The bolt on my ryobi whipper snipper attaching the head to the shaft has been lost or loosened during use. can u easily grab a bolt of same diameter and replace this or is there more to it then that?

exactly what is the question
there are a number of blades available on the market that fits on the ends of straight shaft drives of trimmers that include alligator , fixed blade or chain saw blade
the alligator is a head that has 3 stainless blades that are free to pivot , the fixed is like a triangle and the chainsaw is round with a chainsaw attached to the circumference

Make sure the head is engaged on the drive shaft correctly

no clockwise only

Englebert the most common thing would be that the inner shaft has sheared off. Remove the head and see if it spins then.

Hold the shaft to prevent it from turning, then turn the trimmer head clockwise (viewed from the bottom) The cutter adapter has left hand threads. Hope this helps you out, as this is from the service manual. Best regards and good luck.

Is definitely a left hand thread on the bolt...your machine came with a small rod to hold the head while loosening or tightening the nut which holds the blade....there is a hole and a slot in the machines head to insert the rod....if you've lost it you can probably use a small piece of drill rod about 5/16 diameter or a drill bit to hold it ...turn nut in clockwise direction and it will probably come loose....good luck !!! ;-D

I never heard it called a whipper snipper before but I am on this side of the ocean. you should be able to go to www.ryobiuk.com. Put in your model number and you should find a parts list for it.
HTH
Good Day. mate

This was answered in a previous solution

After much mucking around I noticed a hole in the top shroud of the shaft. I placed a narrow screw driver through the hole until it made contact with the rotating part of the head and pushed until the head would not turn. I thern applied pressure clockwise until the head became loose. I did the reverse to tighten. The head of the unit will rotate anti clockwise to tighten as the shaft rotates clockwise during operation. I hope this explains the method.
Thank you any way for your interest.

Greg