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Thanks so much for the help. Let me clarify. These seem to be driveline vibrations, not clutch chatter. Tremendous vibrations when the clutch is being let out while picking up speed in first or reverse. Second and Third are fine. This is the second clutch kit from MAC's. The first one had the same issues. Showed hot spots on the pressure plate. Are these just bad kits? Any better ones out there?
Now I’m confused. Vibrations while the clutch is being let out is exactly what clutch chatter is in my mind. Vibration after the clutch is fully engaged is drive line.
My 48 has clutch chatter while the clutch is being let out and slipping, but once fully engaged it accelerates smoothly. I suspect an issue with the clutch.
I'm with Bob, at least for now We are probably back to clutch issues. Bench pressing vintage transmissions isn't that much fun so you better give us all the details. So this is clutch attempt #2.
A few random thoughts that may be worthless.
-I have no idea if Mac's clutch kits are known to be problematic, but I'd be researching that. I'd be calling them and trying to talk to an experienced salesperson. There is definitely such as thing as inferior clutch kits for various vehicles. I've run across a few in newer vehicles.
-I suppose an NOS kit is impossible to find?
-I'd still be inclined to put some miles on it. Improves our chances of seeing a problem when you disassemble which is almost certainly going to happen. I'd run it on a hoist so you can observe, though it wouldn't be under load.
-If the transmission comes out the smartest tranny guy I could find would be having a look at it. Fairly minor problems like a worn bearing can allow things to move around and cause problems.
Thanks again everyone. Here's the history. Truck ran, shifted flawlessly. Smooth, no vibrations whatsoever. Needed new clutch because old clutch burnt out during a long "stop and go" parade in 2019. New clutch kit installed in 2020. Vibrations were awful while letting foot off clutch in first or reverse. Mechanic thought original motor mounts were the issue. He replaced motor mounts, but no change, the vibrations still were there. Brought truck to new mechanic this May, 2021. New clutch kit installed. Same vibrations. New mechanic thinks the urethane motor mounts are too stiff, thus the vibrations. I'm no mechanic, but it seems like it's the clutch kit. First clutch kit showed some burnt out spots on pressure plate.See photo.
Urethane bushings are more prone to passing some vibration through but that's not it. I've run solid steel mounts in high performance applications and it's just a minor annoyance. Nothing that ruins your driving experience. That pressure plate isn't perfect but nothing that makes me say yeah that's the problem for sure. If the transmission comes back out get some pics of the flywheel and pilot bearing. Those were replaced/reconditioned right?
Those clutches with the solid release fingers are cheaply made and frequently a problem. The original type Long clutches had adjustable fingers and are a definite step up in quality. The Macs type can be adjusted but not easily, and that should be done at the factory anyway.
The one burned spot on your picture seems more likely to be where a greasy finger was laid on it during assembly. I have had clutch chatter with 2 or 3 high-quality clutches, one time due to oil contamination. Having a taller rear axle doesn't help, I think it is a major contributor. Is your rear axle stock or a taller gear? Light duty 3-sp trans?
That’s a nice truck, it looks like it’s in good shape. I don’t have anything against Mac’s, but I think most recommend Ft Wayne for clutch related parts for our trucks.
If the problem is clutch chatter, jumping while slipping the clutch taking off, was the flywheel refinished before the new clutch was installed? Sometimes if the flywheel has serious hot spots it'll have to be machined more than just a clean-up pass.
If the problem is clutch chatter, jumping while slipping the clutch taking off, was the flywheel refinished before the new clutch was installed? Sometimes if the flywheel has serious hot spots it'll have to be machined more than just a clean-up pass.
I have asked several times but I think he had it installed and may not be sure. That's 50% of the grip surface so it definitely matters. The three fingered pressure plate design is suspect though as Ross mentioned. They are typically used where a more aggressive clutch is desired and this isn't one of those times.
I'm still unclear if this is while engaging the clutch or when fully engaged and moving . The fact that 2 different mechanics have put clutches in it is somewhat curious if it's a bad flywheel or a ridged trans output housing .
If your mechanic replaced the shuddering clutch and didn't machine the flywheel then he deserves a kick in the pants .
I hate to throw rocks because I wasn't there, but if a hot spotted flywheel wasn't a prime suspect for a second mechanic chasing a classic symptom of clutch shudder he did fail pagoda69. If the flywheel was not turned that applies whether it's actually the problem in the end or not. It might be 70 years old.
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We are probably back to clutch issues. Bench pressing vintage transmissions isn't that much fun so you better give us all the details. So this is clutch attempt #2. 
