A Series


Hi there, long time no see.  Lots of stuff happening in the last two weeks. I managed to finish what was missing in the engine and then could not resist to test it for oil pressure. It was nice to hear it spinning for the first time! Oil pressure was good. Then I waited a few days to see if there were any oil leaks – none! Not a drop. Finally I etch primed the entire engine and painted it in the usual “Middle Bronze Green” or “Mowog green” as it is called on the street. It looks awesome! Ok, enough of that, calm down. It looks good, but I haven’t started it yet. I wanted to have the carburetters done by now, but I wasn’t even able to start. I’ll deal with them in the next weeks so I can start the engine and see how it runs. All in all, I’m happy with this build so far. It’s not a 1961 850cc engine, but if we consider what it actually is, we should agree it looks pretty close. CHECK OUT THE REBUILD POSTS SHORT BLOCK PREPARATION FOR THE 850 MINI 850 MINI – SHORT BLOCK FINAL BUILD MODIFYING THE CYLINDER HEAD FOR THE 850 MINI 4 SYNCHRO GEARBOX FOR THE 850 MINI GETTING THE ENGINE READY FOR THE 850 MINI



Since I’ve got nothing better to do, I’ve been out playing with some rockers Now I don’t want to disappoint you, but probably not what you’re thinking.This kind of rockers. Original Mini rockers are very inconsistent and within the same set we can have very different valve lift results due to their geometry. I’ve measured every single rocker I have and confirmed they do vary a lot. The procedure I’ve used was to get a cylinder head on the top of the block, set the (average) rocker geometry, forget about bush wear and then use valve no. 1 (exhaust valve) to test each rocker. This way I was able to swap rockers just by sliding them out of the end of the shaft without the need of undoing any of the pedestal nuts. Having that said, I had to adjust the rocker clearance (0.012″) for each rocker tested. This was a lot of trouble and I’m still not sure why I did that. I guess I was looking for 8 rockers of the same type that were close enough to make a set. I was also looking or hoping (?) for those 8 rockers to be on the high(er) lift range… these aren’t really “high lift” rockers, but some are better than others. I’m pretty positive none of this makes much difference in a mildly tuned road engine, but… OCD sounds like a good justification. While 1275cc pressed steel rockers were showing higher lifts, I didn’t have enough to make up a consistent set. The closer ones were actually a set of Spanish Authi rockers. I don’t know why I have those, but they look good and this is the only set of forged rockers I’ve seen made specifically for small bore engines.  The old bushings were a little bit worn so I managed to swap them and ream the new bushings to 9/16″. I did that on a small lathe I recently purchased and of which I might do a post in the future as I’m facing some challenges with it. Anyway, all the main components are ready to be glued together, so let’s start with the block and gearbox. Previous Next While bolting the gearbox to the engine block is a quick and easy task, it is a significant one. Not only because we might feel that rush like the engine is almost ready, but also because we lose access to the “interior” of the engine. Any mistakes done will give a lot of work to fix. For instance, if we drop a bolt or a washer into the gearbox, or if we forget to fit the O-ring for the oil pressure or the front half moon seal, that means splitting both components again and spend a good amount of time cleaning. Even worse than that is when you take a second look to the workbench and realize the oil ring is right there… but you’re pretty sure you didn’t forget it. Then you start questioning if this is a second ring, or if you actually forgot to fit it. When that happens, and it will happen sooner or later, the right thing to do is to actually go through the work and check everything. And while you’re at it, check if all the rods and mains bolts were torqued to the correct setting. The next step is kind of similar which is the fitting of the clutch housing. That will cover and block access to the drop gears, so before doing that, check if you’re not leaving anything behind. Paper towels, a couple of sockets, your swiss army knife… The idler gear, which is just barely visible through the primary gear hole above, runs between the gearbox face and the clutch housing. It’s function is to transfer the power from the flywheel to the gearbox – it acts like an intermmediate gear to make things spin the correct way. For that reason, it will be subject to some heat and thermal expansion and that’s why it needs some clearance to run – the books quote 0.003 – 0.006″. That clearance is normally checked with the clutch housing mounted to the gearbox – no engine, like the last image here. That’s when we get access to properly check the endfloat/clearance and if needed, easily swap the thick shims that sit at each side of the gear. It is assumed we use the same gasket (thickness of the gasket matters) and torque the bolts to the specified torque setting. I have made that exercise in order to have 0.0045″ of endfloat – that’s the value I wanted for this engine.  However, I tried to measure that now with the clutch housing bolted on to the block and that gap closed up 2 thou. The final endfloat is actually 0.0025″. That’s a bit on the tight side, but the gear still feels right. Also I’m thinking that this 0.0025″ gap is accurate in the sense that nothing else will squeeze it and that measured clearance will actually be there when running. It might actually increase after the gear is pushed hard from side to side. In a way, this reading might be “more correct” than those taken with feeler blades when just the block and clutch housing are involved. Plus there should be a bit of cosine error with the DTI reading…  Well, I’m done trying to convince myself this clearance is OK. The gear feels fine to me, so against my own words above, I’m taking this risk and going forward. Now is the correct time to torque the camshaft nut. In order to be able to do that, the camshaft needs to be locked so it doesn’t spin. We can lock the crank and cam, which are connected through the timing gears, by fitting the flywheel to the other end of the crank and slip an old mains shell between the starter ring teeth and the flywheel housing. Alternatively, there’s a tool with a tooth welded that can be bolted to the starter motor



