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Intro and *clunk* question


hooptylife

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Hello! New Datsun owner here, looking for advice and resources to help me treat my truck right.

A little about the truck and I: its a '79 620, bought it pretty cheap up in LA to use for occassional commuting, hauling the motorcycle, and just enjoying fixing up. Its not in bad condition; it goes, stops, and turns quite well. The body has a few dents on the front end and rusty patches on the bed sides where a previous owner proved his lack of dedication by half-sanding off the original paint. Also, the driver's side floor board is... well, almost gone (my mistake not noticing that before purchase, i was pretty infatuated with this girl).

But before I ensue with body, cosmetic, and interior work, I want to make sure she's mechanically sound and will be a reliable daily driver.

The concern I'd like to address first is a worrisome *clunk* that I hear every time I let out on the clutch, seemingly coming from the rear end. I really can't describe it any better, there's no lurch or anything, just this noise (srry, not the most mechanically experienced here, this is my first serious vehicular project).

I haven't taken it to a mechanic yet, so that should probably happen soon, but if there's anything that I can check to at least diagnose this issue I want to do as much as I can myself. Any ideas here will be appreciated, I have a Haynes manual and a tool box - just point me in the right direction.

Also, anyone know of Datsun shops in the San Diego area? My searches have turned up pretty skimpy.

 

Thanks for your time!

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 The drive shaft is in two parts. Between the front and rear is a support bearing held in place by a rubber surround. Very likely the rubber isolator surrounding the carrier bearing has deteriorated.
 
620driveshaft003Large.jpg
 
 620driveshaft002Large.jpg
 

Without proper support the driveshaft will bump around.

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Thanks for your replies, gents!

I crawled under tonight and found a very wiggly u-joint (the middle one). Ordering the replacement tomorrow.

The center bearing and insulator appear to be in good shape, but my manual doesn't say how to check that bearing. The shaft rotation felt pretty course when I turned it by hand, should I replace that bearing while I have it taken apart this weekend?

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The carrier bearing rarely goes bad... I think only MM had one verified bad one. They don't transmit any power just allow the insulator to hold the spinning driveshaft in place. If the insulator is firm and the driveshaft well supported then it's fine.

 

The U joint is the next likely think to cause a clunk specially if broken. Best replace before it damages the yoke flange that holds it.

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Out of the two I have, one bearing is bad.  I ordered 2 complete replacements, one of which was made wrong (bearing in the wrong location) and bound up from too much fore/aft load on the bearing.  I had one shaft professionally rebuilt and 3 u-joint cups had been spinning in the yoke bores which is BAD.  Especially for someone like me planning to go 8500 rpms.  

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I replaced the broken u-joint tonight, the old bearings in one of the cups were ground up into powder, and completely missing in another cup

But thankfully, the old one came out fine and the new one fit well - no more *clunk*!!

 

One more concern though: while i was under there i noticed the rear dif had some play in it. Once the drive shaft was disconnected in the middle, i could spin the back half a little, maybe 3/4" either direction - not sure if this is normal diffetential behavior or if this means there are worn out gears inside that rear axle. Is this something else I need to dig into before the next joyride down the coast?

 

Oh, and also the back side of the transmission is constantly covered in oil. Its not enough to drip and puddle (what we'd call a "class 2 leak" here in the military), but it caught my eye and depending on what y'all think, might be my next area of attention.

Again, thanks for reading, and any knowledge shared will be carefully considered and much appreciated.

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Not much you can do with the diff but drive it. There will always be some back and forth as there is some clearance or lash.

 

The transmission you should have mentioned earlier as it's an easy fix.... when the drive shaft is out. There is a small oil seal in the very back of the transmission. Just ory out the old one with a screwdriver and start the new one in. Place a block of wood over it and hammer it home. Be sure to grease the seal lips so the spline doesn't tear it going in or run dry on start up.

 

The Nissan part number is 32136-U0100 but any auto store can cross reference this number or just tell them a '79 620 with F4W71B or FS5W71B 5 speed. Less that $6

 

tranny014Large-1.jpg

 

tranny020Large-1.jpg

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I don't know about new. The H-190 is probably the most common differential of all Datsuns and later Nissans. 520/521/620/720/Hardbody/Xterra trucks .  510, 610, 810 and Maximas goons plus the early Roadster, S110 and S12 all had them.

 

Only the '79 620, '80 2wd 720 and '81-'82 Z22 automatics had the 4.11 ratio

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