A short explanation:
There were multiple issues with the Robbe stock head, some had to do with manufacturing others were with the design. The head’s loose fit on the shaft was due to lack of proper manufacturing tolerances and a very liberal QC processes. The only tight fitting part of the head was around the 3mm Jesus-bolt once the Jesus-bolt squeezed the slotted skirt. Robbe anodized the heads’ bores… the 1-Piece Head Block bores were masked off in the anodizing process (a hole’s geometry and sizing cannot be properly controlled in the anodizing process).
Due to the 13mm loose fit above the Jesus-bolt, the Robbe head was wobbling on the shaft which eventually caused the top of the skirt to develop fractures and in some cases break off. When that happened to my Robbe head I decided to design the 1-piece head.
As to loctite’s retaining compounds, I don’t know if they can solve the problem but they definitely can’t hurt. Please note that different compounds are rated for different radial gaps. Also the max strength of the compounds are rated for steel to steel applications and not steel to anodized aluminum.
The 638 doesn’t need primer to bond to the inactive anodized surface, however if I had to choose between the 638 and the 635 I would go with the 635 because of its longer working time.
Hope this helps,