I drove and repaired all of these, some more than others. From a driver’s and mechanic’s viewpoints I would rate them the same: top AEC, then David Brown, Fuller (range change not 6 six speed) and very definitely last ZF. A lot depended on the installation and the amount of abuse or care they had received. Like everything they all had their faults. ERF were head and shoulders above everyone else with their installation; some others were abysmal - the rear engine mountings do not belong on the gearbox and some manufacturers even today still haven’t got the ■■■■■■■ message. I have recently been introduced to the term “Professional Struggler” which so aptly describes what the manufacturers turned the life of a fitter into.
I was never really a fan of the Fuller and they certainly weren’t trouble free. I hated the chrome flat top gear knob it made the feeling of there being a long length of stiff rubber between the hand and the gearbox worse. The advantage of the box was its close ratios allowing faster changes, aided somewhat by the clutch brake. It however suffered from the same fault as the David Brown brake - contamination from over enthusiastic greasing of the release bearing. It had another trick up its sleeve, the lugs securing it to the input shaft broke off, which in those days meant gearbox removal to fix. Since the clutches of the period went out of adjustment quite quickly it often wasn’t working anyway, poor adjustment in the workshop didn’t help hence one reason for the lugs mentioned breaking off when the clutch brake disc(s) ended up trying to stop the weight of the complete vehicle. The range change interlock slug would seize in the side of the gearbox, and the control valve or its pipework caused issues, 5/32" nylon pipes with special expensive olives which were a swine to remove at times in the restricted space available - even better in an 8 wheeler. The main box was really quite good all the troubles being abuse related. The boxes which were difficult to change had burred teeth on the dog clucthes or gears. The same good opinion could not be said of the range change section. The synchro cone wore often because the springs weakened which was the cause of the loud noise on engagement. This led to the one of the three pins breaking and eventually no range change. Left long enough this chewed up the mainshaft output gear. Actually working on the boxes was fine really, it was just getting the things in and out that was a pain in the “hurry up you’re taking too long” atmosphere of a dealers workshop. Definitely not helped by the refusal to buy the manufacturer’s special tools to make the job easier. You didn’t need them strictly but it took longer without. The same attitude meant that the range change repair was expected to be done in situ: so much more difficult than with the box stood on its nose but a financial winner for the dealership who chraged for gearbox removal and replacement.
The ZF fortunately is a more distant memory, There was an AK680 and and AK690 and a smaller box as well. They were all a tricky box to handle as a driver. DAF seemed to make an absolute pig’s ear of installation, my lasting memory is of the well known saying about stirring a rice pudding - it was this combined with a box which required the timing to be down to a milli-second which made things so tricky. Oddly enough since they ■■■■■■ up every other box they fitted in the 200 and 400 range with the reverse gear pattern, the 300 with a ZF was a LOT easier to manage. All suffered IIRC with selector and detent issues and ZF seemed to manage to make the most complicated part of any gearbox even more so. Getting the things apart wasn’t easy either just about everything required a puller or a press and I have vague memories of uncaged needle roller bearings…but I may be mistaken.
The David Brown six speed 06 600 would IIRC have been right at the limit of its torque input capacity behind a ■■■■■■■ 220 and behind a Gardner there would be a lot of gearchanges and maximum torque input if running in hilly country. From a drivers aspect they were a delight in ERFs and Atkinsons, one finger on top of the knob and it was like a hot knife through butter, but you did need to get used to them, they could be unforgiving at first or if in a hurry. The main problem I recall was the casings which wore at the bearing bores. This seemed to be put down to the near constant full torque operation in the hills. The case split in half which made stripping it down so much easier than some rivals. It had a support bearing in the middle of the two shafts which required the gears etc to be pressed or pulled off. The tricky part was fitting the top half of the casing without trapping the mainshaft shims. Like all 'boxes it broke teeth of the shafts and it had a habit of breaking selector shafts which made the job more complicated than on some other makes, since the box had to be split. Seddon Atkinson really manged to ■■■■ this one up with their appalling installation. Not only was the patern reversed, but the previous light action was gone; replaced by the need for a vice-like grip and accompanying effort to move the poxy lever. They went one better and fitted a Lipe Rollway pull type Clutch (probably at the insistance of “sales and marketing”). This entailed a complicated set up procedure: involving much measuring and re-adjustement of the master and slave cylinders as well as the actual clutch itself with different sized measuring blocks, all vastly different from standard practice; to make it work . After that was done the clutch brake could be set up.
AEC or Thornycroft. There were two main six speed boxes the D197 and the D203 for Mercury/ Marshall in the case of the former and Mandator/ Mammoth Major the latter.
These I found a real pleasure to drive - except for one thing which most have probably forgotten : - that infernal buzzing from the gearlever all day long, caused by wear in the change speed box, remote- mounted on the side of the engine. It could be shimmed to tighten things up, but access was difficult to do the job. IIRC there was a similar issue with thee same noise from the David Brown boxes in ERFs and Atkinsons but not quite so bad. Having driven a coach with the smaller box in recent years it has once again reinforced the opinion about installation, abuse and wear in the linkage. With the gearbox half way down the chassis everything is exacerbated, difficulty in hearing an engine so far away makes timing more difficult and as a result over the years bad changes burr the dog clutches and gears, so making the timing more important. Both these boxes suffered from drivers starting off in third gear when empty, the bearings and particulalry the rear one took a dislike to this habit. A circlip locating 2nd gear bush broke quite often which allowed all the gears to move along the shaft, if lucky the box would be stuck in one gear; if unlucky you lost a tooth. The grubscrews holding the selector forks to the selector rail would quite often work loose, again resulting in only being able to select that gear. Early boxes with second gear running on a bush sometimes saw lubrication failure seizing the gear to the bush with similar consequences. However none of this was ever a problem to fix unless the damage ws severe. A hammer and a punch was about all that was needed to get either of these two gearboxes apart. The grubscrew issue could be dealt with beside the road. The 10 speed versions were similar but a very big box indeed to get in and out.
Since we have mentioned Magirus Deutz, that is top of my list for hates. An AK 690 with GV90 splitter in an 8 wheeler is a nightmare, the only sensible way to get the box out is to remove the engine first.