AM 25 Mk I

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The engine in the left hand photo above is the AM25 Mk I (which some astute readers may even have guessed from the page title). For comparison purposes, the right hand photo shows it to the left of the Mk II version. The Allen-Mercury (AM) engine range is covered on the AM35 page, and more information on their designer, Dennis Allen, appears on the AE Engine Range page. This page serves as a quick engine recognition bulletin regarding the visible difference between the Mk I and more common Mk II versions.

The main difference between the Mk I and Mk II is the cylinder hold-down method. On the Mk I, the cylinder liner was attached to the case with four short screws, two of which extended into studs which slipped into holes on the bottom fin of the cooling jacket. The jacket was then retained by jiggling two nuts (6BA) into pockets above the holes and tightening them down, about 5° at a time, provided you had a nice little thin 6BA spanner with a small head. Lacking this tool, a pair of needle nose pliers could be used to quickly and efficiently remove all the flats from the hex nuts and butcher the cooling jacket at the same time. Any wonder the Mk II (and all other engines in the AM range) used long 6BA screws to clamp the liner via pressure on the head. Although simpler and practical, this is heavier and more prone to accidental distortion of the cylinder alignment unless care is taken to gradually tighten the screws in opposing pairs.

 

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