The crankshaft is the cornerstone of any great engine. The journal size and roundness will affect oil pressure and flow.
Great care should be used when bolting connecting rods to the crank. Below are the sizes for the different twin journals, and the acceptable wear limits.
PRE UNIT 500 AND 650
Most pre unit cranks were a three piece design held together by six 1/4" ground bolts. The connecting rod bearings were babbitt material, and not replaceable. Although some shops still can re-babbitt rods, none that we know of will do bores this small.
One solution to these "small journal" cranks is to fit a new set of rods from Thunder Engineering or LF Harris, both of which use insert bearings from the Unit 500 engine. The crankshaft will have to be ground to accommodate the later bearings. Although people may suggest these crankshafts are not strong, there is nothing wrong with this design and the cost of production is what most likely phased it out.
1946-1954 Pre Unit 500cc & 650cc (with babbitt rods)
UNIT CONSTRUCTION 500
Triumph introduced the Unit construction 500 in 1959. It had a plain bushing for the timing side instead of a ball bearing. Because the oil feed is through the bushing, the clearance and fit of the timing side journal must be checked and corrected if necessary.
To make things easier, Triumph used the same size for the bearing journal as the rod journal. Timing side bushings can be bought in .010" increment undersizes, in both metal backed and solid bronze.
In 1968 Triumph introduced a ball bearing to replace the timing side bushing. The rod journal dimensions did not change, and are as follows:
1959-1974 Unit 500cc (with insert bearings)
UNIT CONSTRUCTION 650 & 750
From 1955 to 1972 Triumph maintained the same crankshaft journal size. Mid 1950's Pre Unit 650 cranks were still a three piece design, but had a new larger journal size. When the two piece cranks were introduced, the same E6304 connecting rods with the same journal diameter remained.
Triumph changed the design of the 650 connecting rod in the 1960's, and used a shorter rod for the 750, but all 650 and 750 two piece crankshafts use the same shell bearings.
1955-1978 Unit 650cc and 750cc (with insert bearings)
CLEARANCE CLEARANCE CLEARANCE
There are a lot of things to check when building an engine. One big one if you are using racing cams or mixing parts is cam lobe to flywheel clearance. This is to be checked before the engine is put together, as it is impossible to correct when assembled. A good safety margin to use is .080" camshaft to flywheel for racing.