If you insert a 9Ah battery instead of an 8Ah battery in a Royal Enfield Thunderbird 350cc, it is generally safe to do so. The Ah (ampere-hour) rating of a battery represents its capacity, indicating how much charge it can hold and deliver over a specific period of time.
In this case, using a 9Ah battery instead of an 8Ah battery means you have slightly higher capacity available. The additional 1Ah should not cause any immediate issues or harm to your motorcycle. The higher capacity battery will provide a bit more reserve power and potentially last slightly longer between charges.
It's worth noting that the battery's Ah rating primarily affects its capacity and the duration it can provide power before needing recharging. As long as the physical dimensions and voltage of the battery are compatible with your motorcycle, a slightly higher Ah rating shouldn't cause any negative consequences.
Additionally, it's recommended to consult your motorcycle's owner's manual or a qualified mechanic to confirm the appropriate battery specifications for your specific model to ensure compatibility and optimal performance.
Overall, using a slightly higher capacity battery within reasonable limits should not cause any significant problems, and it may provide a small improvement in power reserve.