The cylinder head is back from the machinist, and looking good. A quick coat of engine paint and Brian’s ready to reassemble it.
Minor issue: One of the heater outlet stud holes was stripped, so the machinist fitted a heli-coil for us. Unfortunately, he didn’t have any imperial heli-coils to hand, so he fitted a metric one. And it’s crooked.
Rather than go back to the machinist, we decided to fix it ourselves.
Some early Minis were fitted with the optional fresh air heater, with the air intake snaking through the engine bay. It was a bit of an afterthought, and makes the engine bay quite crowded.
Since we have a brake booster and a radiator expansion bottle, we’re going to need all the space we can find. Later models moved the fresh air intake to the right side wheel arch, which seems far more sensible. Using parts from a later Mini would be equally sensible, but far less fun than making my own ducts.
I’ve made a polystyrene former and wrapped it in fibreglass with epoxy resin. Once the resin was set, I chipped out the polystrene, then used paint thinners to dissolve any remaining pieces.
Brian’s been hard at work assembling the gearbox and engine block. It’s starting to look like a power unit again.
Engine stands are cool.
The old flywheel was cracked, so we had to buy a new one. Sadly, standard cast iron flywheels are no longer available, so we had to buy a lightweight billet steel one. What a shame.
Here’s a tip: test fit your new clutch to your new flywheel *before* having the engine balanced.
It turns out that changing the flywheel on a Mini is a complete pain in the arse, because the thickness of the flywheel changes the way the clutch works.
A Mini’s clutch is a bizarre beast with an over-centre diaphragm spring on one side of the flywheel and the pressure and friction plates on the other. Changing the thickness of the flywheel changes the static height of the diaphragm spring. If your static height is wrong in either direction you won’t get the full clamping load, but if it’s too high you’ll also destroy the thrust bearings. Adjusting the static height involves machining down the clamp plate, which can change the flywheel balance- so if you do it in the wrong order you end up going back to the machinist a few times. Ugh.
Here’s a more in depth analysis of how it all works, and why it needs to be set up correctly: Minimania’s Clutch FAQ
Modifying the cylinder head on an A-series is fairly straightforward and well documented, since people have been doing it for more than 60 years. I happened to have a copy of David Vizard’s “Tuning BLs A-Series Engine” lying around, so that’s the guide I used.
Tungsten carbide cutters in a die grinder make short work of shaping the cast iron head. Grinding wheels are for sissies.
Once the basic porting and combustion chamber shaping was done, the head was sent back to the engine rebuilders to have the new unleaded valve seats and valve guides installed.
When having an engine machined, it’s a good idea to write down all the sizes and specifications and leave the list with your engine machinist. That way, when you specify 0.002-0.003″ end float and it comes back to you with 0.009″ end float, it’s your machinist’s problem.
A-series engine thrust bearings only come a few thicknesses, and this was well outside the range that could be fixed with standard oversize parts. Once we’d finished shouting, we found that there’s a special, massively oversized thrust bearing available, and our engine machinist was able to mill it down to the custom thickness needed.
Strangely enough, having had this near disaster, we went back to the same machinist for the head work. More on that later.