It is an ordinary bolt, other than VERY short to allow removal with rad
installed. VW says M8 x 12mm. If you get one longer than this the bolt
may be trapped in the radiator by the upper bracket of the van body.

No groove is needed. Air under pressure can escape around the threads. I
have replaced quite a few original radiators and don't recall any with a
grooved bolt. I do not remove the bolt for bleeding, only loosen it a
turn and wiggle it. I do remove the bolt when doing a coolant change to
speed up the process.

Mark

Malcolm Stebbins wrote:
> Is the BOLT that is used to bleed the front radiator any special bolt?
>
> Does it have grooves in the threads to let the air escape?
>
> Is it just an ordinary 13mm bolt?
>
> If a special bolt, do you have one for sale?
>
> Malcolm
>
>
>