Dommi 88 crank end float / shimming ?

Fitting twin roller bearing to a Model 88 is a waste of time and effort unless you are planning to use your bike for serious track racing.

Fit the best roller bearing you can get to the Drive-side and an upgraded ball bearing to the Timing-side. RHP also used to offer a heavier duty ball-race RHP M306 of the notched type that employs 10 balls. There are modern equivalents around.
This set-up will probably outlive you on an 88.

All my 650 and 750 Nortons used an NJ306E C3 on the Drive-side and M306 on the Timing-side. No messing about with shims for me.

Phil is right , the engine designers knew what they were doing. The 500cc motor does not gain anything from the  change to the bearings that the bigger motors needed. Shims are just a bodge anyway and  can break up in use unless provision is made to put them out of harms way. I would rather run the motor on 2 uprated ball races than do what you propose !. It has been done.

Wayne........your choice of bearings is absolutely fine. A quality 8 ball on the timing side will be OK. Fittings these two bearings will save the hassle of shimming up the crankshaft to get the correct end-float.  Plus cut down on internal friction.
The Bracebridge bike crankcases  were machined to give 8 to 10 thou of end-float cold.
The recommended max, for end-float, is 20 thou. 
The late John Hudson told me that he frequently pulled apart Commando engines and found 40 thou gaps.
I have rebuilt dozens of Dominator engines and generally found the crankshaft end-float between 12 and 16 thou. The exception to that was a bottom half glued together with Green Hermatite.

Robert is quite correct about the twin ball bearings engines. This was a trick used by the race lads to cut down on the internal friction that increases when using double rollers. The draw-back being the drive-side bearing always needed replacing after a race day.
 

Wayne hello - Is your 88 an early Wideline (with a small valve head) , a later Wideline or Slimline (with a large valve head and Daytona cam) or an SS with the all important SS cam, h/c pistons, SS pushrods and followers, Competition magneto, Twin carbs etc.  The reason I ask is because having ridden and played with 88s all my life unless you have an 88SS "balancing the motor" will be an unnecessary extra cost.  On most models an MJ30 timing-side ball bearing with C3 clearance will be adequate with the 10-ball notched ball bearing good in the SS.  If you are building a racer or road burner good luck. Howard     

So in 1959 it would have had a Big Valve cylinder head.  These heads are easily identified in that none of the fins on the head are vertical (ie they all lie horizontal) and the inlet ports where the manifold fits are vertical.   Vertical fins behind the exhaust ports signify a Small Valve head and horizontal inlet ports, where the manifolds fit,  signify a semi-downdraught head.  If your new  con-rods are Norton and the pistons are UK made then you shouldn't need to re-balance. If the pistons and rods are much heavier than Norton ones it may be that re-balancing is required, If you decide to split the crankshaft before re-balancing use a set of new crankshaft nuts.     Cheers, howard    

So you're quite correct.  The Thunder Engineering con-rods are heavier than stock 88 and the Gandini pistons are a good bit heavier than Norton and AE.  Are you fitting twin carbs ?  It sounds as though it will make a decent racing engine......food for thought ?   I'm pleased to hear that you've gone for the notched ball race on the timing side.  Many people don't of course and regret it when the 7 balls wear out early.   Cheers, Howard