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620 rattling (1st gear)


Droski

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Hello y'all just bought a 1975 620. The truck rattles upon taking off in first gear. Not sure if it's a clutch or transmission issue or possibly carburetor since its happening upon acceleration from a dead stop. Otherwise the truck runs great. Previous owner said it also happens in reverse. Any help is greatly appreciated

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Working my way towards that. It's the rubber surround that goes bad on the carrier not the bearing. Only bad confirmed bearing was metalmonkey's 620. Without the rubber support the driveshaft will flop around. But more of the shudder or thump than rattle.

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Thanks for the reply's, I would also call it a shudder I guess, the truck shakes. It does not rattle while idling in neutral . Only when I'm releasing the clutch and giving it gas in first.

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Some funny thinks happen in the transition from stopped.... to motion, in either direction. The chassis will absorb a lot of torque in first and reverse because of the transmission ratios that you don't get in higher gears. Weight transfers to the rear and  the front lifts and the rear lowers. Reverse is opposite. Drive shaft angles change. The clutch changes from slipping to gripping.

 

Two things come to mind....

 

s-l1000.jpg

 

The clutch disc has a ring of coil springs that compress and release energy during engagement. One side connects to the center and the other the disc itself. If damaged, it can cause harsh engagement called 'judder'.

 

The other is the driveshaft. Wrapped around the stationery part surrounding the carrier bearing is a rubber surround. These always rot away over time and without this support the driveshaft is unstable under load and will wobble around.

06NRVt4.jpg

 

 

Others are: loose or damaged engine and/or transmission mount rubber isolaters or their mounting bolts.  

Removed leaves* to lower the body or broken leaves in the spring pack, loose U bolts holding them on.

A lowered vehicle  with uncorrected and poor drive line angles.

Bad shock absorbers, worn out or leaking.

 

*a very bad idea.

 

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The truck is lowered on 3 inch blocks in the rear not sure how much on the front. I will check the rubber surround today. Any idea where I can purchase the rubber surround and rubber mounts ? In case that is the problem. Today is day two of owning the truck so I will give you guys an update. Thanks for all the help.

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Big vibration shakes the truck pretty violently for about 1-2 seconds. I started the truck today after leaving it parked overnight and the symptoms were gone, this is probably why I didn't feel the shaking during test drive . It seems like the symptoms come after a long drive, like when I drove it home it was pretty far then I drove to a friends house so I put some miles on it yesterday. Could it be the clutch heating up and not working properly?

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Clutch chatter can also be caused by a flywheel that has been ground incorrectly. You can only verify this by removing the trans and clutch cover and checking the runout on the flywheel clutch surface.

 

Could also be loose flywheel bolts.

 

Could also be a broken or failing clutch cover or disc.

 

Or loose u-bolts on the axle. Bad u-joints. Bad trans mount bushing. The list is endless...

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so that screeching sound described in the earlier posts was just a loose alternator belt. caught that early on.

i did have to replace the timing kit due to a worn out tensioner, but runs amazing other than the slight judder i get on the longer drives. i think its the clutch

 

im going to replace the clutch, pressure plate and flywheel next. any recommendations on a good source for a flywheel for L2OB ?

i already have the other parts , do you guys know if a 74 4speed trans will fit an L20b bolt pattern ?

i happen to have a spare 74 4speed wondering if it bolts up 

 

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'Judder' only happens during clutch engagement not on long drives.

 

The best flywheel is the one on it. Have it surface ground to true it up if you are worried about it.

 

All L series transmissions will bolt to all L series 4 or 6 cylinder engines. (not that you might want to) This is why it's called an L series. Yes the '74 L18 4 speed will fit any year L20B.

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The judder happens during clutch engagement only but not when the engine is cold. Judder seems to get more noticeable and more violent on longer drives. I will get the existing flywheel resurfaced as recommended. Thanks for the info on the transmission I had a feeling it would but didn't want to assume.

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Yeah, when it gets heated up. When you take apart have a good look at the mating pressure plate side also. High spots can form on either surface. The only clutch I had that shuddered when engaging was actually the disc. There are several small coil springs mounted axially that soften the engagement. One had popped out. If the disc is worn down to the rivets, this can cause judder too.

 

s-l1600.jpg

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