The so called 4 synchro gearbox was introduced in the Autumn of 1968 and that was a significant upgrade to the Mini driveability. Up until then only 3 synchro gearboxes were available – the first gear is the non-synchromeshed one. In practice, having a 3 synchro gearbox means we need to stop the car in order to engage first gear. But these old 3 synchro gearboxes still have their place today, mostly on original cars – namely the Cooper S with its famous 22G190 or 22G333 casings – and in my opinion, they’re not really that bad. As long as we remember not to engage first while the car is still moving, a well built 3 synchro gearbox is fine. However, not all 3 synchro gearboxes are the same, they were developed and evolved through time. The first Minis, up to 1962, had the cone synchromeshed type of gears. They’re called like that because they had a bronze wedged ring press fitted in 2nd, 3rd and 4th gears that was supposed to bite the synchro hub in order to help matching the speed of the gear train to the speed of the wheels – that’s what makes a gear change smooth. That didn’t work very well because the bronze rings would wear fast and soon enough the gears would start to crunch and being more difficult to change. To improve on that, the A-type gears were introduced with baulk rings. Those are independent rings that work between the synchro hubs and the gears – if they wear, we can replace them. But A-type gears were a bit loud due to the steep angle the helicoidal teeth were cut, so around 1964 the new B-Type gears were introduced and lasted until the 4 synchro boxes were standard in all engines. There were a few more specific changes done through time, but in generic terms these are the 3 types of gears we might find in an early Mini gearbox. Using a centre oil pick up pipe helps insuring the engine will not get oil starvation on long hard corners, unlike what might happen with the original pick up pipe mounted at one corner. However, when fitting this new part we must check if it fouls the reverse gear, most of the times it will. To fix this problem we need to bend the pipe until it clears the gear. However, this might only be possible if we grind the web in the casing and enlarge the hole where the pipe goes through. Here we can see adequate clearance to the reverse gear. The original 22A145 gearbox from my Mini has cone synchromeshed gears. Those gears are now worn and even if I could replace them, it makes no sense to use that type of gearset when the rest of the engine was upgraded. Using a later type of 3 synchro gearset in the original casing is not straight forward because this case is machined for the early and smaller double-row bearing. I’ve not gone deep into how to convert the box, but I understand that with some machining it is possible. In this case, it’s just too much work with no real benefit, so instead and since I can’t squeeze a 4 synchro gearset in a 3 synchro gear case, I’ve decided to just use a complete 4 synchro box. While this decision helps with the reliability and driveability questions, it certainly raises one which is related with the type of gear lever / gear selection mechanism. While it’s common to see references to the minimum baulk ring to gear stand-off (0.030″), I haven’t seen any about the maximum stand-off. Here I’m showing a NOS 3rd gear with a new baulk ring and a gap that’s just too big. It happened with my first gear as well and the consequence of that was the double-row bearing being clamped against the gear and not the bearing “hat”. The cluster would get locked by the baulk rings pressed against the gears. To resolve this I had to change the baulk rings. This only works, of course, if you use Hello Kitty soft vice jaws. All MK1 850cc Minis had “magic wand” gear levers. Those things go straight into the differential housing through a hole in the floors cut next to the footwell. On the other hand, the Cooper and Cooper S models of the same era, had a different type of differential housing where an alloy extension was attached. That allowed the gear lever to emerge between the seats through a hole cut in a different location in the floors – that’s the “remote” type. By 1968, when the 4 synchro boxes were introduced, the same style of remote extension was standardized in all Minis (with the exception of the Van and Pickup models that now had their own specific type of magic wand gear lever). Being from 1961, my Mini does not have that hole on the floor that allows a remote type gearbox to be used. I could grab a grinder and just open it up, but… I don’t want to. Instead of doing that, I’ll adapt the original 850 magic wand differential housing to a 4 synchro gearbox. Sure I could get the Van / Pickup conversion, but that would be too easy, wouldn’t it? Previous Next The truth is I had a few of these early 4 synchro cases (22G1128) around and after a quick inspection the one that looked better, with no broken fins and less play in the layshaft locating holes, didn’t have any differential cover attached to it. It only made sense I looked for a 3 synchro cover to fit. Swapping Mini differential covers is not recommended because originally both case and cover get the diff bearings holes machined at the same time, as if they were one single piece. Both holes are machined inline and when we swap a cover to another case most often we can feel a lip where both parts meet. Sometimes we can improve on



