Lowlites, Splitties, Thous and Millions Lowlites, Splitties, Thous and Millions
Lowlites - Splitties - Thou's - Millions

Modifying A-Series Engines

You want me to tell you everything there is to know about modifying Morris Minor engines, don't you? Unfortunately for you, I'm NOT going to do it!

For one thing I DON'T KNOW IT ALL! (don't tell anyone I admitted that!). For another, there's just too much to know. The easiest thing to do, for all of us, is for you to go out and buy the BMC Engine Bible, a.k.a '

My first attempt at any engine building whatsoever was going to be a rebore & tidy-up of the standard 948cc engine of my 1962 Minor 1000. What came out of the garage two months later was a Minor that climbed hills like they weren't there, accelerated like a scolded cat and cruised oh-so-happily at 85mph! I even got 100mph out of it down the back straight of the local racetrack!

The following is a few of the things I learned whilst building a Race-Spec Minor Engine!

  • A set of extractors/headers. If possible get a set which exit the Minor engine bay through the standard exhaust aperture. Some extractors exit down the side of the block past the gearbox. Avoid these if you can as the pipe width is restricted and generally results in sharp bends in the pipe which constricts gas flow.
  • A set of double valve springs to suit the head (Mini Cooper).
  • A re-profiled camshaft - I used an original Formula Junior race-cam, but for general road use this is a bit hairy! I ended up using a modern version of the BMC 731 camshaft - this is a much better bet for the road.
  • Double row (Duplex) timing chain and sprockets. Try finding a set from an Austin Allegro. These chains are stronger and less likely to stretch and therefore result in a quieter engine. You may be able to find a set which includes a chain tensioner - this is a good thing as it theoretically eliminates all timing chain rattle.
  • Lightened flywheel. This allows the engine to rev more freely, but does affect the torque available to pull up hills etc, however, this is easily offset by the increase in horsepower. Note that flywheels should be lightened on the inside face (facing the block) and avoid cutting into the surface that bolts to the crankshaft. This can cause serious vibrations if this surface is not perfectly flat!



  • When fitting the Imp pistons to the Minor con-rods I replaced the gudgeon pin circlips with custom made Teflon bushes. I had to do a rebore after one of the original circlips broke and the pieces scored up and down the bore! Teflon bushes mean no more broken circlips! If not Teflon, then you should at least fit new gudgeon pin circlips.

    When fitting the duplex timing chain, you have to replace the two bolts behind the crankshaft sprocket with countersunk Allen key screws. I simply used an oversize drill bit at slow speed to countersink the holes in the front engine plate.

    Make sure you prime the oil pump with some heavy grade or engine rebuild oil as you do the rebuild. If you don't, there will be an air blockage and the oil pump won't be able to do anything when it comes to starting up your new engine for the first time. If you forget (as I did the first time!), then it is possible to inject some oil down the oil gallery at the back of the block on the exhaust side. This oil gallery is located just in front of the oil pump, so (hopefully) it will drain into the pump and get things moving.