Rebuilding an engine from a pile of bitsThis is the first of a series of articles under the general theme of ‘adventures in classic mini ownership’. Basically a diary of how I waste my evenings and weekends tinkering with mini related projects... These were originally written by me for our local mini club newsletter, but I thought may be of wider interest here. I’m re-posting with prior permission of Mark.
When I bought my 1967 Mini Cooper 1275 ’S’ last year, I was pleasantly surprised how good a condition it was in after 50 years. The body looked mostly standard except for later alloy wheels and plastic arches, but the powertrain definitely was not: a M.E.D. 1380cc A+ engine, straight through exhaust, K.A.D. quick-shift gearstick and a sports steering wheel.
I really wanted to experience the original character of finally driving the fabled Cooper S from my childhood dreams, so I decided that my long-term project would be to remove all the bolt-on aftermarket parts and do a sympathetic restoration back to original 1967 factory specification. Or, alternatively, as one of my friends, put it: “So you want to undo 50 years of progress, and reinstall the unreliability!”
Fortunately, some original parts that had been removed by the previous owner were included as part of the sale, in boxes and a wooden engine crate that accompanied the car. I bought this car sight-unseen off eBay with no prior inspection, so there was no telling what else would be included...
Opening the crate, I found most of the components that made up a 1275 short engine: Crank, Pistons, Rods, block, flywheel, camshaft and followers, duplex timing gear, front plate, tappet and timing chain covers. Looking up the forging numbers in the AKD showed these were all genuine Cooper S Mk1 parts! So that box of bits would be my starting point for the restoration.
But first, I needed to know why the engine had been swapped out. I got no history with the car, so this would have to be figured out from scratch.
The camshaft certainly wasn’t from a 1275 ‘S’. It was the correct pin drive, but it had 3/8” lobes so was obviously from a small bore engine. It also showed only moderate wear on the lobes - unlike the retaining nut which looked like it had been removed with a cold chisel!
Interestingly, best as I could tell the block was in fairly good condition. It was bored +20 (1293cc) matching the pistons, but with moderate wear - no ‘step’ in the top of the bores, no scoring, and about 2 thou ovalled front to back as best as I could measure using bore gauges and a vernier caliper.
So evidently the engine had not run too much after last being overhauled. On the other hand, the crankshaft main journals were badly scored and worn, and there was evidence from pulled-up threads in the front end of the block flange that it had ‘bucked’ under torque. So whatever failed, it was at high revs - maybe the engine had suffered oil pressure failure and the crank seized while driving...
That camshaft seemed suspicious - best guess is that a pin drive Cooper 998 oil pump and camshaft were fitted when the block was last rebuilt. But why would that cause the engine to fail so catastrophically? Curiouser and curiouser.
After some googling, I learned that the 1275 block is deeper front to back, so its AEG410 oil pump has a longer driveshaft than the 998. I did not have the old pump to confirm this theory, but using the wrong oil pump would explain the symptoms, the consensus being that the shorter pump would shear.
Now that I had a good guess what was wrong, I set about fixing it. I sent the crankshaft out to have the main and rod journals reground, and ordered a correct 510-profile 1/2” lobe camshaft. I was unable to source a peg drive, so I bought the spider drive version and the matching ‘deep nose’ 1275 oil pump used on the Mk2 S. The only other major component I needed was cylinder head. Fortunately an online vendor had big-valve AEG163 heads available rebuilt and ready to use.
Since I had no idea whether this engine was even going to run, I didn’t want to invest too much cash in a full rebuild up front. I knew the bores were worn but probably good enough to ‘go again’ so I decided to reuse the +20 pistons since they were in ok condition, but I re-ringed them +30 over size then gapped down with a hand file to the correct tolerance.
Basically I made my own equivalent of ‘reclaimer’ rings, which used to be available to repair older engines, to hopefully take up some of that 2 thou of bore ovalling. My thinking was that if the engine proved to be usable in the car, a rebore or sleeving back to standard could easily be done in future.
For the other issues, I drilled out and tapped the pulled threads on the block flange to fit a helicoil, and drilled/tapped the oil pump mounts to accept the later two-bolt style from the Mk2, using grubscrews with loctite to seal the now unused three-bolt holes. I fitted a new IWIS timing chain, but reused the original gears.
A lick of MOWOG green paint over the battleship grey POR15 that a previous owner had inflicted on the block, sunshine yellow for the fan pulley, and it was ready to go back in the car for the first time in nearly 20 years.