A naturally aspirated engine, like the A-series, is only as efficient as the air and fuel it can burn to produce power. Let me say that again: the more air and fuel we can squeeze into the cylinders and burn efficiently, the more power we’re able to get. Hopefully, we stop before something breaks. Mini engines are known to have restrictive cylinder heads, thus limiting their capability to fill up the cylinders which in turn limits their power output. This is especially true with the standard 850cc (2A629) and 998cc (12A1456) small bore heads that have small ports and valves. In theory, these heads can be replaced with off-the-shelf heads like the 12G202 (with slightly bigger ports and inlet valves), the 12G206/129295 (with even bigger ports and inlet valves) or a big bore head to make our small bore engine breathe better, but then we have a few things to consider: Mechanical compatibilityWhile all unmodified small bore heads are interchangeable, a big bore head (or a small bore head with bigger valves) in a small bore block might require some attention. If the exhaust valves are bigger than the original 1″ valves, they will not clear the cylinder walls. So depending on how much they open, they might hit the top of the block and if that happens, sooner or later those valves will bend and brake. To prevent that, the easiest solution is to get some clearance by milling a pocket on the side of each bore. Compression ratioA standard 850/998 head has 24.5cc chambers, while the best flowing small bore head – the 12G295 – has 28.3cc. This 3.8cc difference is significant and if a direct swap happens, we will loose compression resulting in a less efficient power stroke and therefore losing power. On the other hand, the big bore head has just 21.4cc chambers and if used as is in a small bore engine, the compression ratio will increase. This increased compression might/will lead to excess of heat building up and that’s when detonation (pinging/knocking) starts to happen. Then all sorts of bad things will follow. To address this, we might skim the head to decrease the chamber volume and therefore increase the compression ratio; or we might need to grind the chamber a little bit to increase the volume, lowering the compression. Air velocityIn order to have good idling and a quick response in low revs, road engines normally have short duration cams. That means the inlet valves open late and close early in each cycle and that “duration”, which is the period the inlet valve is open, leaves just a small window to fill up the cylinders with air and fuel. Then there’s the behavior of the valve. It starts to lift off the seat slowly, following the cam lobe profile and not allowing much air to flow (the head of the valve is still covering the inlet throat), then continues to rise as the piston is going down faster, until it reaches max lift where it allows most air to flow – but! – it will only stay there for a fraction of the time. Then it starts to make the opposite journey returning back to the seat. We don’t get much airflow while the valve is opening or closing – it’s a progressive event and doesn’t work like a switch. Now that we understand our window is very small and why filling the cylinder has to be fast, we might also understand that having large ports probably isn’t such a very good idea. Air velocity is slow on a large pipe. But there’s more to it: a cylinder head port is not a straight pipe subject to the same constant pressure… and then there’s the presence of fuel, which is heavier than air… and let’s not forget that we need good fuel atomization. All the dynamics involved make this a very complex subject. If you’re curious, like I am, go and make a few searches online – there’s lot of information about this and by people better qualified than I.  Let’s just say that we might want to question if that big head with huge valves is the ideal for our small Mini engine. Of course, all of this ends up being a compromise, but if we take one thing from this subject, it should be this: heads and cams go hand in hand and bigger is not always better. So I’m looking for a slight increase in performance for this 1047cc engine, but I’m also looking for reliability and driveability. That’s why I’ve chosen such a mild camshaft. Another requirement I have is looks: I don’t want this engine to look too much out of place in a 1961 car. For that reason, a big bore head is out of question and because I don’t want to skim 0.080″ (2mm) off of a 12G295 to bring the compression ratio back up, so is that cylinder head as well. After going through a number of heads, I’ve decided to try a standard 998 head and make some modifications to suit the engine. People have been doing this since the early days and I thought that choosing a cheap head would be wise given this would be my first time modifying a cylinder head. If something goes wrong, I’m ruining a head that’s considered scrap anyway. Also, having small ports means more meat to play with.  But this one revealed a suprise. Someone had been playing with it before. Previous Next After some cleaning, I grabbed a manifold gasket, a cylinder head gasket and a template I made for the new chamber shape. Those were useful to layout the new shapes on the head. Then I grabbed my mini-grinder, some cheap stones and started shaping.  I will not describe how I’ve done this as I don’t want to turn this post into a “how to modify cylinder heads”. Like I said, I’ve never done this before, I’m just familiar with some theory and curious about it. I’m not looking to dispute anything with anyone. 



