Just to add to this cam bearing discussion from the 'way-back machine':
I just measured the front cam bearing bore in 4 heads and while I need to make my measurements again to get down to the last .001", these bores are all over the map. Here is what I have now:
- '75 head: 2.111"
- '75 head: 2.107"
- Early 3 bearing head with triangle: 2.105"
- Early 3 bearing head with triangle: 2.097"
Now the '77 dealer parts manual lists 2 different OD bearings for the front cam bearing position: 2.11" and 2.07". And every other bearing position in both 3 and 4 bearing head also has 2 different OD bearings listed, with one that is .04" smaller than the other.
So these 4 heads would all use the larger of the 2 different bearing OD's from the Opel parts catalog. Durabond lists their front bearing OD as being 2.1069" So the 1st head in the list above looks like the Durabond front bearing would have too small of an OD, and that bearing would 'fall through' the front bearing bore if nothing else was done. I need to check that particular head again, but it sounds like the problem that the OP ran into... he needed an original Opel front cam bearing with the larger OD.
BTW, I will make a suggestion on what to do about any bearing bores that are slightly too big for the Durabond bearings: Use Permatex High Temp Sleeve Retainer compound. (PN 64000). It is rated by Permatex to be used with diametrical gaps of up to .007" and stand temps up to 400F. One of its explicit uses is for bearings in loose bores. Also, it can be used on valve guides so it can take all the heat that the cam bearings will ever see.
One thought about assembly with this compound: apply it and then insert the cam (with a thin coat of light grease on the cam journal) to make sure all is lined up. Just make sure that it does not get on the cam journal: The mfr's instruction for diassembly reads something like 'heat the parts to 500F and apply pulling force to disassemble'! LOL
For the others, it looks like there will be a variation in the front bearing interference fit with the Durabond cam bearing set. No wonder some folks report that their cam bearings needed some shaving for the cam to fit in. That by itself is not unusual in many other applications.