Jim,

The entry level engines like the TT GP 42 and the OS .46 LA are smaller physically than their ball bearing counterparts, as you have pointed out. The significant measurement is the distance required between mounting beams: typically about 32mm for the GP and LA versus 36mm or so for the ball bearing .40s and .46s. Apart from this, I'd imagine there is not a lot required to fit a ball bearing engine where once a GP or LA was installed.

My recommendation is to try the GP, see how you go and if necessary upgrade to a .46.

If you are in the market for a new ball bearing engine, check the prices. Tower's prices indicate that the premium to go from a .40 ball bearing to its .46 equivalent is very small: GMS .40 to .47, $5; OS .40 FX to AX46, $5 and TT .40 Pro to TT .46 Pro, $4.

Call me old fashioned, but a 15% upgrade in capacity for $5 seems like a cracking deal to me!

Let us know what you decide.