Engine transplants - initial information and engine choices 

Terminology - 
BBU - Big Bore Unit 
SBU - Small Bore Unit 
Bodge - English term for 'make do' engineering- assured to fail at an in opportune 
Dizzy - Distributor 
NOTE; this information is largely for transplanting large-bore units into small-bore engined cars. 
Is this familiar? - You take the small one out; you put the big one in. In, out, in, out, shake your fist about, you do the hokey-cokey and you turn around, that’s what it’s all about...kinda sums up the situation many folk find themselves in when attempting to endow their beloved Min with a more impressive turn of speed. Engine swaps - particularly to big-bore options - is possibly the most common form of performance enhancement carried out by Mini owners. On the surface it seems a pretty straightforward job - whisk out the standard SBU and whack in a BBU. Probably take - ooooh - a good weekend once a suitable BBU's been sourced. 
The nightmare tends to go like this - start Friday night straight home from work instead of going out on the beer with your mates. Saturday 9.00am down the local Mini spares specialist getting the odd engine mount/gasket/oil seal, back home to swap all ancillaries over, shove it in the hole, start connecting everything back up. Aaargh, this hose is JUST too short, and that one kinks too much. Where’s the heater tap connection? Humph, these heater pipes are the wrong diameter. I’ll just tweak this up, down, tape it, bodge, bodge, bodge. Sunday a.m. - flash down to the local do-it-all accessory shop to buy over-priced parts that aren’t quite right to complete more bodging. Finish connections, exhaust fitting, and start it up. Or not. Damn, which way round did those wires go? Hello, what’s this one with no connector on? Spark? Yeah. Fuel? Yeah. Oh God - HEELLLPPPP!! I need the car to go to work tomorrow morning!! 
Let's try and ease the agony. To cover absolutely all combinations and peculiarities of each and every engine type available would take a full book. Just the engine types and how to identify them is considerable. This, therefore, is not going to be a definitive bible. The 'Engine transplant -' series is to be a guide through the jungle of engine/gearbox/component selection and fitment problems. To start the ball rolling, here’s a brief résumé on suitable engines. In the main, A+ derivatives will be dealt with first, as these are most common. 
Basic engine selection 
An abundance of Metros in the scrap yards means that there’s plenty of 1275cc engines available. These are not the only source however. Allegros and Austin 1300’s (Austin Americas in USA) are also a ready source, although a little rare nowadays. Avoid the offer of a cheap Maestro 1275 unit - it's a whole different ball game! What must be born in mind is the type on engine you are swapping it for. The advent of the A+ engines brought with it a number of different types of ancillary fitments not interchangeable with its aged forerunners. So unless an engine with absolutely all ancillaries is sourced, stick with swapping like for like - pre 1979 cars with pre A+ engines, post 1979 cars with A+ engines. I’m not saying swapping an A+ for a pre A+ is impossible, just more involved. The significant and relevant differences are detailed in the relevant article. 
Pre-A+ engines are those without the external stiffening webs that run across the sides of the block, the distributor retained by an oval collar-clamp secured to the block by two bolts. These two points are the easiest to identify without any delving. A+ types have the stiffening webs and the dizzy’s retained in the block by a forked plate and one bolt. 
Engines can be identified by the engine number embossed onto a plate riveted to the block on a platform just below the temperature sender. Any number starting ‘12H’ denotes a 1275cc capacity. I know - Cooper S 1275s started with ‘9F, 9FD, or 9FE’, but the likelihood of one of these cropping up is extremely rare. I’m covering the most popular stuff. If there’s no plate, look at the side of the block underneath the carb/exhaust manifolding - 1275s are a solid casting here unlike the SBU that has the two tappet chest plates bolted to it. No engine number plate could also mean it’s been rebuilt at some time, so possibly to be avoided unless the builder is known or a full strip-down prior to fitment is planned. 
It’s also possible to tell from this number what the compression ratio and final drive fitted is - but the list is extensive, precluding me from unfurling it here! If you are lucky, the number may include the letters ‘HC’ (high compression) or ‘LC’ (low compression) - the higher compression ratio (CR) generally giving better performance, and therefore more desirable. If you only have the number, Mini Spares/Mania AKM2 parts catalogue is an excellent source of identification details, as is Chris Rees ‘Complete Mini’, and the ubiquitous Haynes manuals. There’s also one on the Metro by Rob Golding I think. 
By far and away the easiest way to identify what you are likely to be getting. Economy variants of Metros mainly had the lower spec engines (8.8:1 CR), the HLE/plusher types better ones (9.4:1 CR), and MG the best (10.3:1 CR) and most popular. Don’t discount the Turbo engine just because you don’t want to get involved with all the turbo paraphernalia - it’s essentially a high compression engine (9.4:1) with superior grade pistons and a turbo bolted on. Unfortunately the camshaft is just the standard, run of the mill 1275 A+ type. The only point to bear in mind if going for the turbo unit is that many suffered gearbox failures, so it may be prudent to use just the engine fitted to your Minis gearbox. Power outputs are a bit variable, but generally go from 50-odd hp on the lower performance jobs to 74hp for the MG. That’s why it’s the popular choice!! The extra power comes from a special cam, Cooper S-size inlet valves, careful profiling of inlet throats, high CR, and efficient manifolding. 
Allegros, standard Austin 1300s/AAs, and 1275GTs all shared the same engine - the 8.8:1 CR job, power being around the 50-odd hp mark. However, the Austin 1300GT and Vanden Plas version used an up-rated engine. 9.75:1, S-sized inlet valves, and twin 1.25” carbs. They also had the extra cylinder head stud and bolt at either end of the rocker cover as used on the S. In fact the last few Mk3 Ss used this engine build. 
If buying a modified engine of any sort, the basic identifying features still stand, as does their swap-ability with regard to ancillaries. 
NOTE; It is entirely feasible to use the pre-Verto flywheel and clutch assembly with the later pre-engaged (integral solenoid) type starter motor. The best way to accomplish satisfactory fitment is to fit the narrow ring gear of either the Verto flywheel (part no. PSF10003, 129 teeth). However, the standard pre-Verto ring gears (107 teeth) will also work OK with the pre-engaged starter, albeit somewhat noisily that will shorten ring gear and starter bendix life. Where the standard, wide ring gear (0.50-in wide) is fitted, a 0.125-in spacer MUST be fitted between the starter and the transfer gear case to prevent the starter bendix from being permanently engaged. Washers to the value of 0.125-in will be Ok for a short period, a proper, full spacer plate duplicating the starter mounting plate is necessary to ensure long life of the starter. Failure to do this will cause extensive damage to ring gear and starter at the least. The thin ring gear (0.345-in wide, part no.12G2613) such as used on the ultra light and steel lightweight flywheels is used, no spacers are required. 
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