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Sounds like stretched clutch cable, so you're still getting a bit of drive with the clutch in. Forward gears are all synchronized. Clutch wear would be going the other way, where you're not getting enough engagement, this sounds like too much engagement.
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Clutch is probably dragging (remaining engaged)
Hopefully an adjustment will solve the problem.
Check for adequate free-play (pedal movement Before clutch dis-engages)
Reverse is always a little tricky, as you are changing rotation direction in the transmission, and momentum / engagement will keep tranny churning in the direction of engine rotation.
Maybe you can do without reverse!
I drove one vehicle for 9 yrs without reverse gear.
This could be a sign your clutch is not adjusted right, and not releasing "enough" to allow shifting into the unsynchronized reverse gear these have. There should be only a small amount of slack as you start pushing the clutch pedal down, look in a shop manual. If you don't have a shop manual, get one. A lot of old timers like a lot of slack in the clutch linkage because as the clutch wears, the slack diminishes. If you don't feel a reduction in force when you have the clutch fully disengaged, and this holds for every vehicle I have ever seen with a manual, you have excessive slack in the clutch linkage.
I rather doubt your shifter linkage has anything to do with this.
A temporary work-around would be to engage any forward gear, holding the clutch down, and then shift into reverse. This is a good practice anyway.
You note that you drive the car frequently, but if you let it sit for a few weeks for whatever reason, you may need to "unstick" the clutch. I recommend starting in neutral, just a little less drag on the starter and no thrust on the main bearing. If, after starting, it's balky to put into gear, you may be able to engage 4th say but not 1st. Reverse would definitely grind. After engaging 4th you should be able to get into 1st. If the clutch is bad stuck, you may have to put the car in gear, I would recommend put it in 4th, hold the clutch down and start it. Be ready for the car to pull forward a bit.
Finally if you get a clutch really bad stuck, you can put the rear axle up on jack stands, start the car in 4th with the clutch held down, and, still holding the clutch down, hit the brakes.
You didn't ask about lube, but in my experience Redline MTL works great in these old trannys.
Sounds like stretched clutch cable, so you're still getting a bit of drive with the clutch in. Forward gears are all synchronized. Clutch wear would be going the other way, where you're not getting enough engagement, this sounds like too much engagement.
This vintage of GM car has a mechanical linkage to the clutch, not a cable.
But you are exactly right that the clutch is engaging too much. Too much slack in the clutch linkage.
More unsolicited advice for the OP - If you don't mind that it does not look original, and making sure it will fit, dropping in an HEI type distributor is a good upgrade. You need to put the ignition wire to this on a relay, to bypass the integral ballast resistor. If HEI won't fit or you want to maintain original appearance, an aftermarket point replacement like the Mallory Unilite is also a good upgrade. An adjustable (Ford type) vacuum can is also a good upgrade, can get you a bit better mileage.
OK, I forgot to mention that your clutch linkage may be failing. I had a 68 GTO and it had a fairly stiff clutch, the "bell crank" that was rotated by the pull rod from the clutch pedal and which actuated a push rod to the clutch actuator on the bell housing developed a crack. Was able to make a weld repair. You might get a new part from a yard or maybe Year One or similar vendors. If you have trouble DM me, there is an excellent yard nearby that specializes in vintage GM cars. Knowing GM, that clutch linkage was probably used on many different models and for a long time as well. Of course any part in the linkage could be failing, but on my car, it was the bell crank.
If you are suddenly having the clutch grab closer and closer to the floor, suspect linkage problems. This would be like a clutch cable stretching, but these solid mechanical linkages don't stretch, they stay the same or they crack and break.
Or maybe bad luck, it's something with the throwout bearing or something else inside the bell housing. That said, pulling the MT out of this car and replacing the clutch is not that big a job, either DIY or find a good old-time shop.
Sounds like stretched clutch cable, so you're still getting a bit of drive with the clutch in. Forward gears are all synchronized. Clutch wear would be going the other way, where you're not getting enough engagement, this sounds like too much engagement.
not in a 1970 imp! thems is linkages!
OP: gum inside the clutch housing is my guess and not enuf drag on the input shaft, reverse would not be synchronized M21? yank your M code off the build sheet under the rear seat.
IIRC the spec would be 90W GL4/5 but it got modernized over the years to 75-90, some use 70-140, and gm started speccing SAE 30 with additives (specially if lim slip) and the new sexy today is ATF - which will NOT protect that unless you rebuild it with modern parts.
when you take up the clutch, when does it start dragging? immediate? the last 1inch or two of pedal should be totally free. if this is fairly virginal, I would expect EVERY bushing to be ovalized.
This vintage of GM car has a mechanical linkage to the clutch, not a cable.
But you are exactly right that the clutch is engaging too much. Too much slack in the clutch linkage.
More unsolicited advice for the OP - If you don't mind that it does not look original, and making sure it will fit, dropping in an HEI type distributor is a good upgrade. You need to put the ignition wire to this on a relay, to bypass the integral ballast resistor. If HEI won't fit or you want to maintain original appearance, an aftermarket point replacement like the Mallory Unilite is also a good upgrade. An adjustable (Ford type) vacuum can is also a good upgrade, can get you a bit better mileage.
Thanks Mitch.
The engine is a GM Performance turn key crate ZZ4 with an HEI coupled with a Richmond Super T10 four speed.
Car originally had a six cylinder, three on the tree and the entire drive train was replaced (from radiator to ring and pinion) back in 2011 and has apprx 28K miles on it.
The body, chassis and interior has an original approx 54K miles.
Now that I'm in the south I use it as a regular driver.
You may have got heavy clutch springs with that driveline update, these sometimes eventually fatigue the linkage.
Enjoy that car!
I would start by adjusting the clutch to correct and small amount of slack (do have some slack, just not too much) and see if the grinding goes away. See if the clutch stays adjusted.
Or, for that matter, a bushing in the linkage may be so worn that it prevents complete disengagement of the clutch. Just a little drag in the clutch can keep the input shaft turning enough to make it hard to get into reverse.
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