The AEC Monocontrol epicyclic gearbox was used in both close-coupled and island-mounted form.
As best I can determine, the Regal IV (later variants), Regal V, Regal VI, Regent V all used only the island-mounted variant.
The Swift/Merlin used only the close-coupled variant.
But the Reliance seems to have been a mixed bag.
The MU (and by extension, the 2MU) had a close-coupled gearbox, as shown in this except from a 1956 May sales brochure:
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AEC Reliance #541 195605 p.05.JPG [ 98.36 KiB | Viewed 1065 times ]
The HMU (and by extension, the 2HMU) also had a close-coupled gearbox, as shown in this excerpt from a 1956 sales brochure:
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AEC Reliance Overseas #545 195607 p.04.jpg [ 991.67 KiB | Viewed 1065 times ]
There is no reason to suppose that this configuration did not apply to both the 16’0” and 17’6” wheelbase variants.
I do not have any “hard” information about the Reliance 470 4MU/5MU and Reliance 590 2U/4U variants. My best estimate, from interpolation between the earlier MU/2MU and the following 6MU/6U models is that they had the island-mounted gearbox. Specific information would be welcome, though.
In respect of the Reliance 691 6U, both the home (18’7” wheelbase) and export (17’6” and 18’7” wheelbase) variants had island-mounted gearboxes, as shown in these excerpts from respective sales brochures dated 1966 August.
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AEC Reliance 691 Home 196608 p.04.jpg [ 478.57 KiB | Viewed 1065 times ]
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AEC Reliance 691 Export 196608 p.04.jpg [ 1.43 MiB | Viewed 1065 times ]
Of note is that the 17’6” wheelbase 6U had an island-mounted gearbox, whereas the 17’6” wheelbase HMU/2HMU had had a close-coupled gearbox.
It seems reasonable to assume that the 6U inherited its configuration from the preceding 2U/4U.
In respect of the 6MU, the only datapoint that I have is that from a 1970 July specification sheet for the home model. In the optional Monocontrol case, this was described as having a separately-mounted gearbox. As both the 16’2” and 18’7” wheelbase variants were covered, we may assume that this applied to both.
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AEC Reliance 505 Home #0055 197007 p.02.gif [ 729.22 KiB | Viewed 1065 times ]
The 18’7’ wheelbase 6MU variant probably inherited its island-mounted gearbox from the 4MU. That the 16’2” wheelbase variant had an island-mounted gearbox is somewhat surprising. Two possibilities come to mind. One is that the 16’4” wheelbase 6MU did not simply inherit the close-coupled gearbox of its 2MU predecessor, but instead was configured the same way as its longer wheelbase brethren. Another is that the 16’4” wheelbase 6MU did have the close-coupled gearbox, but that when the chassis was reconfigured with a 16’2” wheelbase and longer front overhang, the gearbox placement was changed. I suspect that there is some chance of finding out what was the Monocontrol gearbox placement on a pre-1970 short Reliance 505.
The 17’6” wheelbase export 6MU could have followed either the 2HMU (close-coupled) or 17’6” wheelbase 6U (island-mounted). On balance, the latter seems to have been more likely.
That 1970 July Reliance 505 specification shows that the Monocontrol gearbox was available in either 4-speed or 5-speed underdrive form. I am not sure when the 5-speed option was first offered on AEC chassis, although I think it was in the LMC era.
The 5-speed version of the Wilson gearbox was in the SCG lexicon quite early on. When Guy started licence manufacture in 1948(?), it went abundance and built the 4-speed, 5-speed overdrive and 5-speed underdrive forms, each in both spring-operated and internally air-operated preselector form. At the time Daimler was building both the 4-speed and 5-speed underdrive preselector forms, both spring operated, and AEC was building the 4-speed preselector form, mostly with internal air operation but with spring operation at customer request. Leyland started with the 4-speed direct air operated model, and AEC also added this type to its range. Leyland had added a 5-speed underdrive version by 1958. Daimler first moved to the internally air-operated preselector form, 4-speed only as far as I know, and then to the 4-speed direct air operated model as the Daimatic. I am not sure if it added a 5-speed option before the BL era. Guy was last to adopt the direct air-operated form, and when it did, it offered the full range, namely 4-speed, 5-speed underdrive and 5-speed overdrive variants.
Note that at various times Daimler offered its spring-operated preselector gearboxes variously with hydraulically assisted mechanical, external hydraulically operated and external air operated gearchanges,. In all cases the basic spring-operated gearbox itself was unchanged. External hydraulic and air operation required pressure to effect gear disengagement, rather than pressure to effect and hold gear engagement. As I understand it, AEC had tried external vacuum operated gearchange with spring operated gearboxes on the 1930s Q model, but rather unsuccessfully. Assuming the use of air-suspended vacuum equipment, I guess that outcome was not too surprising. Better might have been vacuum-assisted mechanical, perhaps adapting a brake master servo unit for the purpose.
Cheers,