Sometimes there's a simple fix for gear grinding with a SM-420. Some folks think "newer is better" when it comes to lubricant and they use super-slippery synthetic gear lube like 85W-140 instead of the specified 90 weight mineral oil. That prevents the synchronizers from working properly to put the brakes on those heavy gears during shifting. If you're using synthetic lube, drain it, flush the trans by idling for a for a few minutes with a couple of quarts of Diesel fuel in it, then drain and refill it with 90 weight. You might be surprised at how much better it shifts.
Jerry