After careful preparation of each individual short block component, the final assembly was fairly quick and straight forward. This type of engine is not complicated, in fact it’s quite simple, but focus during the rebuild is still the name of the game to ensure we don’t forget something simple like doing the final torque on a bolt.  So this time, a detailed build sheet was created for this engine as things were moving forward. The idea here was to have enough data to avoid unconfortable questions or doubts when/if troubleshooting something later. Things like clearances, dimensions and orientation of parts, torque figures, some bolts lengths, replacement part numbers and a checklist of things done to ensure nothing essencial is missed. Oh, and of course, lots of photos. All 6 mains bolts were replaced with their ARP equivalents – Kit # 743-2500. These are 7/16″ bolts, 20 TPI, 2.5″ long, just like the original bolts. This means we don’t need to cut/turn them to length like we have to on Cooper S ARP studs. The recommended torque setting for an original bolt is 60-65lbft. For the ARP, the recommended value is 70lbft with ARP Ultra-Torque lube. I’ve used that with no issues. A note here: this engine, even if a little modified, does not actually require this grade of bolts, but since I had them and the crank had some work/effort put into it, I thought it wouldn’t hurt to have them.  After each step is considered final, like having the final torque applied, we need to check if that component still behaves correctly and hasn’t affected other components. For instance, a crank should rotate easily with no dragging or binding anywhere. Crank endfloat should be measured again (vertically) to ensure nothing changed from the dry build to the final build. Piston ring gaps should be checked / corrected if they’re too small. When a ring gap is too small (less than 10 thousands in this case) the ends of the ring can start to bind as they grow due to thermal expansion and both ends can eventually get welded. The rule of thumb is to have a gap of 0.004″ per inch of bore. With the rods and pistons in the block, the next step is to fit the camshaft. The one I used was an old cam reground to a modern high lift / short duration type of spec. The cam has a profile close to Swiftune’s SW5 but with a little bit more lift and duration. This will ensure driveability and good manners while having that extra bite compared to the original cam.  The camshaft is the big driver when it comes to how an engine behaves and how we feel it. There’s hundreds of profiles out there so the game here is to find one that suits the engine and our driving style. Then there’s other things, like fueling, ignition timing and gear ratios, which will influence how/if an engine is able to deliver its full potential. This is why sometimes we might find 2 engines with similar specs but one does not perform exactly like the other.  For this engine I’m choosing to be on the mild side of the fence instead of going to the ‘race’ side, so I’m not expecting anything besides reliability and some, non-measurable, fun. Previous Next One of the issues I had was the cam gear binding the cam thrust plate. That was identified when I tried to measure the camshaft endfloat and started to get different results according to the rotation of the gear (0.001″ to 0.0035″).  Turns out that some of the new cam gears available today have a generous radius (see first photo where that radius was blued) and if the thrust plate is not centered correctly, the gear will bind. Although all thrust plates can be fitted with a little offset, I didn’t verify this problem with original cam gears – the ones I have don’t have that radius.  The solution for this was to center the thrust plate using the gear and a plastic zip tie to take up the clearance evenly – see second photo. Then both parts were fitted at the same time and the bolts were torqued using a socket through the cam gear holes. After centering the plate the gear and the zip tie can both be removed. If the gear didn’t offered access through the bolts, blueing is a good way to check if the plate is correctly centered. And just like that it’s ready for cam timing.



