Project Mini Cooper S | Magazine Series Part 4: Some Assembly Required

Update by Tim Suddard to the Austin Mini Cooper S project car
Sep 3, 2020

[Editor's Note: This article originally ran in the November 2013 issue of Classic Motorsports; for more updates on our 1967 Mini Cooper S, visit here.]

Now that our Mini Cooper S had a solid, rust-free foundation, we could turn to the mechanical work. One small issue, however: The powertrain came to us disassembled and in boxes. We had some detective work to do.

Some 25 years ago, the previous owner decided to rebuild the original engine and replace the transmission’s synchros. His first step was to pull and dismantle the engine and transmission–and that’s as far as he got.

When we obtained the car, the drivetrain parts were still wrapped in newspaper from the 1980s. Fortunately, everything was neatly organized and marked. The only things missing were the correct air cleaner housing and carburetors.

We sourced the carburetors from The Winner’s Circle. We have been dealing with this BMC A-series shop for years. DJ’s Minis had one of the early air cleaner housings in their secret stash of used Mini parts. That air cleaner housing is quite rare, so we dodged a bullet when DJ’s found it.

The news on the engine’s internals was quite good. Though a bit scuffed up, the cylinder bores were still standard. The engine had never been overbored.

The block and head were correct to the car. And unlike most early 1275cc Mini Cooper S heads, ours was not cracked. According to A-series expert Dave Anton, the head was savable despite having received numerous valve jobs in its life.

However, Dave suggested that we leave this rare and valuable head on our spare parts shelf and let him build a killer head from some of his stock. His shop, Advanced Performance Technology, has been building fast BMC engines for decades.

We declined, deciding that this car needed to stay stock–or at least nearly stock. Our plan is to carefully restore this Mini. If anything is going to be changed in the name of increased performance or durability, we want it to be unseen by the untrained eye.

Rebuilding the Lower End

The previous owner didn’t do much work on the Mini over the years, but he did collect parts. In fact, the car came with many of the bits we needed.

Included in this stash was a new-old-stock set of Hepolite 0.040-inch-over pistons. While we didn’t need to overbore our block, a set of good, free pistons is a set of good, free pistons. Besides, by overboring the engine that amount, we’d be increasing the engine’s displacement by 35cc, giving us a total of 1310cc. As the old adage goes, there is no replacement for displacement.

We took our block and crank to Gerald & Charlie’s Auto Machine Shop. They’re good machinists with reasonable rates. They’re also located in Sanford, Florida, not too far from our home office. They boiled and line-bored the block, then machined it for 0.010-inch-under main bearings. They also installed new freeze plugs and cam bearings.

The Cooper S has nicer rods than a stock Mini, and the machine shop reworked ours. After years of constant back-and-forth forces, the rod journals can get a little oblong. Resizing these openings makes them perfectly round again, and this is an important step in the engine building process.

Once the parts came back home, we assembled the engine in our shop using plenty of Red Line’s Assembly Lube. We also carefully checked and rechecked every piece as it was installed, constantly making sure the rotating assembly moved freely.

In addition to the pistons, a few other aftermarket parts found their way inside our engine. Our Hepolites are now surrounded by well-reputed Deves piston rings sourced from Mini Mania, their U.S. distributor.

We also used Automotive Racing Products rod bolts, head studs and other associated hardware. Dave told us that the stock BMC hardware crystallizes and gets brittle over time; as a result, it will not take the torque needed to secure the head to the block. Good hardware, especially when it comes to rod bolts, is easy insurance when building an engine. We use ARP’s pieces on virtually every engine we build.

Head Case

While our cylinder head had survived a gauntlet of valve jobs, the last one or two had not been done well. Dave installed some hardened valve seats that would better tolerate unleaded fuel.

Dave started the building process by milling the head’s mating surface so it would perfectly fit against our block. The top of the block was also slightly decked, and Dave figured we’ll have about a 9.8:1 compression ratio when we’re all done. Stock is 9.75:1.

The Cooper S has longer valve stems than a standard Mini, meaning oversized pieces aren’t really an option. Since there’s not much room for bigger valves anyway, Dave installed new, stock-sized valves: 1.40-inch intakes and 1.22-inch exhausts. To get the most out of this stock configuration, he also performed a three-angle valve job while cleaning up and matching the ports.

Our original rocker shaft was pretty worn out, but fortunately Mini Mania had a new one in stock. We didn’t need new rocker arms, though, since the Cooper S came with forged steel pieces–the lesser Minis received pressed steel units.

Dave recommended a little hotter camshaft, steering us toward his 266 grind. The lift is 0.270 inch, and basically this is a step up from the factory 1275cc Cooper S piece.

Thanks to modern design techniques, this cam promises excellent drivability and low-end power–two things that seem to be missing from many 1960s aftermarket camshafts. As Dave explained, this camshaft grind is his own design, not just a copy of an out-of-date piece originally released decades ago.

To finish off the valvetrain, he installed Advanced Performance Technology double valve springs. These will work better with the performance camshaft and also help keep the valves from floating at high rpm.

Advanced Performance Technology specializes in the BMC A-series engines found in the Mini, and they can build a cylinder head for any need. They also have the expertise and dyno equipment to meet any goal. What’s it cost? Depending on the specifics, budget $1000 to $2000 for one of their heads.

And the Rest

An engine needs more than pistons and valves to operate. We had some ancillaries to tackle, too.

Our ignition system now features a PerTronix Ignitor along with a stock cap and plug wires. These parts, plus our cool-looking Lucas Sport Coil, are available from Mini Mania.

We also found the parts to rebuild our original starter and generator at Mini Mania. You could argue that it’s easier to just buy rebuilt units, but we would rather redo these items ourselves: We’ve found rebuilders to be inconsistent.

The stock Mini Cooper S is rated at 75 horsepower and good for about 55 at the wheels. Dave told us we could expect about 64 horsepower at the wheels. We’ll also have complete drivability and reliability, and no concours judge will be able to tell that we modified the engine.

Once the car is back together and on the dyno, we’ll see how this all turns out.