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Sorry for the simplicity of this site but I thought I'd make the pictures available first and then worry about the presentation later.
These pictures are of the very first BMW K Twincam head converted Mini. I completed it in 1989 and it was first seen at the combined East/West US Mini Meet that year. The engine configuration is a thick cored 850 block offset bored to 68mm, stroked with a 998 crank; using 'S' rods. The completed displacement is 1071cc by pure accident -its not the same bore and stroke configuration of an 'S' 1071.
The car was a daily driver for several years and even completed a cross country trip in 1991 from Seattle to Chicago and back for Mini Meet East Chicago. It was parked for many years after that and the later model Canadian body deteriorated severely. I recently resurrected it as a daily driver, but I've decided to swap the original 998 motor back in and freshen up the Twincam to be placed in another shell.
Hope you enjoy the photos - Gregg
1. Pictures of engine in car - daily driving configuration with air cleaner plenum and remote filter box removed.
c. Header view
e. From the top
f. The whole car. License plates shown were temporary - used to say TWNCAM - now has TWINCAM.
2. Pictures of engine freshly out of car.
b. Detail showing external oil pressure feed to head and return drain to tappet cover
c. Detail showing where that pressure feed comes off the main oil galley
g. The all important heater control valve
h. More of the business end showing she's actually a Tri-cam.
h. The air plenum sitting on top of the motor
i. Carbs set on top of the motor
Engine dissassembly photo's 11-6-02
b. Galling detail for all those who like looking at nasty wounds.
c. Valve cover modification to create end sealing plate.
g. Head upside down showing bar which seals off bottom of BMW's cam drive chanmber.
h. Blanked off oil return galleys.
i. Block with head gasket still in place. Very studdley if I may say so myself.
j. The BMW head gasket by itself.
k. Block with head gasket removed.
l. In this photo you can see the faint images of various plugs I installed in the block deck.
m. An Allen-nut typical of what I used to hold down the head. Fits in a small counterbore area.
o. Because of the uneven bore spacing here's an example of an untouched piston.
p. Cog wheels and adapter shafts.
q. Here's what one of the cam adapter shafts looks like.
r. Crank timing gear captured by modified balancer pulley.
s. Water pump pulley spacers so that things lined up.
Here's a link to Matt Read's Miniman site who's similarly bit with the Twincam A series bug. Check out his 16 valve head conversion project. It makes me tired just looking at all the stuff he does.
Reasons the Twincam was built from an 850 block (But no reasons why someone shouldn't try it on a 1275)
1. Their cheap and plentiful. Wouldn't feel bad if one ended up scrap.
2. Small bore engines have inner tappet walls; since the entire rear row of head studs had to be repositioned, its best they're anchored in a structurally sound area. Big bore Austin America blocks don't have any structure in this area and have a thin block deck. I don't know what the situation is with A+ big bore blocks.
3. The smaller starting bore means that you can offset the bore to achieve consistent bore spacing and match the bore center to the combustion chamber center. The BMW head has consistent bore spacing. The Mini block does not. Even after offsetting, they're still not perfect, but they're a lot better than they would have been without doing anything.
4. On small bore blocks there's a reasonable amount of wall between cylinders 2 and 3; reduced likelihood of blown head gaskets.
5. 1275 blocks are taller than 850 blocks. With the 850 block the head just fits under a standard hood. I barely have room between the hood and head for a throttle cable, that's it.
6. I wouldn't have been able to use the BMW head gasket with a big bore block as there would have been significant gasket overhang into the bores. A custom head gasket out of annealed copper of the like would have been possible, but it was just one more complication I wished to avoid.