Two years into the production of the Mini, the 1961 850cc engine is still considered a very early engine where most of the issues known from the first years of production are found. Add 30, maybe 40 years of use and one can expect replacing a lot of parts in a rebuild.  IS IT WORTH IT? Even if those parts are found in new or good condition, this engine will always have reliability issues related with the very early type of gearbox using cone synchromesh gears, the wet crank tail and the early clutch. Sure, there are conversion sets, but in the end an engine like this will also have little power and while that might be fun for a while, it also gets tiring. I believe that for this specific Mini, where originality is a third priority, there are better options. This engine has been in my family since I can remember. It used to have a cylinder head on top, but it seems that in the last 10 / 15 years the bores were left exposed and even started collecting water from a broken window.  A few details tell us this would have been originally from a 1965 / 1966 ADO16 car, although it had (strangely) a Mini type remote diff housing that looked very original.  Anyway, I had the idea of using this 1098cc as the basis for the new engine. They are famous for their low down torque and the 1098cc engine is the biggest of all “small bores”, so capacity wise, the bigger I can fit in my Mini and still make it look somewhat “correct”. I want to avoid going “big bore” and keep looks similar to the original 850 engine. However two things happened pretty much at the same time that made me change the plan. First, the 1098 crank was heavily scored. Second, I had the chance to get a good 998 crank with some work done by S.H. Engineering. Since the 998 engine shares the same specification of cylinder block with the 1098  – the main difference between both engines is the crankshaft and pistons (in the 1098 the piston pin is in a lower position in relation to the top to accommodate the longer stroke of the crank) – this 12A497 block was still an interesting candidate. A FEW minutes IN THE MAGIC POT AND THE BLOCK waS READY Previous Next Actually, not so fast.  Selecting single parts from different engines means none of them had been intimate before. In fact, I don’t think any of the main components that make this new engine were ever in the same neighborhood. When this happens, it’s our job to get things started and make some introductions; check how they are getting along and how they play before they actually get married.  No assumptions and careful measurements are definitely in demand. So before dipping the block in the electrolysis bath and spend some money in machining, the viability of the block had to be addressed. First cleaning the bores. A rebore to +0.020″ would definitely not clear wear and pitting, +0.040″ was still questionable, so since I had to buy pistons, I went straight off to +0.060″. That increases capacity from 998cc to 1047cc. Then checking how the crank fits. That means checking crank journals sizes and their corresponding housings in the block. Clearances were within spec, including bearings crush figures. The holes that feed the main journals had to be enlarged in the block as they were only half aligned with the bearing holes. This is quite usual. A dry build of the crank allowed to confirm it runs straight – less than 0.001″ run out in all journals. The crank end float was also addressed at this stage: 0.045″ – 0.005″ clearance was the target and the new thrust washers needed a skim. Then all plugs removed, both from oil and water galleries to ensure we can clean all the gunk. Finally all threads chased and inspected for broken bits of bolts/studs at the bottom (believe it not, it’s not unusual). Previous Next This was the first time I tried electrolysis in an engine block and I was surprised to see how well it worked. All the heavy rust was gone and the block was pretty clean afterwards. No visible damage in any areas, including the alloy 1100 tag at the back of the block and cam bearings which hadn’t been removed at that time. But, as soon as it started to get dry, flash rust started developing very fast, so if you’re doing this, have a can of WD40 at hand. After the block was clean it was sent to the machining shop. Some holes on the top of the block were heavily corroded (water passages), so they were welded  and closed a little bit. New cam bearings were installed, a rebore of +0.060″ was done (actually the size was piston size + clearance) and a fresh skim of the top left 5 thousands of clearance between the top of the block and the flat top pistons at TDC.  The oil gallery ends were tapped – 14mm at both ends of the block and 5/16″ at the bottom. Doing this, the brass plugs could be replaced with grub screws as that’s just makes it easier to clean up the gallery in case of future need. Previous Next Meanwhile I was getting the rotating assembly ready for dynamic balancing. This is generally good practice, especially when using new parts or used parts from another engine. In this case, I was using a new flywheel / clutch and also a new crank pulley and damper combination (the same used in the Cooper S). These parts do not come balanced off the shelf and if they happen to have a small inbalance, it might be reflected as a big inbalance as the revs go higher and centrifugal force increases. Getting this assembly balanced will reduce the chance of having those big vibrations and improve the overall smoothness of the motor. This is done, of course, up