The BMC A-series Motors

The motors as used in the production of Austin Healey Sprite, MG Midget, Morris Minor, Mini Cooper and many others can also be found in a wide variety of special home built race cars and even more common in various ‘kit cars’ including things like the Lotus 7A and many formula Juniors.

So what do you need to know if you are going to rebuild it yourself? What do you need to know if you are simply going to have a local shop do the whole thing? And just as important, if you are going to do it yourself, how are you gong to pick a machine shop.

 

If the starting point for this motor is an already race prepped motor, then the choice of machine shop is the only critical decision you have to make. And don’t kid yourself, even if this was a good race motor, it can be screwed up by a poor choice of machine shop. Your first choice should always be not just a shop you have used in the past but someone that has had previous experience with these motors. If you have to start from scratch to find a machine shop perhaps your asking a few questions up front will save in real headaches (and dollars) in the long term. Let’s start at the top- if the cylinder head will be needing new valve seats, do they have a source for them as they should? Do they know the clearance requirements for the valve guides? Do they recommend 4 or 8 valve stem seals? (Race motors with new guides, etc. should only have seals on the intake valves.) Do they understand the different hone patterns for use with cast iron piston rings vs chrome rings? Do they have the capability to line ‘hone’ the crank bore. This will be critical if you are to keep the original rear seal from leaking. While a line ‘bore’ can control and correct this seal area, it will also move the crankshaft closer to the camshaft. This results in a lose timing chain and but camshaft timing control. Does the machine shop want you to tell them what piston to cylinder wall clearance they should be setting? While these may sound silly to some of you, you would be blown away by how many shops accept the job from a customer only to then pick up the phone and call the likes of Mini Mania to help them do what they should already know how to do!

 

If you are starting with a stock or street motor with the objective of building a race motor the first thing you need to consider is how many races you want to run before rebuilding. Obviously balanced with this is the consideration of how much HP is enough. If you want you engine to run to 8,000 RPM with a compression ratio of 15:1, you better plan to open the engine up every 3 or 4 races. If you are conservative in your needs and wants, you can expect to get at least 20 races from a rebuild. I neither case does it mean that the results would be such that the cost would then be extreme- it simply means it is time to check everything and refresh bearing, etc. Remember that if you are attempting to keep you motor is top condition it will be necessary to do a ‘valve job’ twice as often as you need to replace the rings!

 

So how do you spec the motor for the builder or yourself? The single most common overwhelming problem we see are people that focus on a single component and believe it’s selection will result in a fantastic upgrade! Too often we hear about the choice of a 300 degree duration camshaft and when asked about the expected compression ratio we hear ‘Oh’ about 10:1’! A motor is an assembly and not merely a collection of parts. Each component must be chosen to work with each other. The A-series motors have now been around for years and have so many ‘tweaks’ applied to them that there is little reason for having to try to re-invent the wheel. Most principals of tuning applies to the entire range of motor sizes so we will focus for a minute on the very popular 1275cc.