Converting an Automatic Block to Manual Transmission Block
This tech tip offered by posters on the Mini Mania/Mini Spares Message Board:
An hour on a milling machine table will make that block indistinguishable from a manual block. Here's what needs to be done:
Drill out the 7/16" dipstick hole.
Remove the Oil Pressure Relief Valve Seat. The seat (which sort of looks like a metal spool) is cross drilled so you either need to braze or weld up the cross drilled openings, or simply replace the seat with a new "manual transmission block" one. Leave the seat now because the main oil feed hole from the seat area down through the block and into the oil pump area can only be drilled (at a slight angle in order to miss the #4 Cylinder wall) with the valve removed..
Mill a flat bottom hole, 3/8" dia., 1/4" deep for the oil pump's outlet port.
Drill the 5/16" dia. hole from the pressure relief valve to the 3/8" dia. hole you just drilled for the oil pump outlet port.
Drill the 5/16" dia. hole from the filter head pressure feed outlet to intersect the horizontal 5/16" passage from the pressure relief valve.
Make and then install a 1/4" thick spacer between the oil pump and the block because the Automatic block's oil pump recess is bored deeper than the manual block's.
A Couple of Notes:
Be aware that the oil pump spacer must be fitted before the main 5/16" oil feed hole is drilled! (You have been warned!) This is because the newly drilled hole often intersects the spacer. It is most important to seal this area on final assembly, to prevent oil leakage.
Don't forget the relief valve seat is different on an auto, it is cross drilled. As noted above, this needs to be pulled out and the X-hole filled, or a manual seat fitted.
Some of this machine work is tricky, particularly around the oil pump.
Problem is, the back of auto block is bored deeper than stock for the BIG auto pump. Auto blocks don't have the main oil pump drilling as they use an external pipe from the pump to the auto valve block.
On all 1275 blocks, the pump oilway alignment is closer to the rear block surface than on small bore blocks to clear the bigger bores. Note: By "rear block surface" we're talking about the surface of the block that faces the clutch) This means that this oilway, when drilled thru to the pump mount for this conversion, usually cuts thru the intersection of the spacer and the pump mounting face. It's best to set up and drill this hole on a milling machine to ensure it doesn't wander during drilling.
The way to seal this intersection is ensure the pump spacer is a real neat fit in the block, and use Loctite 515 Master Gasket when fitting it.
More Details and Photos:
http://home.earthlink.net/~roygmisc1/co ... vert1.html
http://members.tripod.com/austin_america/id82.html