Merc 300XS engine to jackplate attachment bolts

I have 1/2" diameter bolts that attach my motor to the jackplate. The holes on the motor are larger than 1/2" and the holes in the jackplate are 1/2" (.500). Seems like the bolts are two small and should be same size as the diameter of the holes on the motor to ensure there's no movement up/down or side to side. Does anyone know what diameter hole's are on the motor and should I ream the holes on the jackplate the same size as the holes on motor and install larger diameter bolts to get rid of the slop. What type/grade of bolts are used for this installation.

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Don't do anything to the holes period. Use 1/2-20 Stainless Steel bolts and Ny-Loc nuts (when available) with flat washers. The oversized holes are just that, there not ment to take anything other than the 1/2" bolts. Opening up holes on the motor will just weaken that location.....................

I'm not talking about opening the holes on the motor. They came from the factory with larger than half inch. If I was guessing they are 9/16 inch. I'm talking about opening the 1/2" holes on jackplate to match the holes already on the motor and installing bolt with close tolerence fit.

As I said earlier, and regarless of their location (motor or J/Plate) DON'T TOUCH ANY OF THE HOLES, use them as is and torque to factory spec's. When hanging the motor onto the J/Plate, leave the motor on the hook and align the holes, instert the bolts, washers and nuts. Tighten down the nuts until they just make contact drawing the motor up to the J/Plate (use a little 2 cycle oil on the threads) and then loosen slightly just to remove any load on the nuts. Drop the motor slowly to rest on the bolt shanks, check for level (motor to boat), if ok tighten down to factory spec's, remove hook.........................your done !!!!!!!!!!

I put 9/16" grade 8 bolts and lock nuts in the top holes of the motor. The bolts went into the motor without drilling but he holes in the plate were about .030 smaller than 9/16" which required drilling. I left the bottom bolts at 1/2" as I feel there is more stress on the top bolts. Did the same thing to the top bolts where the plate attaches to the spacers. Put 5/8" grade 8's thru the transum to spacer on the top and bottom. I shouldn't have to worry about breaking a bolt.

i would use grade 8 as well

You'll hear nearly as many opinions on this subject as you do on "liquids". http://www.bassboatcentral.com/smileys/Laugh.gif

I agree that in theory, high-quality Grade 8 9/16" mounting bolts, washers and locknuts SHOULD provide improved retention.

However, in my shop, I utilize ONLY the FACTORY MOUNTING BOLT KITS, available in various lengths. This is STRICTLY for liability reasons... I can't afford to have to "back up" the integrity of "misc hardware choices" that are "safety related". http://www.bassboatcentral.com/smileys/wink.gif