Blade bolts breaking

[Dr Red Shift]

I have a 1 yr old john deere z425 48" with less than 300 hours. As of today I have replaced all three of the (OEM) blade spindle shafts. The bolts just keep shearing off. After the first one snapped (right side) the dealer thought they were over tourqued-so I began using a tourque wrench versus an air compressor. After the second bolt snapped (left one) the service dept suggested stronger bolts, so I bought the stronghest available, again tourqued to spec., Today the third bolt snapped on the middle shaft. So far the bolts have not snapped on the replacement spindels.

Anybody else ever heard of or had a similar problem? I do regular weekly maintenance including the blades.

Only good part is I've learned to change out the spindel in under 15 min.

Thanks