the bolt out for the shaft to part from the coupling. Have covered about 150 miles I checked the car over and found the shaft was actually loose and with a bit of a tug the coupling popped off the shaft on the rack
But the bolt was still tight
So have I got a dodgy rack / coupling or is an 8mm bolt wrong and I need to use a 3/8 or similar bolt
Hope this makes sense lol cheers