JonnyPlus2 wrote:Hi All;
In his book Tuning Twin Cam Fords, on pg. 34 David Vizard states "Having acquired these (steel main caps), they must be inline-bored to 0.015" oversize to accommodate the Vandervell competition bearings".
My machinist thinks that the oversize bore is unnecessary to ensure that the mains are true. Does anyone know why Vizard spec'ced 0.015" oversize? BTW the main caps are from QED and I used their socket head cap screws.
Thanks much.
Best regards,
Jonny
To remove a set of caps (or start with a block without them) and fit new caps and retain the original bore size and position requires either a lot of luck that the size and alignment of replacements are absolutely spot-on, or a very skilled machinist to bore the replacement mains caps without taking any more material out of any surface of the original main bearing (half)bore in the block, and make round holes. A couple of thousandths of an inch here or there will make out-of-round holes.
So with that method and its problems it is sometimes wise to make a new bore slightly oversize to make sure the holes are perfectly round and where there aren't any spots where the machining tool has "hit or miss" in various places due to set-up / alignment issues.
The other way to keep the original bore size is to machine /surface grind the platforms where the mating faces for the mains caps sit (the faces on the block, not on the caps) in the block down by a small amount (perhaps 0.005" - 0.010" or so, as a guess), then machine the mating faces of the mains caps themselves down slightly, making the caps shorter, and then carefully set-up to machine an original size hole - taking material from both the original block bearing bores and the replacement caps - this method means the main bearing bores will be slightly higher in the block once finished (ie towards the head and maybe even moved sideways one way or another). In doing so you are moving the centre line of the crank slightly in the block, but hopefully no more than a negligible amount so as not to affect crank position and therefore how much room the pistons have before being proud above the block face (and you then start to run into other issues). If it doesn't go too well the crank centreline may move more than a small amount.
The question you may want to ask your machinist is how round will the holes be and how far will the crank have moved etc in the block after the centreline is changed, if they choose that method.
There are people who can do it either way, but making perfectly round and true to size holes / bores once finished is another story.