7. I'm just a small bore kinda guy!
Modifications to Block
The most extensive metal cutting modifications were made to the block. The head is too busy with valves, ports, tappets, bearings and coolant passages to change much of anything. It's probably even worse with the 16V head. The 3 front stud locations in the block were spot on and did not move. Amazing! The remaining stud locations, pushrod holes and cooling holes were tapped and plugged with cast iron pipe plugs, 23 locations in all. The block deck was then cut flush and 27 new stud and water passage locations were drilled to match the head. I ended up using the BMW head gasket in the final assembly so it served as a useful pattern during these modifications. All this work was done by hand with the aid of a drill press. Offset boring of the block was performed by a marine machine shop here in Seattle. They were the only ones I could find that would attempt the job and they charged a small fortune. I had taken another 850 block to a weld shop and they'd sliced the deck off with an automatic hack saw so I could judge the cylinder wall thickness. The offset boring meant I had uneven cylinder wall thickness'. I'm certain there's a good deal of undesirable distortion of the bore due to that variation but it was a matter of tradeoffs. The offset boring also meant that the bores were no longer centralized over their respective crank throws. I dealt with that situation by using Cooper 'S' connecting rods. They have nearly the exact amount of offset built into them to compensate for the bore shift. The offset boring exposed the cross drilled oil passage that Vizard warns about. That had to be plugged, driving the machining costs up. To safely boost displacement I installed an A+ 998cc crankshaft. That required some machine shop modifications to the block thrust surfaces.
Modifications to the head
On the K bikes the head is oriented with its intake ports facing up and its exhaust ports facing down. Unlike a standard Mini, where pushrod holes form the drain path for excess oil the BMW's valve cover cavity was the primary pathway for oil drainage away from the tappets. However when the head is mounted in a horizontal fashion provisions have to be made for proper oil drainage. I purchased some very long "aircraft" drill bits and bored holes along each side of the camshaft bearing saddles. This provided a pathway for the oil to flow towards the cam drive end of the head. At that location an external pipe connected to the front tappet cover for final passage into the block. There were several additional holes that had to be plugged in the head but for the most part it was largely unmodified and probably could be reinstalled on a K bike with very little work.
Cam drive
Both the Mini and the K bike use an oiled chain drive system. A custom chain drive didn't seem like a simple affair especially considering the Mini's water pump location, right smack in the middle of it all. A belt drive system seemed much more achievable. I had a Mini after market belt drive kit which I utilized primarily for it's oil sealing plate. The cog wheels I used are inexpensive Ford Racing parts from their 2.3L SOHC engines and have numerous timing adjustment key-ways to allow for pretty simple cam timing adjustments. The head has an aluminum end plate I fabricated by hand with 2 oil seals installed. The camshafts are brought out through those oil seals with short extension shafts made from discarded Ford 2.3L camshafts. A tensioner, also from a Ford 2.3L engine was mounted to the front engine plate to complete the package. The belt is from a VW Rabbit. This all worked out very well as I was able to use a stock water pump, stock fan and stock radiator.
Loads of other detail stuff
The Mini thermostat is mounted in its head, the BMW's is mounted in its block, so I ended up fabricating my own thermostat housing by capturing a thermostat between the compression nuts of some 1-1/4 plumbing sink pipes. I think the actual thermostat was from a Renault. The headers were about the only used BMW part I incorporated in the project. They were after market Luffmeister porcelain coated pipes. If I had mounted them the way they were made they would have pointed straight through the right side inner fender. I ended up cutting them into numerous pieces and welding them back together to get them routed downwards. I remember it was not easy gas welding glass coated metal. At the time I could not afford the BMW fuel injection components. A friend of mine had a set of carbs off a Honda 650 Nighthawk. I had to extensively modify the linkages and fuel pipes so the carb spacing matched up with the BMW head. The head is set up for direct port electronic fuel injection. It would be really nice to get that setup going. The carbs blocked access to the standard Mini dipstick so the one you see is from some Oldsmobile I found in a junkyard. The BMW head did not have any provision for an oil filler cap so I brazed a section of pipe onto a fuel pump blanking plate and brought that out top with a plumbing supply dishwasher downspout. It was topped with a Toyota rubber oil cap. The air cleaner setup took a long time to figure out. I ended up fabricating a plenum chamber from some aluminum plate ands a 16" long loaf bread pan. The plenum was connected to a Honda CRXsi remote air cleaner box through a 3" smooth bore RV sewage hose. It looked pretty factory despite the hodgepodge of components.
Credits: Jim Fletcher, Ken Temkin, Rob Volk, John Jacobs and members of MINI (Minis in Northern Illinois) and SAMOA (Seattle Area Mini Owners Association), but most importantly Shuko and Loren.
Gregg Temkin
twincam@lojomo.com