Shown above is a pair of Tobin-Arp B-138 main bearing boring bar locating fixtures for use when align boring the crankshaft main bearings in the Model A and B Ford engine block.
The Tobin-Arp uses a 1-1/8" diameter boring bar, whereas the Kwik-Way and the LEMPCO use a 1-1/4" bar. These fixtures are used with both the TA-14 and TA-15 Tobin-Arp machines.
These locating fixtures insert into the cam bearing bores at the front and rear of the engine block to locate and support the boring bar centerline, according to the camshaft bore centerline location, and the pan rail datum surface.
In this way, the Tobin-Arp fixtures directly locate and control the centerline dimension between the engine crankshaft and camshaft.
This centerline placement strategy is important to maintain proper engagement and clearances between the crankshaft and camshaft direct-mesh timing gears.
The Tobin-Arp boring bar locating fixtures also do not interfere or obstruct the main bearing caps and bolts on the Model A and B cylinder block.
In theory, this is a reliable enough strategy for locating the main bearing boring bar (to the camshaft centerline) in an engine design having a direct camshaft and crankshaft gear engagement like a Model A Ford. However, the cam and crank alignment and engagement could be compromised when using this strategy if setting up on a block having worn camshaft bearing bores.
If the block has excessive wear in either the front or rear camshaft bores, this method of locating off of the camshaft bores can introduce error in the proper placement of the boring bar in the block. This worn condition is more likely to be found on the front cam bore on Model A's and B's.
Measure cam bores for egg shape. Check your work carefully and confirm position before cutting.
Visit the links below on Ford Garage for more information about establishing and maintaining proper crankshaft and camshaft timing gear engagement and clearances.