The engine block is the heart of the beast and careful measuring and drilling/boring can pay in easier assembly and replacement of parts. I start with a truing up of the mill head so all the cuts and boring are straight. Also i always index off the same point. For the Twincam this will be the #2 cylinder bore. To get it on the mill i leave one main cap dowel in on each end of the engine and place the block on the mill with the water pump side on the right and the dowels in the table t nut groove. i will pull the engine down with a stud in the #3 bore and one on each side to get the block stable. Here is where i kind of assume some things. First i assume the crank bores are straight and i assume that #2 is in the correct spot because if its not then at least all of the cylinders will be out the same amount. When you are measuring things on the engine you will notice that things will be out of spec and the bores and studs will be out one way or another .005. These old british engines are just not that accurate. Some things will be more like .015
Now we are ready to bore. I start by indexing #2 then moving the Bridgeport digital read out (DRO) over 3″ to start on #1 and take it out with increasingly more .010 cuts. I will end up at 2.845 on all four bores. Now i know this is not 76mm. Why because i know the bore walls are about .180 to .220 thick and if you add up the 73mm overbore of .97 and the fact that i moved the bore .028 this is .097/2+.028 this is about .076 thinner than stock bore wall and a .200 wall – .076 = .124 if i went directly the full 76mm i could end up under .100 wall thickness something i don’t want. Besides the reason for moving the bores is to make sure the cylinder is not shrouding the valve as much as possible but since we are at 73mm bore this is 2.5mm larger than the BMW stock bore of 70.5. All of this juggling has us clear of the valves and with minimal bore offset. The cylinder bore is now only 2.845 the rest of it will be done at the engine machine shop to get the final plateau finish at 2.874.
Here is a video of the offset boring http://s1346.photobucket.com/user/Hydrolastic/media/Twincam/IMG_2191_2_zpsynrs0hzz.mp4.html?sort=3&o=3
and one of the final to 2.845
Next post will be about moving the studs